(a)(1) water; hydrologic factors, such as the frequency, duration,
magnitude, timing, and rate of hydrologic connections, including
shallow subsurface flow; the size, density, or number of waters that
have been determined to be similarly situated; landscape position and
geomorphology; and climatological variables such as temperature,
rainfall, and snowpack. The functions in this rule are indicators that
are tied to the chemical, physical, or biological integrity of
paragraph (a)(1) waters, including contribution of flow; trapping,
transformation, filtering, and transport of materials (including
nutrients, sediment, and other pollutants); retention and attenuation
of floodwaters and runoff; modulation of temperature in paragraph
(a)(1) waters; or provision of habitat and food resources for aquatic
species located in paragraph (a)(1) waters.
    Section IV.C of this preamble also provides guidance on
implementation of each provision of this rule. In implementing this
rule, the agencies generally will consider first if a water qualifies
as a paragraph (a)(1) water (i.e., a traditional navigable water, the
territorial seas, or an interstate water). If a waterbody is determined
to be a paragraph (a)(1) water, then it is jurisdictional with no need
for further evaluation. If a water is not a paragraph (a)(1) water, the
agencies generally will consider next whether any of the exclusions in
paragraph (b) of this rule apply to the water. The exclusions in this
rule do not apply to paragraph (a)(1) waters, and therefore, a
traditional navigable water, the territorial seas, or an interstate
water cannot be excluded under this rule, even if the water would
otherwise meet the criteria for an exclusion.\85\ If a water does not
qualify as a paragraph (a)(1) water and the agencies determine that an
exclusion is applicable (e.g., waters that meet the waste treatment
system exclusion, wetlands that qualify as prior converted cropland),
the water would not be jurisdictional under this rule. If the water is
not a paragraph (a)(1) water, and an exclusion under paragraph (b) does
not apply, then the agencies generally will determine next if the water
can be assessed under paragraphs (a)(2) through (4) of this rule. If
the water does not meet the criteria for paragraphs (a)(1) through (4),
the agencies generally will assess next if the water is jurisdictional
under paragraph (a)(5) of this rule. When assessing the jurisdictional
status of waters after the effective date of the final rule, regulators
and the public should use the definition of ``waters of the United
States'' established by this rule. For example, when assessing whether
a stream is a jurisdictional tributary, regulators and the public
should consider the provisions related to tributaries in the final
rule.\86\ If a water is not jurisdictional under paragraphs (a)(1)
through (5) of this rule, then the water does not meet the definition
of ``waters of the United States.''
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    \85\ See also discussion of the waste treatment system exclusion
in section IV.C.7.b of this preamble, infra.
    \86\ The agencies will continue to evaluate potential
enforcement actions using the regulations in place when the alleged
violation occurred. For example, if a person excavated a ditch while
the pre-2015 regulatory regime was in effect and the person complied
with the terms of the pre-2015 regulatory regime, today's final rule
does not create new liability. See United States v. Lucero, 989 F.3d
1088 (9th Cir. 2021) (explaining that the 2020 NWPR did not apply
retroactively to the defendant's violations, which occurred before
the 2020 NWPR became effective).
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    It is important to note that some aquatic resources can potentially
be assessed for jurisdiction under multiple categories of this rule.
For example, certain streams, rivers, lakes, ponds, wetlands, and
impoundments can be assessed as traditional navigable waters or
interstate waters under paragraph (a)(1)(i) or (a)(1)(iii) of this
rule. Other streams, rivers, lakes, ponds, and impoundments are
situated such that they are part of the tributary system and can be
assessed under paragraph (a)(3) of this rule. The agencies will assess
intrastate lakes and ponds, streams, and

wetlands under paragraph (a)(5) of this rule only if they do not fall
within paragraphs (a)(1) through (4). In any case, the agencies will
identify the provision or provisions of the rule under which a
determination of jurisdiction is made.
    Section IV.C of this preamble provides increased clarity and
substantial guidance to assist in implementing the relatively permanent
standard and significant nexus standard. See sections IV.C.4, IV.C.5,
and IV.C.6 of this preamble for additional information on how the
agencies will implement these standards for tributaries, adjacent
wetlands, and waters assessed under paragraph (a)(5) (these sections
include guidance on identifying waterbodies on the landscape,
determining which waters are ``relatively permanent, standing or
continuously flowing,'' identifying waters with a ``continuous surface
connection'' under the relatively permanent standard, and identifying
which waters are ``similarly situated'' and ``in the region'' under the
significant nexus standard).
    As is typical after a rule is promulgated, the agencies have
entered into a joint agency coordination memorandum to ensure the
consistency and thoroughness of the agencies' implementation of this
rule, which is available in the docket for the final rule. See Docket
ID No. EPA-HQ-OW-2021-0602. As part of these coordination procedures,
EPA and Corps field staff will coordinate on all draft approved
jurisdictional determinations based on the significant nexus standard,
and the agencies will follow a process for elevating a subset of these
determinations to EPA and Corps headquarters for review as necessary.
That coordination will be enhanced for waters assessed under paragraph
(a)(5), and headquarters at the agencies will review all draft approved
jurisdictional determinations \87\ for paragraph (a)(5) waters based on
the significant nexus standard. After nine months, the agencies will
reevaluate this requirement and assess the implementation and
coordination memorandum approach. See section IV.C.6 of this preamble
for additional discussion.
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    \87\ An approved jurisdictional determination is a Corps
document stating the presence or absence of ``waters of the United
States'' on a parcel or a written statement and map identifying the
limits of ``waters of the United States'' on a parcel. See 33 CFR
331.2.
---------------------------------------------------------------------------

    The agencies note that Congress exempted or excluded certain
discharges from the Clean Water Act or from specific permitting
requirements. This rule will not affect any of the exemptions,
including exemptions from section 404 permitting requirements provided
by section 404(f), such as those for normal farming, ranching, and
silviculture activities. 33 U.S.C. 1344(f); 40 CFR 232.3; 33 CFR 323.4.
This rule will also not affect the existing statutory or regulatory
exemptions or exclusions from section 402 NPDES permitting
requirements, such as for agricultural stormwater discharges and return
flows from irrigated agriculture, or the status of water transfers. 33
U.S.C. 1342(l)(1), (l)(2); 33 U.S.C. 1362(14); 40 CFR 122.2, 122.3(f).
In addition, where waters are covered by the Clean Water Act, the
agencies have adopted measures to simplify compliance with the Act such
as general permits and tools for expediting the permitting process
(e.g., mitigation banks, in-lieu fee programs, and functional/
conditional assessment tools). The agencies intend to continue to
develop general permits and other simplified procedures to ensure that
projects, particularly those that offer environmental or public
benefits, can proceed with the necessary environmental safeguards while
minimizing permitting delays.
    Finally, with respect to determining whether a water meets the
definition of ``waters of the United States,'' under case law and the
Corps' existing regulations ``[u]nauthorized discharges into waters of
the United States do not eliminate Clean Water Act jurisdiction, even
where such unauthorized discharges have the effect of destroying waters
of the United States.'' 33 CFR 323.2 (1987). Thus, for example, an
unpermitted discharge of fill material into a jurisdictional adjacent
wetland that destroys all wetland characteristics does not render that
water no longer jurisdictional. Nor does an authorized discharge,
filling in a part of a tributary, for example, sever jurisdiction
upstream, provided that the upstream waters meet the definition of
``waters of the United States'' absent the unauthorized discharge.
2. Traditional Navigable Waters, the Territorial Seas, and Interstate
Waters
a. This Rule
    The agencies are not making changes to the text or substance of the
provisions of the 1986 regulations covering traditional navigable
waters, the territorial seas, and interstate waters. The agencies are
consolidating these three categories of waters into one paragraph at
the beginning of the regulatory text. While combined into one
paragraph, each category will remain distinct in separate
subparagraphs. The agencies have concluded that this non-substantive
change streamlines the regulatory text and increases clarity. This
consolidation requires corresponding changes to cross references and
the numbering of other provisions in the rule. These changes increase
clarity by reducing the number of cross references necessary and make
practical sense because the jurisdictional status of other categories
of waters relies on their connection to traditional navigable waters,
the territorial seas, or interstate waters. For example, the definition
of ``significantly affect'' refers simply to ``the chemical, physical,
or biological integrity of waters identified in paragraph (a)(1) of
this section'' rather than requiring multiple cross-references to three
separate paragraphs. This streamlining is not a substantive change and
does not alter the agencies' longstanding interpretation and
implementation of these provisions.
b. Summary of the Agencies' Consideration of Public Comments and
Rationale for This Rule
    The agencies have concluded that the non-substantive change
consolidating traditional navigable waters, the territorial seas, and
interstate waters into paragraph (a)(1) streamlines the regulatory text
and increases clarity. These changes increase clarity by reducing the
number of cross references necessary and make practical sense because
the jurisdictional status of other categories of waters relies on their
connection to traditional navigable waters, the territorial seas, or
interstate waters. The rationale for retaining each of these three
water types is provided in the relevant subsections below.
    Some commenters expressed support for the categorical protection
and consolidation of traditional navigable waters, the territorial
seas, and interstate waters. One commenter stated that the
consolidation is ``consistent with the history and text of the law.''
Several commenters opposed the consolidation of the traditional
navigable waters, the territorial seas, and interstate waters
provisions into one jurisdictional category, arguing that the
categories of waters are distinct and therefore should remain separate.
The agencies agree that each of these provisions is a distinct category
but disagree that consolidating them into one paragraph has any effect
on distinguishing the types of waters which fall within each category.
Further, the agencies have kept the text of each category the same as
in the 1986 regulations and have established separate subparagraphs for
each category to ensure there is no confusion. The jurisdictional
standards for each of

the three categories are different, so the agencies will clearly
identify the subparagraph under which a particular water is
jurisdictional. A water which meets the test for traditional navigable
waters under the Clean Water Act, for example, will be identified as
jurisdictional under paragraph (a)(1)(i). Note that some waters may
fall into more than one category of paragraph (a)(1) waters (e.g., a
water may be both a traditional navigable water and an interstate
water, such as Lake Tahoe, or a water may be both a traditional
navigable water and part of the territorial seas, such as the Pacific
Ocean).
    A commenter stated that the protection of traditional navigable
waters, the territorial seas, and interstate waters should not be
affected by any exclusions that the agencies may include in this rule.
The agencies agree and the text of this rule is clear that the
exclusions do not apply to paragraph (a)(1) waters. See also section
IV.C.7 of this preamble. The Clean Water Act fundamentally protects
these three categories of waters: traditional navigable waters are
clearly encompassed within the defined term ``navigable waters''; the
territorial seas are explicitly mentioned in the definition of
``navigable waters''; and, as discussed further below, interstate
waters, by definition, are waters of the ``several States'' and are
unambiguously ``waters of the United States.'' While the agencies have
authority to draw lines excluding some aquatic features from the
definition of ``waters of the United States,'' the Clean Water Act
provides no such authority to the agencies to exclude waters in these
three unambiguous types of ``waters of the United States'' under the
statute. Even if jurisdiction over one or all of these categories of
waters were ambiguous, the agencies have concluded that since these are
the fundamental waters that Congress intended to protect under the
Clean Water Act, and that have had longstanding and unequivocal
protection, with the exception of the 2020 NWPR, it is reasonable to
establish unequivocal jurisdiction over these waters. Further, the
agencies have concluded that there are no policy, practical, or
technical bases to apply the exclusions to these paragraph (a)(1)
waters given their crucial role in the statutory regime.
    Some commenters expressed support for consolidating just
traditional navigable waters and territorial seas into a single
category of jurisdictional waters. A commenter added that this approach
is logical because these two types of waters are the only types of
waters that are explicitly referenced in the operative sections of the
Clean Water Act. The commenter asserted that combining these waters
into one category would make the rule clearer and easier to administer.
Similarly, a couple of commenters expressed concerns that the proposed
rule too broadly categorized what is considered a ``foundational''
water. The 2020 NWPR consolidated the categories of traditional
navigable waters and the territorial seas in the definition of ``waters
of the United States'' into a single paragraph in the regulatory text
in order to streamline the text but deleted the interstate waters
category. 85 FR 22280, 22338, 22340 (April 21, 2020). The agencies
agree that combining these waters into one category makes the rule
clearer and easier to administer. However, the agencies have also
combined interstate waters into the same paragraph because, as
discussed above, protecting all three categories of waters is a
fundamental aim of the Clean Water Act. See section IV.C.2.b.iii of
this preamble (discussing protection under the Clean Water Act of
interstate waters in the same manner as traditional navigable waters
and the territorial seas). Under this rule, the jurisdictional status
of the other categories of waters relies on their connection to any one
of these three categories of waters--a traditional navigable water, the
territorial seas, or an interstate water (and, where required, meeting
either the relatively permanent standard or the significant nexus
standard). Therefore, the agencies have concluded that streamlining the
rule by including all three categories of these waters in one paragraph
is reasonable and appropriate.
    A commenter suggested that the agencies provide a definition of
``foundational waters.'' The commenter suggested that ``if the common
shorthand is that the waters used for commerce, the interstate
waters[,] and the territorial seas are the `foundational waters[,]'
then the additional term `foundational waters' should be defined as
such.'' The commenter asserted that this would make the rule text
easier to understand and use. The agencies are not providing a
definition for ``foundational waters'' because they are not using the
term ``foundational waters'' in the rule text. The agencies used the
phrase ``foundational waters'' in the preamble to the proposed rule
simply for convenience and readability rather than writing the phrase
``traditional navigable waters, the territorial seas, and interstate
waters'' repeatedly. As discussed above in this preamble, in light of
the new consolidated paragraph that groups those three categories of
waters together, the agencies will simply refer to those waters as
``paragraph (a)(1) waters'' in this preamble.
i. Traditional Navigable Waters
(1) This Rule
    The Clean Water Act, the 1986 regulations, the 2015 Clean Water
Rule, the 2019 Repeal Rule, and the 2020 NWPR all include within the
scope of ``waters of the United States'' traditional navigable waters,
defined by regulation as ``all waters which are currently used, or were
used in the past, or may be susceptible to use in interstate or foreign
commerce, including all waters which are subject to the ebb and flow of
the tide.'' E.g., 33 CFR 328.3(a)(1) (2014). With respect to
traditional navigable waters, the text of the 1986 regulations and the
text of the 2020 NWPR are identical. The agencies did not propose to
amend the longstanding text defining ``traditional navigable waters''
and are not making changes to the text in this rule. As discussed
above, the agencies are consolidating three categories of waters into
one paragraph at the beginning of the regulatory text, and with this
consolidation, ``traditional navigable waters'' are identified in
paragraph (a)(1)(i) of this rule.
    The agencies also are not making changes to their longstanding
interpretation of traditional navigable waters for purposes of Clean
Water Act jurisdiction. Thus, these paragraph (a)(1)(i) waters include
all of the ``navigable waters of the United States,'' defined in 33 CFR
part 329 and by numerous decisions of the Federal courts, plus all
other waters that are navigable-in-fact (e.g., the Great Salt Lake,
Utah and Lake Minnetonka, Minnesota). To determine whether a waterbody
constitutes a paragraph (a)(1)(i) water under the regulations, relevant
considerations include the agencies' regulations; prior determinations
by the Corps, by EPA, and by the Federal courts; and case law. The
agencies will determine whether a particular waterbody is a traditional
navigable water based on application of those considerations to the
specific facts in each case.
    As noted above, the paragraph (a)(1)(i) waters include, but are not
limited to, the ``navigable waters of the United States.'' A water body
qualifies as a ``navigable water of the United States'' if it meets any
of the tests set forth in 33 CFR part 329 (e.g., the waterbody is (a)
subject to the ebb and flow of the tide, and/or (b) the waterbody is

presently used, or has been used in the past, or may be susceptible for
use (with or without reasonable improvements) to transport interstate
or foreign commerce).
    Traditional navigable waters also include ``all waters that are
currently used, or were used in the past, or may be susceptible to use
in interstate or foreign commerce, including all waters which are
subject to the ebb and flow of the tide.'' Some examples of waters that
will be considered traditional navigable waters, and thus
jurisdictional under this provision of this rule include: waters
currently being used for commercial navigation, including commercial
waterborne recreation (for example, boat rentals, guided fishing trips,
or water ski tournaments); waters that have historically been used for
commercial navigation, including commercial waterborne recreation; or
waters that are susceptible to being used in the future for commercial
navigation, including commercial waterborne recreation. See ``Waters
that Qualify as Traditional Navigable Waters Under Section (a)(1) of
the Agencies' Regulations,'' \88\ available at https://www.epa.gov/wotus/waters-qualify-traditional-navigable-waters-under-section-a1-agencies-regulations.
---------------------------------------------------------------------------

    \88\ ``Waters that Qualify as Traditional Navigable Waters Under
Section (a)(1) of the Agencies' Regulations,'' began as ``Waters
that Qualify as Waters of the United States Under Section (a)(1) of
the Agencies' Regulations'' in Appendix D to the U.S. Army Corps of
Engineers Jurisdictional Determination Form Instructional Guidebook
(available at https://usace.contentdm.oclc.org/utils/getfile/collection/p16021coll11/id/2316) that was published in 2007
concurrently with the 2007 Rapanos Guidance and thus is often simply
referred to as ``Appendix D.'' The Rapanos Guidance was updated in
2008, but Appendix D has remained unchanged since 2007. Paragraph
(a)(1)(i) of this rule was paragraph (a)(1) of the regulations in
place when the guidance was issued, but the text of that provision
has not changed through the various rulemakings defining ``waters of
the United States,'' and the agencies have continued to use the
guidance for determining whether a water is a ``traditional
navigable water.'' See 80 FR 37054, 37074 (June 29, 2015) (2015
Clean Water Rule); 85 FR 22250, 22281 (April 21, 2020) (2020 NWPR).
There have been no substantive changes to the guidance since it was
issued on May 30, 2007. In 2021, EPA and the Army established
``Waters that Qualify as Waters of the United States Under Section
(a)(1) of the Agencies' Regulations,'' as a standalone guidance
document when rescinding a memorandum on traditional navigable
waters finalized after the 2020 NWPR. However, for clarity the
agencies have updated the title to ``Waters that Qualify as
Traditional Navigable Waters Under Section (a)(1) of the Agencies'
Regulations'' and deleted references to the Rapanos Guidance. The
agencies will continue to use this guidance to determine whether a
water is a ``traditional navigable water'' for the purposes of the
Clean Water Act and the agencies' implementing regulations. This
document is available at https://www.epa.gov/wotus/waters-qualify-traditional-navigable-waters-under-section-a1-agencies-regulations.
---------------------------------------------------------------------------

2) Summary of the Agencies' Consideration of Public Comments and
Rationale for This Rule
    Supreme Court decisions have not questioned the inclusion of
traditional navigable waters in the definition of ``waters of the
United States.'' See, e.g., SWANCC, 531 U.S. at 172 (``The term
`navigable' has at least the import of showing us what Congress had in
mind as its authority for enacting the CWA: its traditional
jurisdiction over waters that were or had been navigable in fact or
which could reasonably be so made.'').
    Some commenters voiced support for the agencies' decision to
interpret the scope of traditional navigable waters consistent with the
agencies' longstanding approach in the document known as ``Waters that
Qualify as Waters of the United States Under Section (a)(1) of the
Agencies' Regulations.'' A commenter added that such an interpretation
is consistent with the agencies' longstanding guidance and is familiar
to Tribal and State co-regulators as well as the general public.
Another commenter stated that the agencies' reference to ``Waters that
Qualify as Waters of the United States Under Section (a)(1) of the
Agencies' Regulations'' would create additional confusion during the
implementation of this rule. The agencies are maintaining their
longstanding approach to traditional navigable waters for purposes of
the Clean Water Act as reflected in this well-established document. The
agencies have used this guidance since 2007 and through a number of
rulemakings. The 2020 NWPR continued use of this guidance, stating,
``because the agencies have not modified the definition of `traditional
navigable waters,' the agencies are retaining [`Waters that Qualify as
Waters of the United States Under Section (a)(1) of the Agencies'
Regulations'] to help inform implementation of that provision of this
final rule.'' 85 FR 22281 (April 21, 2020). Given the longstanding use
of the guidance, the agencies do not think it will cause confusion to
continue to use it. To provide additional clarity, however, the
agencies are maintaining this document as standalone guidance titled
``Waters that Qualify as Traditional Navigable Waters Under Section
(a)(1) of the Agencies' Regulations,'' with minor edits to the title
and to reflect that the Rapanos Guidance is no longer in effect,
simultaneously with this rule.
    After the 2020 NWPR was promulgated, the agencies issued a
coordination memorandum that created some confusion. ``U.S.
Environmental Protection Agency (EPA) and U.S. Army Corps of Engineers
(Corps) Process for Elevating and Coordinating Specific Draft
Determinations under the Clean Water Act (CWA)'' (hereinafter, ``TNW
Coordination Memorandum''). The memorandum established an
implementation process by which the agencies elevate to their
headquarters certain case-specific and stand-alone Clean Water Act
traditional navigable water determinations concluding that a water is
``susceptible to use'' solely based on evidence of recreation-based
commerce. Id. The TNW Coordination Memorandum merely required enhanced
coordination for such determinations and did not state that a
``susceptible to use'' determination could not be solely based on
evidence of recreation-based commerce. On November 17, 2021, the
agencies rescinded the TNW Coordination Memorandum but kept in place
the ``Waters that Qualify as Waters of the United States Under Section
(a)(1) of the Agencies' Regulations.'' \89\ A few commenters asserted
that recreational activities are sufficient evidence to demonstrate
that a water is susceptible to being used in the future for commercial
navigation, thereby qualifying waters supporting recreational
activities as traditional navigable waters for purposes of the Clean
Water Act. Alternatively, several commenters asserted that recreational
activities are not sufficient evidence to demonstrate that a water is a
traditional navigable water. The Supreme Court has been clear that
``[e]vidence of recreational use, depending on its nature, may bear
upon susceptibility of commercial use.'' PPL Montana v. Montana, 565
U.S. 576, 600-01 (2012) (in the context of navigability at the time of
statehood); id. at 601 (``[P]ersonal or private use by boats
demonstrates the availability of the stream for the simpler types of
commercial navigation.'' (quoting United States v. Appalachian Elec.
Power Co., 311 U.S. 377, 416 (1940))); id. (noting that the ``fact that
actual use has `been more of a private nature than of a public,
commercial sort . . . cannot be regarded as controlling''' (quoting
United States v. Utah, 283 U.S. 64, 82

(1931))). Therefore, the agencies are maintaining their longstanding
position that commercial waterborne recreation (for example, boat
rentals, guided fishing trips, or water ski tournaments) can be
considered when determining if a water is a traditional navigable
water.
---------------------------------------------------------------------------

    \89\ U.S. Environmental Protection Agency and U.S. Department of
the Army. ``Recission of June 30, 2020 Memorandum `U.S.
Environmental Protection Agency (EPA) and U.S. Army Corps of
Engineers (Corps) Process for Elevating and Coordination Specific
Draft Determinations under the Clean Water Act (CWA).'' November 17,
2021. Available at https://www.epa.gov/system/files/documents/2021-11/nwpr-tnw-coordination-rescission-memo_signed-11.17.2021.pdf.
---------------------------------------------------------------------------

    Some commenters stated that the agencies must ensure that
traditional navigable waters are not limited to just the waters that
the agencies have determined to be ``navigable waters of the United
States'' under section 10 of the Rivers and Harbors Act of 1899. Other
commenters stated that the agencies should limit the scope of
traditional navigable waters to the section 10 waters under the Rivers
and Harbors Act of 1899. The agencies are not changing their
longstanding position that the traditional navigable waters for
purposes of the Clean Water Act include, but are not limited to, the
section 10 waters under the Rivers and Harbors Act of 1899, and include
any of the waters that constitute traditional navigable waters under
relevant judicial decisions. See ``Waters that Qualify as Waters of the
United States Under Section (a)(1) of the Agencies' Regulations.'' \90\
The scope of the Rivers and Harbor Act of 1899 is generally narrower
than the scope of the Clean Water Act. See, e.g., 1902 Atlantic Ltd. v.
Hudson, 574 F. Supp. 1381, 1392-93 (E.D. Va. 1983) (explaining that
``[t]he term `navigable waters of the United States' as used in the
Rivers and Harbors Act of 1899 has a substantially different, and more
limited, meaning than the term as used in the Clean Water Act'' and
that ``the term has a more limited meaning, consistent with the
concepts of `navigation' and `navigability' as of 1899''). The scope of
``navigable waters of the United States'' under the Rivers and Harbors
Act of 1899 is thus more limited than the scope of traditional
navigable waters for purposes of the Clean Water Act and as established
in paragraph (a)(1)(i) of this rule. The Corps' regulations reflect the
difference and under the Corps' regulations, ``navigable waters of the
United States'' (i.e., waters that are subject to section 10 of the
Rivers and Harbors Act of 1899) are limited to ``those waters that are
subject to the ebb and flow of the tide and/or are presently used, or
have been used in the past, or may be susceptible for use to transport
interstate or foreign commerce.'' 33 CFR 329.4. Therefore, there are
numerous waters that have been determined to be traditional navigable
waters for purposes of the Clean Water Act, or navigable for other
purposes under Federal law, but which are not ``navigable waters of the
United States'' under section 10 of the Rivers and Harbors Act of 1899.
For example, the Supreme Court has found that the Great Salt Lake met
the test for navigability for purposes of the ownership of the bed of
the Lake at the time of Utah's statehood, even though it was not part
of a continuous waterborne highway of interstate commerce, but the
Court of Appeals for the Tenth Circuit found that evidence insufficient
to establish that the Lake is covered by the Rivers and Harbors Act of
1899. See Utah v. United States, 403 U.S. 9 (1971); Hardy Salt Co. v.
Southern Pacific Trans. Co., 501 F.2d 1156 (10th Cir. 1974). The Corps
has determined the lake to be a traditional navigable water for
purposes of the Clean Water Act based on the Supreme Court's finding
that the water in the past met the test for navigability. The
distinction the agencies have drawn between section 10 waters and
traditional navigable waters for purposes of the Clean Water Act is
entirely consistent with Supreme Court case law. The Supreme Court in
Kaiser Aetna rejected the notion ``that the concept of `navigable
waters of the United States' has a fixed meaning that remains unchanged
in whatever context it is being applied.'' Kaiser Aetna v. United
States, 444 U.S. 164, 170 (1979). Instead, the Court cautioned that
``any reliance upon judicial precedent must be predicated upon a
careful appraisal of the purpose for which the concept of
`navigability' was invoked in a particular case.'' Id. at 171 (internal
quotation marks omitted) (emphasis in original). The Supreme Court
further stated that the ``cases that discuss Congress' paramount
authority to regulate waters used in interstate commerce are
consequently best understood when viewed in terms of more traditional
Commerce Clause analysis than by reference to whether the stream, in
fact, is capable of supporting navigation or may be characterized as
[a] `navigable water of the United States.''' Id. at 174. More
recently, the Supreme Court has cautioned ``that the test for
navigability is not applied in the same way in [different] types of
cases[,]'' referring, for example, to cases arising under the Federal
Power Act, Clean Water Act, and title disputes. PPL Montana v. Montana,
565 U.S. 576, 592 (2012).
---------------------------------------------------------------------------

    \90\ See supra note 88.
---------------------------------------------------------------------------

    A number of commenters stated that the agencies' interpretation of
traditional navigable waters was inconsistent with the test for
navigability in The Daniel Ball, 77 U.S. 557 (1870), with the
discussion of navigability in SWANCC, and with the plurality and
Justice Kennedy's opinions in Rapanos. The agencies disagree. None of
the opinions in Rapanos addressed the test for traditional navigable
waters; rather, they simply cited to The Daniel Ball--the beginning of
a long line of cases addressing navigability. As the Supreme Court has
explained: ``The Daniel Ball formulation has been invoked in
considering the navigability of waters for purposes of assessing
federal regulatory authority under the Constitution, and the
application of specific federal statutes, as to the waters and their
beds.'' PPL Montana, 565 U.S. at 592 (citing The Montello, 20 Wall.
430, 439 (1874); United States v. Appalachian Elec. Power Co., 311 U.S.
377, 406 & n.21 (1940) (Federal Power Act); Rapanos, 547 U.S. at 730-31
(plurality opinion) (Clean Water Act); id. at 761 (Kennedy, J.,
concurring in judgment) (same)). In PPL Montana, the Supreme Court was
clear that the test for navigability has evolved since The Daniel Ball;
it depends upon the authority being exercised by the Federal Government
and is a case-specific inquiry. ``It should be noted, however, that the
test for navigability is not applied in the same way in these distinct
types of cases.'' 565 U.S. at 592. Of particular relevance for
traditional navigable waters for the Clean Water Act, ``federal
regulatory authority encompasses waters that only recently have become
navigable, see, e.g., Philadelphia Co. v. Stimson, 223 U.S. 605, 634-
635, 32 S.Ct. 340, 56 L.Ed. 570 (1912), were once navigable but are no
longer, see Economy Light & Power Co. v. United States, 256 U.S. 113,
123-124, 41 S.Ct. 409, 65 L.Ed. 847 (1921), or are not navigable and
never have been but may become so by reasonable improvements, see
Appalachian Elec. Power Co., supra, at 407-408, 61 S.Ct. 291. With
respect to the Federal commerce power, the inquiry regarding navigation
historically focused on interstate commerce. See The Daniel Ball,
supra, at 564. And, of course, the commerce power extends beyond
navigation. See Kaiser Aetna v. United States, 444 U.S. 164, 173-174,
100 S.Ct. 383, 62 L.Ed.2d 332 (1979). . . . Indeed, `[e]ach application
of [the Daniel Ball] test . . . is apt to uncover variations and
refinements which require further elaboration.' Appalachian Elec. Power
Co., supra, at 406, 61 S.Ct. 291.'' PPL Montana, 565 U.S. at 592-93.
Thus, the agencies' interpretation of traditional navigable waters for
purposes of the Clean Water Act is consistent with The Daniel Ball as
applied by the Supreme Court.

ii. Territorial Seas
(1) This Rule
    The Clean Water Act defines ``navigable waters'' to include ``the
territorial seas'' in section 502(7). The Clean Water Act then defines
the ``territorial seas'' in section 502(8) as ``the belt of the seas
measured from the line of ordinary low water along that portion of the
coast which is in direct contact with the open sea and the line marking
the seaward limit of inland waters, and extending seaward a distance of
three miles.'' The territorial seas establish the seaward limit of
``waters of the United States'' and are clearly jurisdictional under
the Clean Water Act.
    The Clean Water Act, the 1986 regulations, the 2015 Clean Water
Rule, the 2019 Repeal Rule, and the 2020 NWPR all included ``the
territorial seas'' as ``waters of the United States.'' This rule makes
no changes to ``the territorial seas'' provision and retains the
provision in the regulatory text, consolidated in paragraph (a)(1).
(2) Summary of the Agencies' Consideration of Public Comments and
Rationale for This Rule
    As described above, the Clean Water Act explicitly defines the
agencies' jurisdiction to include ``the territorial seas.'' This rule
confirms the agencies' jurisdiction over these waters, consistent with
Congress's direction. A commenter stated that if the agencies combine
traditional navigable waters, the territorial seas, and interstate
waters into one category of waters in this rule, the agencies should
clarify that the territorial seas represent a distinct basis for
jurisdiction and are not a type of traditional navigable water. The
agencies agree with this commenter that the territorial seas are an
independent category of jurisdictional waters. However, in the preamble
to the proposed rule, the agencies also stated that the territorial
seas are a type of traditional navigable water. While most portions of
the territorial seas are also traditional navigable waters, the
agencies are clarifying in this rule that portions of the territorial
seas that may not be navigable or capable of being used in interstate
or foreign commerce are still jurisdictional if they meet the
definition of the ``territorial seas'' in the Clean Water Act. The
agencies did not intend to exclude any portion of the territorial seas
as the term is defined in Clean Water Act section 502(8), 33 U.S.C.
1362(8). To avoid any confusion, this rule continues to list
traditional navigable waters and the territorial seas as separate
categories of jurisdictional waters.
iii. Interstate Waters
(1) This Rule
    This rule retains the longstanding categorical protections for
interstate waters, regardless of their navigability, that were
established by the earliest predecessors to the 1972 Clean Water Act
and remained in place except during the time period the 2020 NWPR was
in effect. Interstate waters are, by definition, waters of the
``several States,'' U.S. Const. Article I, section 8, and are
unambiguously ``waters of the United States.'' In addition, categorical
protection of interstate waters is the construction of the Clean Water
Act that is most consistent with the text of the statute, including
section 303(a), its purpose and history, Supreme Court case law, and
the agencies' charge to implement a ``comprehensive regulatory
program'' that protects the chemical, physical, and biological
integrity of the nation's waters.
    The agencies interpret interstate waters under this rule to mean
``all rivers, lakes, and other waters that flow across, or form a part
of, State boundaries'' based on precursor water protection statutes and
practice. See 33 U.S.C. 466i(e) (1952) (codifying Pub. L. 80-845
section 10(e), 62 Stat. 1161 (1948)). Interstate waters thus include
waters that cross or form a part of State boundaries with other States
and with other countries (Canada and Mexico). Examples of such waters
include portions of the Amargosa River, which flows from Nevada into a
dry playa in Death Valley, California, and the Great Dismal Swamp, a
wetland which crosses the border between Virginia and North Carolina.
The Amargosa River is not a traditional navigable water and does not
otherwise flow to a traditional navigable water or the territorial
seas, but under the agencies' pre-2015 regulations and the final rule,
the portion of the Amargosa River that crosses the California/Nevada
border is an interstate water. Tributaries to interstate waters like
the Amargosa River and wetlands adjacent to interstate waters and their
tributaries are critical sources of life in desert climates. Interstate
waters also include waters that meet the definition of a traditional
navigable water or are tributaries of traditional navigable waters or
the territorial seas, such as the portions of the Ohio River and
Mississippi River that cross or serve as State boundaries; the portions
of the Rio Grande that cross State boundaries (Colorado/New Mexico) or
that cross the border or serve as the border between the United States
and Mexico; and Lake Champlain, which crosses the New York/Vermont
border and crosses the border between the United States and Canada.
    Because, as explained below, the Clean Water Act unambiguously
includes interstate waters, they are fundamental to the Act in the same
manner as traditional navigable waters and the territorial seas. Even
if the text of the Clean Water Act does not unambiguously resolve the
question of jurisdiction over interstate waters, the agencies have
concluded that it is reasonable to construe the statute to protect
interstate waters without need for further assessment based on the
history of the statute, Supreme Court case law interpreting the Act,
the legislative history, and the objective of the Act to restore and
maintain the integrity of the nation's waters. Therefore, this rule,
like the 1986 regulations, provides Clean Water Act protections for
interstate waters in the same manner as for traditional navigable
waters and the territorial seas, and the following waters that meet the
relatively permanent standard or significant nexus standard based on
their connection to interstate waters are ``waters of the United
States'': tributaries to interstate waters, wetlands adjacent to
interstate waters or to their jurisdictional tributaries, and paragraph
(a)(5) waters.
    Interstate waters may be streams, lakes or ponds, or wetlands. The
longstanding definition of ``waters of the United States'' includes
interstate wetlands. As discussed in section IV.A.2.b.ii of this
preamble, the Clean Water Act's statutory text, structure, and history
establish that adjacent wetlands are ``waters of the United States''
covered by the Act. And, while the Supreme Court's focus in Riverside
Bayview was on adjacent wetlands, the Court's unanimous conclusion that
section 404(g)(1) provides express textual evidence ``that the term
`waters' included adjacent wetlands,'' 474 U.S at 138, is informative
for interstate wetlands as well. For more than 45 years the agencies
have concluded that waters, for purposes of the Clean Water Act,
include wetlands. The agencies have also, for more than 45 years,
concluded that some of those wetlands are ``waters of the United
States,'' and among those wetlands are interstate wetlands. Because the
agencies consider wetlands to be waters, the rationale for covering
interstate waters based on the history of the statute, Supreme Court
case law interpreting the Act, legislative history, and the objective
of the Act applies with full force to interstate wetlands.

    Under this provision of the rule, consistent with the pre-2015
regulatory regime, lakes, ponds, impoundments, and similar lentic (or
still) water resources, as well as wetlands, crossing State boundaries
are jurisdictional as interstate waters through the entirety of their
delineated extent.
    For streams and rivers, the agencies will determine the upstream
and downstream extent of the stream or river crossing a State boundary
or serving as a State boundary that should be considered the
``interstate water'' using stream order. Stream order is a common,
longstanding scientific concept of assigning whole numbers to indicate
the branches of a stream network. Under this method, for rivers and
streams, the ``interstate water'' extends upstream and downstream of
the State boundary for the entire length that the water is of the same
stream order. See section IV.C.4.c.ii.1 of this preamble for additional
information about stream order.
(2) Summary of the Agencies' Consideration of Public Comments and
Rationale for This Rule
    Until 1972, the predecessors of the Clean Water Act explicitly
protected interstate waters independent of their navigability. The 1948
Water Pollution Control Act declared that the ``pollution of interstate
waters'' and their tributaries is ``a public nuisance and subject to
abatement.'' 33 U.S.C. 466a(d)(1) (1952) (codifying Pub. L. 80-845
section 2(d)(1), 62 Stat. 1156 (1948)). Interstate waters were defined
without reference to navigability: ``all rivers, lakes, and other
waters that flow across, or form a part of, State boundaries.'' 33
U.S.C. 466i(e) (1952) (codifying Pub. L. 80-845 section 10(e), 62 Stat.
1161 (1948)).
    In 1961, Congress broadened the 1948 statute and made the pollution
of ``interstate or navigable waters'' subject to abatement, retaining
the definition of ``interstate waters.'' 33 U.S.C. 466g(a) (1964)
(codifying Pub. L. 87-88 section 8(a), 75 Stat. 204, 208 (1961)). In
1965, Congress required States to develop water quality standards for
``interstate waters or portions thereof within such State.'' 33 U.S.C.
1160(c)(1) (1970) (codifying Pub. L. 89-234 section 5, 79 Stat. 903,
908 (1965)); see also 33 U.S.C. 1173(e) (1970) (retaining definition of
``interstate waters''). In the 1972 Clean Water Act, Congress abandoned
the ``abatement'' approach initiated in the 1948 statute in favor of a
focus on permitting for discharges of pollutants.
    While the term ``navigable waters'' is ambiguous in some respects,
interstate waters are waters that are clearly covered by the plain
language of the definition of ``navigable waters.'' Congress defined
``navigable waters'' to mean ``the waters of the United States,
including the territorial seas.'' Interstate waters are, by definition,
waters of the ``several States,'' U.S. Const. section 8, and
consequently, are unambiguously ``waters of the United States.'' The
1972 Clean Water Act thus reflects Congress's recognition that the
degradation of water resources in one State may cause substantial harms
in other States. The Supreme Court has recognized that ``the power
conferred by the Commerce Clause [is] broad enough to permit
congressional regulation of activities causing air or water pollution,
or other environmental hazards that may have effects in more than one
State.'' Hodel v. Virginia Surface Mining & Reclamation Ass'n, 452 U.S.
264, 282 (1981).
    In addition, the text of the 1972 Clean Water Act specifically
addresses ``interstate waters'' regardless of their navigability.
Namely, section 303(a) of the 1972 Clean Water Act uses the term
``interstate waters'' and provides that pre-existing water quality
standards for ``interstate waters'' remain in effect unless EPA
determined that they were inconsistent with any applicable requirements
of the pre-1972 version of the Act. 33 U.S.C. 1313(a)(1). That plain
language is a clear indication that Congress intended the agencies to
continue to protect the water quality of interstate waters without
reference to their navigability. Excluding ``interstate waters'' as an
independent category of Clean Water Act jurisdiction would disregard
the plain language of section 303(a).
    The Supreme Court has concluded that the 1972 Clean Water Act was
``not merely another law `touching interstate waters,''' but rather
``occupied the field through the establishment of a comprehensive
regulatory program supervised by an expert administrative agency.''
City of Milwaukee v. Illinois, 451 U.S. 304, 317 (1981) (``City of
Milwaukee''). Thus, the 1972 amendments superseded the Federal common
law of nuisance as a means to protect interstate waters in favor of a
statutory ``all-encompassing program of water pollution regulation,''
id. at 318, and they did not curtail the scope of protected waters.
    Even if the text and history of the statute and Supreme Court case
law interpreting the Clean Water Act do not unambiguously resolve the
issue, the situation addressed by the Supreme Court in the City of
Milwaukee case highlights the reasonableness of the agencies'
interpretation that the Act protects interstate waters. The City of
Milwaukee litigation involved alleged discharges of inadequately
treated sewage from Milwaukee, Wisconsin sewer systems directly into
Lake Michigan, which also borders Illinois. As the Supreme Court noted,
prior to passage of the Clean Water Act, these discharges would have
had to be resolved through litigation, in which the courts must apply
``often vague and indeterminate nuisance concepts and maxims of equity
jurisprudence.'' Id. at 317. However, the Clean Water Act replaced this
unpredictable and inefficient approach with ``a comprehensive
regulatory program supervised by an expert administrative agency.'' Id.
The Court reiterated that view in Arkansas v. Oklahoma, stating in the
context of an NPDES permit for a discharge of pollutants to interstate
waters that, while the Clean Water Act may place some limits on
downstream States' participation in the permitting process, those
limits ``do not in any way constrain the EPA's authority to require a
point source to comply with downstream water quality standards.'' 503
U.S. 91, 106 (1992) (emphasis in original).
    The potential for interstate harm, and the consequent need for
Federal regulation, is particularly clear with respect to waterbodies
that span more than one State. The alternative interpretation would
leave interstate waters that do not fall within any other provisions in
the definition of ``waters of the United States'' without Federal
protection. Parties in different States would need to resolve concerns
about upstream discharges in non-jurisdictional waters through
litigation using ``often vague and indeterminate nuisance concepts and
maxims of equity jurisprudence.'' City of Milwaukee, 451 U.S. at 317;
see also 85 FR 22286 (April 21, 2020) (acknowledging in the 2020 NWPR
that ``remedies for pollution disputes among States that do not
implicate CWA sections 319(g), 401, or 402 would likely derive from
federal common law under the Supreme Court's original jurisdiction.
Remedies for disputes between a State and a public or private party
would likely derive from State or federal common law and be heard by
State or Federal courts'' (citations omitted)). Restoration of
longstanding protections for interstate waters, regardless of whether
they are navigable-in-fact, enables the agencies to address interstate
water quality issues efficiently and effectively. The agencies
interpret interstate waters to encompass all waters that Congress has
sought to protect since 1948: all rivers, lakes, and

other waters that flow across, or form a part of, State boundaries.
Public Law 80-845, sec. 10, 62 Stat. 55, at 1161 (1948). These waters
need not meet the relatively permanent standard or significant nexus
standard to be jurisdictional under the final rule.
    EPA has interpreted the Clean Water Act to cover interstate waters,
with the exception of the 2020 NWPR, since 1973. 38 FR 13528 (May 22,
1973) (providing that the term ``waters of the United States'' includes
``interstate waters and their tributaries, including adjacent
wetlands''). In the final rule promulgated in 1977, the Corps adopted
EPA's definition and included ``interstate waters and their
tributaries, including adjacent wetlands'' within the definition of
``waters of the United States.'' The preamble to that rule provided an
explanation for the inclusion of interstate waters: ``The affects [sic]
of water pollution in one state can adversely affect the quality of the
waters in another, particularly if the waters involved are interstate.
Prior to the FWPCA amendments of 1972, most federal statutes pertaining
to water quality were limited to interstate waters. We have, therefore,
included this third category consistent with the Federal government's
traditional role to protect these waters from the standpoint of water
quality and the obvious effects on interstate commerce that will occur
through pollution of interstate waters and their tributaries.'' 42 FR
37122, 37127 (July 19, 1977).
    Because the Clean Water Act unambiguously includes interstate
waters, they are fundamental to the Act in the same manner that
traditional navigable waters and the territorial seas are. Traditional
navigable waters, the territorial seas, and interstate waters cannot be
protected without also protecting the waters that have a significant
nexus to those waters. This rule protects interstate waters in the same
manner as it protects traditional navigable waters and the territorial
seas. Thus, the following waters that meet the relatively permanent
standard or significant nexus standard based on their connection to
interstate waters are ``waters of the United States'': tributaries to
interstate waters, wetlands adjacent to interstate waters or to their
jurisdictional tributaries, and paragraph (a)(5) waters. The agencies
received multiple comments on the proposed rule in favor of the
categorical inclusion of interstate waters as ``waters of the United
States,'' as well as multiple comments arguing that categorical
inclusion of interstate waters is inconsistent with the Clean Water
Act. Several commenters asserted that asserting categorical
jurisdiction over interstate waters is legally permissible, with some
arguing that the statutory language unambiguously demonstrates that the
Clean Water Act protects all interstate waters. One commenter stated
that the agencies' failure to protect all interstate waters in the 2020
NWPR ``was an abdication of a core premise of the Clean Water Act's
cooperative federalism.'' One commenter added that Federal jurisdiction
over interstate waters protects State sovereignty, rather than
threatening it, and quoted Justice Scalia's plurality opinion in
Rapanos that ``the Act protects downstream States from out-of-state
pollution that they cannot themselves regulate.'' 547 U.S. at 777.
Several of the commenters discussed downstream pollution to demonstrate
their general support for including interstate waters as a
jurisdictional category. Many of these commenters added that including
interstate waters in the definition of ``waters of the United States''
helps reduce the burden of increased pollutants from out-of-state,
upstream discharges.
    Commenters opposed to the categorical inclusion of interstate
waters stated that such an approach unlawfully reads the notion of
navigability out of the Clean Water Act. A few commenters asserted that
pursuant to SWANCC, Riverside Bayview, and Rapanos, interstate waters
or interstate wetlands can only be jurisdictional if they are navigable
or connected to navigable waters. In support of their arguments, some
commenters cited the 2020 NWPR and the order of the U.S. District Court
for the Southern District of Georgia remanding the 2015 Clean Water
Rule. Georgia v. Wheeler, 418 F. Supp. 3d 1336, 1358-59 (S.D. Ga. 2019)
(concluding that the categorical inclusion of interstate waters exceeds
the agencies' statutory authority because it ``reads the term
navigability out of the CWA''). For the reasons articulated above, the
agencies conclude that the interpretation of the agencies' authority
over interstate waters articulated in the 2020 NWPR and in Georgia v.
Wheeler is inconsistent with both the text and the history of the Clean
Water Act, as well as Supreme Court case law.
    A few commenters disagreed with the agencies' proposal to determine
jurisdiction over tributaries to interstate waters, wetlands adjacent
to interstate waters or their jurisdictional tributaries, and paragraph
(a)(5) waters, by applying the relatively permanent or significant
nexus standards to analyze their connection to the interstate water.
Alternatively, a few commenters supported interstate waters being
treated like traditional navigable waters and the territorial seas for
purposes of determining the jurisdictional status of tributaries to
interstate waters, wetlands adjacent to interstate waters or their
jurisdictional tributaries, and paragraph (a)(5) waters. The agencies
have concluded that, since interstate waters are clearly jurisdictional
under the statute, the statute requires the same protections for them
as the Clean Water Act does for traditional navigable waters and the
territorial seas. As the scientific support for protecting tributaries,
adjacent wetlands, and paragraph (a)(5) waters that satisfy the
relatively permanent or significant nexus standard is the same for
interstate waters as it is for traditional navigable waters and the
territorial seas, the agencies have reasonably defined ``waters of the
United States'' to protect such tributaries, adjacent wetlands, and
paragraph (a)(5) waters.
    In the proposed rulemaking, the agencies requested comment on
approaches for implementing the interstate waters provision, including
approaches for determining the upstream and downstream extent of a
stream or river crossing a State boundary or serving as a State
boundary that should be considered the ``interstate water.'' Several
commenters stated that the entire length of a waterbody that is of the
same stream order as the point that crosses State lines should be
considered an interstate water, and therefore jurisdictional. These
commenters added that where a river or stream itself forms the
boundary, the entire length of stream forming the boundary should be
considered an interstate water, and therefore jurisdictional. These
commenters also added that any additional reach of the stream that is
the same stream order as the portion forming the boundary should also
be jurisdictional. One commenter stated that this stream order approach
is well-understood and consistent with the longstanding pre-2015
regulatory regime and stated that it is also consistent with
longstanding accepted scientific practice. Alternatively, a few
commenters voiced opposition or concern for using stream order to
determine the reach of an interstate water, with one commenter stating
that the approach is restrictive and another stating that it could be
too expansive. The agencies agree with commenters who stated that
stream order is an appropriate approach for determining the upstream
and downstream limits of an interstate water that is a stream or river.
The agencies conclude that this

approach is reasonable and provides a method that is transparent, well-
understood, predictable, and easy to implement. This approach is
consistent with longstanding practice under the pre-2015 regulatory
regime and thus is familiar to the agencies and the public.
Additionally, this method is consistent with the agencies' approach to
characterizing tributary reaches based on stream order for purposes of
applying the relatively permanent standard in this rule (see section
IV.C.4.c.ii of this preamble), and the agencies' approach to
characterizing tributary reaches based on stream order to delineate the
catchment for purposes of applying the significant nexus standard in
this rule (see section IV.C.4.c.iii of this preamble).
(3) Waters That Cross a State-Tribal Bundary
    The agencies requested comment in the proposed rule on whether
interstate waters should encompass waters that flow across, or form a
part of, boundaries of federally recognized Tribes where these waters
simultaneously flow across, or form a part of, State boundaries. See
Public Law 80-845, sec. 10, 62 Stat. 1155, at 1161 (1948). The agencies
also sought comment on how to identify ``Tribal boundaries'' for
purposes of implementing the interstate waters provision, such as
boundaries associated with a Tribe's reservation or boundaries
associated with the term ``Indian country'' as defined at 18 U.S.C.
1151.
    Multiple commenters expressed support for treating waters that
cross or serve as State/Tribal boundaries as interstate waters, with
some commenters stating that waters that cross or serve as boundaries
between the lands of different Tribes (i.e., Tribal/Tribal boundaries)
should also be deemed interstate waters under the rule. Other
commenters did not support treating waters that cross or serve as
State/Tribal boundaries as interstate waters. Some commenters provided
input on which boundary should be considered a Tribal boundary for
purposes of the interstate waters category, with many of those
commenters expressing a preference for using ``Indian country'' as
defined at 18 U.S.C. 1151 to delineate Tribal boundaries. A few
commenters suggested that a category broader than ``Indian country''
should be used to adequately reflect Tribal interests and rights.
    As evidenced by the feedback the agencies have received, the issue
of how to address ``Tribal boundaries'' for purposes of implementing
the interstate waters provision is of great importance to Tribes as
well as various stakeholders. The agencies recognize the range of views
expressed on this issue to date, including support for interpreting
Tribal boundaries to include all waters that flow across, or form a
part of, Indian country boundaries; support for finding that interstate
waters include waters outside of Indian country that flow into areas
where Tribes exercise treaty or other rights; opposition to interstate
waters generally including waters that flow across, or form part of,
Tribal boundaries; and views in between. The agencies also acknowledge
commenters who raised questions regarding implementation of potential
interpretations of interstate waters as applied to Tribal boundaries.
    The agencies have considered the input received during pre-proposal
Tribal consultation and the public comment period for the proposed rule
and, at this time, are continuing to evaluate the issue of interstate
waters and Tribal boundaries, including what should appropriately be
considered ``Tribal boundaries'' for purposes of identifying interstate
waters under the Clean Water Act. The agencies have weighed the
benefits of addressing this issue now, based on the record currently
before them, versus undertaking additional analysis and outreach to
Tribes to gain a better understanding of Tribal boundaries as related
to interstate waters and related implications via a separate process,
described below, to avoid delaying the entire rule.
    Based on the agencies' evaluation of the comments received and the
benefits of further analysis and outreach, the agencies have decided to
conduct additional analysis and outreach to inform a future action
related to considering designating waters that cross a State/Tribal
boundary as interstate waters under the definition of ``waters of the
United States.'' The agencies recognize the importance of this issue to
Tribes and are fully committed to directly engaging with Tribal
governments as the agencies continue to evaluate this aspect of the
scope of ``waters of the United States.''
    Accordingly, the agencies will address this issue in a subsequent
action after completing additional analysis and essential outreach and
engagement activities with Tribes and interested stakeholders. Although
the agencies are not taking a position on this specific issue at this
time, a water that crosses a State/Tribal boundary may be
jurisdictional if it otherwise falls within this rule's definition of
``waters of the United States.''
3. Impoundments
a. This Rule
    Consistent with the proposal, this rule retains the provision in
the 1986 regulations that defines ``waters of the United States'' to
include impoundments of ``waters of the United States.'' Impoundments
are distinguishable from natural lakes and ponds because they are
created by discrete structures (often human-built) like dams or levees
that typically have the effect of raising the water surface elevation,
creating or expanding the area of open water, or both. Impoundments can
be natural (like beaver ponds) or artificial (like reservoirs).
    The agencies' implementation of the paragraph (a)(2) impoundments
category \91\ is based on two primary principles. First, as a matter of
policy, law, and science, impoundments do not render ``waters of the
United States'' no longer ``waters of the United States.'' Second, as a
matter of policy and science, if an impounded water has the
characteristics of another jurisdictional water, then the impoundment
is jurisdictional. Based on these principles, in implementing this rule
the agencies consider paragraph (a)(2) impoundments to include (1)
impoundments created by impounding one of the ``waters of United
States'' that was jurisdictional under this rule's definition at the
time the impoundment was created, and (2) impoundments of waters that
at the time of assessment meet the definition of ``waters of the United
States'' under paragraph (a)(1), (a)(3), or (a)(4) of this rule,
regardless of the water's jurisdictional status at the time the
impoundment was created. Waters that are jurisdictional under paragraph
(a)(5) are the exception to these two implementing principles. The text
of this regulation states that they are not covered by paragraph
(a)(2). Therefore, waters that are jurisdictional under paragraph
(a)(5) do not categorically retain their jurisdictional status as
``waters of the United States''

under paragraph (a)(2).\92\ However, a subsequently impounded
jurisdictional paragraph (a)(5) water may still be determined to be
jurisdictional if it meets the requirements of a category of ``waters
of the United States'' other than paragraph (a)(2) at the time of
assessment (i.e., as a traditional navigable water, the territorial
seas, an interstate water, a jurisdictional tributary, a jurisdictional
adjacent wetland, or a paragraph (a)(5) water).\93\
---------------------------------------------------------------------------

    \91\ Impounded waters may be jurisdictional under provisions
other than the paragraph (a)(2) impoundments provision. For example,
they may be impoundments that are traditional navigable waters and
would be jurisdictional under paragraph (a)(1), or they may be
impounded adjacent wetlands and meet the requirements to be
jurisdictional under the paragraph (a)(4) adjacent wetlands
provision. To provide clarity in this preamble, when the agencies
are discussing the subsection of impoundments that are
jurisdictional under paragraph (a)(2) because they are impoundments
of ``waters of the United States,'' the agencies will refer to
``paragraph (a)(2) impoundments.''
    \92\ When an approved jurisdictional determination does not
exist for an impounded water that the agencies conclude based on its
characteristics could only be jurisdictional under paragraph (a)(5),
the paragraph (a)(2) impoundments provision does not apply and the
water will be assessed under another jurisdictional category.
    \93\ For example, if a stream that is not part of the tributary
system of a paragraph (a)(1) water, but which is assessed under
paragraph (a)(5) and is determined to meet the significant nexus
standard, is lawfully impounded subsequent to the jurisdictional
determination, the stream is not automatically jurisdictional as a
paragraph (a)(2) water under this rule. However, the impounded
stream may still meet the significant nexus standard under paragraph
(a)(5) or the impounded stream may develop the characteristics of a
traditional navigable water and become jurisdictional under
paragraph (a)(1).
---------------------------------------------------------------------------

    Consistent with the 1986 regulations, under this rule tributaries
may be tributaries to paragraph (a)(1) or (a)(2) waters. Tributaries to
paragraph (a)(2) impoundments, and wetlands adjacent to such
tributaries, are jurisdictional if they meet either the relatively
permanent standard or the significant nexus standard. Additionally,
wetlands adjacent to paragraph (a)(2) impoundments are jurisdictional
if they meet either the relatively permanent standard or the
significant nexus standard. In order for a tributary to a paragraph
(a)(2) impoundment to meet the relatively permanent standard, the
agencies must be able to trace evidence of a flowpath (e.g., physical
features on the landscape, such as a channel, ditch, pipe, or swale)
directly or indirectly through another water or waters, downstream from
the structure that creates the paragraph (a)(2) impoundment to a
paragraph (a)(1) water. When evaluating a wetland adjacent to a
paragraph (a)(2) impoundment under the relatively permanent standard,
field staff would assess whether the impounded water is relatively
permanent, standing or continuously flowing, and then determine whether
the wetland has a continuous surface connection to the impoundment.
When evaluating a wetland adjacent to a jurisdictional tributary to a
paragraph (a)(2) impoundment when the jurisdictional tributary meets
the relatively permanent standard, field staff would determine whether
the wetland has a continuous surface connection to the tributary. See
section IV.C.4.c and section IV.C.5.c of this preamble for additional
information on evaluations under the relatively permanent standard for
tributaries and adjacent wetlands. For a tributary to a paragraph
(a)(2) impoundment, a wetland adjacent to a paragraph (a)(2)
impoundment, or a wetland adjacent to a tributary to a paragraph (a)(2)
impoundment, that is assessed under the significant nexus standard, the
significant nexus must be to a paragraph (a)(1) water. See sections
IV.C.4.c and IV.C.5.c of this preamble for additional information on
significant nexus evaluations for tributaries and adjacent wetlands.
b. Summary of the Agencies' Consideration of Public Comments and
Rationale for This Rule
    The agencies have determined that as a matter of law, science, and
policy, impoundments do not de-federalize a water, and therefore
impoundments of ``waters of the United States'' remain ``waters of the
United States.'' The Supreme Court has confirmed that damming or
impounding ``waters of the United States'' does not make those waters
non-jurisdictional. See S.D. Warren Co. v. Maine Bd. of Envtl. Prot.,
547 U.S. 370, 379 n.5 (2006) (``S.D. Warren'') (``[N]or can we agree
that one can denationalize national waters by exerting private control
over them.''). While S.D. Warren addressed the meaning of the word
``discharge'' rather than the definition of ``waters of the United
States,'' the Court's conclusion regarding the jurisdictional status of
a dammed river supports the agencies' longstanding interpretation of
the Clean Water Act that ``waters of the United States'' remain
``waters of the United States'' even if impounded, as reflected in the
1986 regulations and continued in this rule. Essentially, the action of
creating an impoundment cannot on its own render ``waters of the United
States'' no longer jurisdictional.\94\ The Court of Appeals for the
Ninth Circuit has similarly found that ``it is doubtful that a mere
man-made diversion would have turned what was part of the waters of the
United States into something else and, thus, eliminated it from
national concern.'' United States v. Moses, 496 F.3d 984, 988 (9th Cir.
2007), cert. denied, 554 U.S. 918 (2008).
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    \94\ Note that a Clean Water Act section 404 permit may
authorize impoundment of a water such that the water is no longer
jurisdictional, for example, to create a waste treatment system that
is excluded from the definition of ``waters of the United States.''
In such circumstances, the water is analyzed under the regulatory
exclusion where applicable, not under the impoundments provision of
the definition.
---------------------------------------------------------------------------

    Asserting Clean Water Act jurisdiction over impoundments also
aligns with the scientific literature, as well as the agencies'
scientific and technical expertise and experience, which confirm that
impoundments have chemical, physical, and biological effects on
downstream waters through surface or subsurface hydrologic connections.
As discussed in section III.C of the Technical Support Document,
impoundments are typically built to maintain some level of hydrologic
connection between the water that is being impounded and the downstream
tributary network. For example, water may pass from a reservoir to the
downstream side of an impoundment by passing through a main spillway or
outlet works, passing over an auxiliary spillway, or overtopping the
impoundment. Indeed, berms, dikes, and similar features used to create
impoundments typically do not block all water flow. Even dams, which
are specifically designed and constructed to impound large amounts of
water effectively and safely, generally do not prevent all water flow,
but rather allow seepage under the foundation of the dam and through
the dam itself. See, e.g., International Atomic Energy Agency, 2003,
``Investigating Leaks in Dams & Reservoirs.'' INIS-XA-616. Vienna,
Austria (``All dams are designed to lose some water through
seepage.''); U.S. Bureau of Reclamation, ``Safety of Dams.'' Provo Area
Office (last updated July 1, 2017) (``All dams seep, but the key is to
control the seepage through properly designed and constructed filters
and drains.''); Federal Energy Regulatory Commission, 2005, ``Chapter
14: Dam Safety Performance Monitoring Program.'' Engineering Guidelines
for the Evaluation of Hydropower Projects. (``Seepage through a dam or
through the foundations or abutments of dams is a normal condition.'').
Further, as an agency with expertise and responsibilities in
engineering and public works, the Corps extensively studies water
retention structures like berms, levees, and earth and rock-fill dams.
The agency has found that all water retention structures are subject to
seepage through their foundations and abutments. See section III.C of
the Technical Support Document.
    Paragraph (a)(2) waters include impoundments created in waters that
were jurisdictional under this rule's definition at the time the
impoundment was created, as well as impoundments of waters that at the
time of assessment are jurisdictional under paragraph (a)(1), (a)(3),
or (a)(4) of this rule regardless of

the water's jurisdictional status at the time the impoundment was
created.\95\ This is generally consistent with the agencies'
longstanding approach to impoundments. See U.S. Army Corps of Engineers
Jurisdictional Determination Form Instructional Guidebook (2007) at 58,
available at https://www.usace.army.mil/Missions/Civil-Works/Regulatory-Program-and-Permits/Related-Resources/CWA-Guidance/
(hereinafter, ``2007 Corps Instructional Guidebook''). The agencies
have concluded that it is appropriate based on relevant case law,
science, and as a practical matter to interpret ``waters of the United
States'' to include both impoundments of waters that qualified as
``waters of the United States'' under this rule's definition at the
time of impoundment, and impoundments of waters that at the time of
assessment meet the definition of ``waters of the United States''
(other than waters jurisdictional under paragraph (a)(5)). As discussed
above, waters that qualified as ``waters of the United States'' at the
time of impoundment (other than waters jurisdictional under paragraph
(a)(5)) remain ``waters of the United States.'' And impoundments of
waters that at the time of assessment fall within one of the other
categories of ``waters of the United States'' in this rule (other than
waters jurisdictional under paragraph (a)(5)) are jurisdictional under
paragraph (a)(2).
---------------------------------------------------------------------------

    \95\ See infra for a discussion of impoundments of waters that
are jurisdictional as paragraph (a)(5) waters, which are treated
differently under this rule.
---------------------------------------------------------------------------

    The agencies received a variety of comments on impoundments during
the public comment period. Some commenters supported the agencies'
inclusion of impoundments of ``waters of the United States'' as a
separate category of jurisdictional waters. A few commenters stated
that the relatively permanent standard and significant nexus standard
should also apply to impoundments for the purposes of jurisdiction.
Some commenters agreed with the proposed rule's approach to not include
impounded paragraph (a)(5) waters in the impoundments category. Many
commenters requested the agencies provide greater clarity about the
definition of impoundments.
    After consideration of public comments and for the reasons
described above and in section III.C of the Technical Support Document,
the agencies affirm in this rule that impoundments of ``waters of the
United States'' remain ``waters of the United States,'' except for
impoundments of paragraph (a)(5) waters, which the agencies find are
better assessed under other categories of this rule. As discussed
above, paragraph (a)(2) impoundments of ``waters of the United States''
legally remain ``waters of the United States,'' so the agencies are not
requiring an additional determination of their jurisdiction under this
rule. While the agencies are not defining ``impoundment'' in this rule,
in this preamble the agencies are providing additional clarity below
about the types of impoundments that are and that are not considered
``waters of the United States'' under paragraph (a)(2). Additionally,
section IV.C.3.c of this preamble provides implementation guidance for
identifying impoundments on the landscape.
    As in the proposed rule, impoundments of waters that are determined
to be jurisdictional under paragraph (a)(5) are not included in this
rule as paragraph (a)(2) impoundments. As discussed above, impoundments
of paragraph (a)(5) waters would need to be assessed for jurisdiction
in their current state under paragraph (a)(1), (a)(3), (a)(4), or
(a)(5) of this rule. Thus, if a water is determined to be
jurisdictional under paragraph (a)(5) and is then later lawfully
impounded, it is not jurisdictional by rule under the paragraph (a)(2)
impoundments provision. Instead, the impoundment of a paragraph (a)(5)
water would itself need to be assessed in its current state to
determine whether it is jurisdictional under one of the provisions of
the rule besides paragraph (a)(2). Impounded paragraph (a)(5) waters
will most likely continue to not meet any of the other categories of
jurisdictional waters and will therefore need to be re-assessed under
paragraph (a)(5). However, if, once impounded, such a water became, for
example, a traditional navigable water, it would be jurisdictional
under paragraph (a)(1) of this rule. This approach in this rule is
consistent with the agencies' careful approach to jurisdiction over
paragraph (a)(5) waters. For example, as discussed in sections IV.C.4
and IV.C.5 of this preamble below, the ``tributaries'' category does
not include tributaries to paragraph (a)(5) waters and the adjacent
wetlands category does not include wetlands adjacent to paragraph
(a)(5) waters. This change from the 1986 regulations reflects the
agencies' consideration of the jurisdictional concerns and limitations
of the statute as informed by SWANCC and Rapanos.
c. Implementation
    Under this rule, for the reasons discussed above, impounding a
water that meets the definition of ``waters of the United States''
generally does not affect such water's jurisdictional status,
consistent with pre-2015 practice. See 2007 Corps Instructional
Guidebook at 58. A water can be found to be a jurisdictional
impoundment under paragraph (a)(2) of this rule if (1) the impounded
water met the definition of ``waters of the United States'' based on
this rule's definition at the time the impoundment was created \96\
(other than an impoundment of a paragraph (a)(5) water) or (2) the
water that is being impounded, at the time of assessment, meets the
definition of ``waters of the United States'' under paragraph (a)(1),
(a)(3), or (a)(4), regardless of the water's jurisdictional status when
the impoundment was created. The agencies also note that over time an
impoundment of a water that does not initially meet the definition of
``waters of the United States'' can become jurisdictional under another
provision of the regulation; for example, an impounded water could
become navigable-in-fact and covered under paragraph (a)(1)(i) of this
rule. This approach to implementation of impoundments is generally
consistent with pre-2015 practice. This section of the preamble
provides information for determining jurisdiction for impoundments
under paragraph (a)(2) and for determining jurisdiction for tributaries
of impoundments, wetlands adjacent to impoundments, and wetlands
adjacent to tributaries of impoundments.
---------------------------------------------------------------------------

    \96\ Note, however, if an impoundment is a waste treatment
system constructed prior to the 1972 Clean Water Act amendments, it
is eligible for the exclusion under paragraph (b) of this rule so
long as the system is in compliance with currently applicable Clean
Water Act requirements, such as treating water such that discharges,
if any, from the system meet the Act's requirements. See section
IV.C.7.b of this preamble.
---------------------------------------------------------------------------

i. Determining the Presence of a Paragraph (a)(2) Impoundment
    Impoundments are distinguishable from natural lakes and ponds
because they are created by discrete structures (often human-built)
like dams or levees that typically have the effect of raising the water
surface elevation, creating or expanding the area of open water, or
both. Impoundments can vary in size, with some being very small and
others being very large, like Lake Mead, a reservoir on the Colorado
River that is created by the Hoover Dam. Paragraph (a)(2) impoundments
under this rule can include both natural impoundments (like beaver
ponds) and artificial impoundments (like reservoirs). Paragraph (a)(2)
impoundments under this rule can be located off-channel (i.e.,

an impoundment with no outlet or hydrologic connection to the tributary
network) or in-line with the channel (i.e., an impoundment with a
hydrologic connection to the tributary network).
    An impoundment is jurisdictional under paragraph (a)(2) of this
rule if the impounded water met the definition of ``waters of the
United States'' based on this rule's definition when the impoundment
was created (other than impoundments of paragraph (a)(5) waters). To
determine if an impoundment meets this criterion, the water would be
assessed to see if the water was jurisdictional as a paragraph (a)(1)
water, tributary, or adjacent wetland based on this rule's definition
at the time it was impounded. Tools that can be used for such
assessment are discussed further in sections IV.C.4.c and IV.C.5.c of
this preamble. Historic aerial photographs, maps, and geospatial
datasets may be particularly useful in helping to determine if a water
was jurisdictional under paragraph (a)(1), (a)(3), or (a)(4) of this
rule at the time the impoundment was created, especially where such
materials depict the aquatic system before and after the impoundment
was created. Similarly, planning, engineering, and design documents, if
available, may provide useful information.
    Paragraph (a)(2) waters also include impoundments of waters that at
the time of assessment are jurisdictional under paragraph (a)(1),
(a)(3), or (a)(4) of this rule regardless of the water's jurisdictional
status at the time the impoundment was created. This approach is
consistent with pre-2015 practice. See 2007 Corps Instructional
Guidebook at 58. A water that is impounded may not meet this rule's
jurisdictional criteria at the time the water was originally impounded,
but the water may meet this rule's jurisdictional criteria at the time
of the assessment (in some cases, many years later). This is because
aquatic resources generally can evolve over time as aquatic landscapes,
precipitation and other climatic patterns, and other environmental
conditions change, or due to human-caused changes (e.g., stream
modification, filling in of wetlands, water withdrawals, or effluent
discharges). Impounded waters may be particularly likely to evolve as
the surface waters are raised or expanded behind the impoundment. To
determine if an impoundment is jurisdictional based on such changes,
the impounded water would be assessed to see if it is a traditional
navigable water, the territorial seas, an interstate water, a
jurisdictional tributary, or a jurisdictional adjacent wetland. Tools
that can be used for such assessment are discussed further in sections
IV.C.4.c and IV.C.5.c of this preamble.
    In assessing if an impoundment of a paragraph (a)(1) water is
jurisdictional under paragraph (a)(2), the agencies would assess
whether the water that is being impounded met the requirements to be a
paragraph (a)(1) water under this rule either at the time of
impoundment or at the time of assessment. Impoundments of paragraph
(a)(1) waters that continue to meet the requirements under paragraph
(a)(1) remain paragraph (a)(1) waters.
    In assessing whether an impoundment of a tributary is
jurisdictional under paragraph (a)(2), the agencies would first assess
if the tributary either met this rule's definition of ``waters of the
United States'' at the time the impoundment was created or if the
tributary meets this rule's definition of ``waters of the United
States'' at the time of assessment. For impoundments of tributaries
that met this rule's definition of ``waters of the United States'' at
the time the impoundment was created, the agencies must be able to
demonstrate that at the time the impoundment was created, there was
evidence of a flowpath (e.g., physical features on the landscape, such
as a channel, ditch, pipe, or swale) directly or indirectly through
another water or waters, downstream from the structure that created the
impoundment to a paragraph (a)(1) water. Thus, an impoundment of a
tributary that met this rule's definition of ``waters of the United
States'' at the time the impoundment was created could currently be
located off-channel (e.g., due to changes in hydrology) or in-line with
the channel, but the flowpath would only need to be traceable at the
time the impoundment was created. For impoundments of tributaries that
meet this rule's definition of ``waters of the United States'' at the
time of assessment, the agencies must be able to at the time of
assessment trace a flowpath directly or indirectly through another
water or waters, downstream from the structure that creates the
impoundment to a paragraph (a)(1) water. Thus, impoundments of
tributaries that meet the definition of ``waters of the United States''
at the time of assessment will always be in-line with the channel due
to the flowpath requirement. This is consistent with the agencies'
approach to tributaries under the final rule. See section IV.C.4. of
this preamble. As with assessment of tributaries under this rule, while
the physical flowpath from the paragraph (a)(2) impoundment to the
paragraph (a)(1) water must be traceable, there is not a need to
demonstrate that flow from the impoundment reaches the paragraph (a)(1)
water. For an off-channel impoundment (i.e., an impoundment with no
outlet to the tributary network), such as an impoundment of a
jurisdictional adjacent wetland, such a flowpath is not required. Under
the final rule, adjacent wetlands do not require a flowpath to the
tributary network, and similarly, impoundments of such adjacent
wetlands do not require a flowpath. The agencies would only need to
determine that the impoundment was created in a water that is currently
jurisdictional under paragraphs (a)(1) through (4) or that the
impoundment was created in a water that was jurisdictional under
paragraphs (a)(1) through (4) at the time the impoundment was created.
    In assessing whether an impoundment of an adjacent wetland is
jurisdictional under paragraph (a)(2), the agencies would need to
determine that the impoundment was created in an adjacent wetland that
was jurisdictional at the time the impoundment was created or that is
currently jurisdictional at the time of assessment. Such impoundments
of adjacent wetlands may be located either off-channel or in-line with
the channel, and do not require a traceable flowpath that is required
for impoundments of tributaries. This is because under the final rule,
adjacent wetlands do not require a flowpath to the tributary network,
and similarly, impoundments of such adjacent wetlands do not require a
flowpath.
    Because impoundments can be jurisdictional under other categories
of ``waters of the United States'' under this rule, field staff may
document that the impoundment is jurisdictional under other categories.
For example, if an impoundment is itself a traditional navigable water,
part of the territorial seas, or an interstate water, the agencies
would typically determine that the impoundment is a paragraph (a)(1)
water, rather than asserting jurisdiction under paragraph (a)(2) of
this rule. Field staff may document any such waters as jurisdictional
under the relevant provision of the rule rather than documenting that
it is jurisdictional as a paragraph (a)(2) impoundment.
    Finally, as discussed above in section IV.C.3.b of this preamble,
waters that are jurisdictional under paragraph (a)(5) and that are
subsequently impounded do not categorically retain their jurisdictional
status as ``waters of the United States'' under paragraph (a)(2). If
the impoundment of the paragraph (a)(5) water does not meet the
jurisdictional standards under one of

the other categories of ``waters of the United States'' in this rule
(i.e., as a paragraph (a)(1) water, jurisdictional tributary, or
jurisdictional adjacent wetland), the impoundment would be re-assessed
as a paragraph (a)(5) water. Implementation of waters assessed under
paragraph (a)(5) is discussed in section IV.C.6.c of this preamble.
ii. Determining Jurisdiction for Tributaries of Impoundments, Wetlands
Adjacent to Impoundments, and Wetlands Adjacent to Tributaries of
Impoundments
    Tributaries of paragraph (a)(2) impoundments are jurisdictional, as
with all tributaries under this rule, when they meet either the
relatively permanent standard or the significant nexus standard. In
order to determine if a water is a tributary of a paragraph (a)(2)
impoundment, the same tools and methods can be used that are discussed
in section IV.C.4.c.i of this preamble to trace the flowpath to the
impoundment. Field staff would then determine if the tributary should
be evaluated under the relatively permanent standard or the significant
nexus standard. For tributaries assessed under the relatively permanent
standard, the agencies must be able to trace evidence of a flowpath
downstream from the structure that creates the impoundment to a
paragraph (a)(1) water. To meet the latter standard, the significant
nexus must be to a paragraph (a)(1) water. Implementation of the
relatively permanent standard for tributaries is discussed in more
detail in section IV.C.4.c.ii of this preamble. Implementation of the
significant nexus standard for tributaries is discussed in section
IV.C.4.c.iii of this preamble.
    For tributaries of paragraph (a)(2) impoundments that are evaluated
under the relatively permanent standard, field staff would determine if
the tributary has flowing or standing water year-round or continuously
during certain times of the year, see section IV.C.4.c.ii of this
preamble, and then determine whether there is evidence of a flowpath
downstream from the structure that creates the impoundment to a
paragraph (a)(1) water. As with all tributaries under the rule, there
is no requirement under the relatively permanent standard for
relatively permanent flow for the entirety of a tributary's flowpath to
a downstream paragraph (a)(1) water. See id. Thus, under the relatively
permanent standard for tributaries of paragraph (a)(2) impoundments,
field staff would not need to determine that flow occurs over, through,
around, or underneath the structure that creates the impoundment.
Instead, the agencies will document that flow occurs from the tributary
to the impoundment, either directly or indirectly through another water
or waters, including non-jurisdictional features, as described in
section IV.C.4 of this preamble, and that there is evidence of a
flowpath downstream of the structure (e.g., physical features on the
landscape, such as a channel, non-jurisdictional ditch, pipe, or swale)
to a paragraph (a)(1) water, either directly or indirectly through
another water or waters. For example, a tributary may flow through
another stream that flows infrequently, and only in direct response to
precipitation, and the presence of that stream is sufficient to
demonstrate that the tributary flows to a paragraph (a)(1) water.
    If a wetland is adjacent to a paragraph (a)(2) impoundment and that
wetland is evaluated under the relatively permanent standard, field
staff would, only for purposes of determining whether the adjacent
wetland meets the relatively permanent standard, assess whether the
impounded water is relatively permanent, standing or continuously
flowing. Next, field staff would determine whether the wetland has a
continuous surface connection to the paragraph (a)(2) impoundment,
consistent with section IV.C.5 of this preamble. If the paragraph
(a)(2) impoundment is not relatively permanent, standing or
continuously flowing, then field staff will assess the adjacent wetland
under the significant nexus standard.
    If a wetland is adjacent to a tributary to a paragraph (a)(2)
impoundment, and the tributary meets the relatively permanent standard,
the wetland would be assessed for whether it has a continuous surface
connection to the tributary, consistent with section IV.C.5 of this
preamble. If the adjacent wetland does not have a continuous surface
connection, it will be assessed under the significant nexus standard.
If the tributary does not meet the relatively permanent standard, then
field staff will assess the adjacent wetland under the significant
nexus standard. To apply the significant nexus standard to tributaries
of paragraph (a)(2) impoundments, wetlands adjacent to those
tributaries, or wetlands adjacent to paragraph (a)(2) impoundments, the
agencies will assess if the waters of interest significantly affect the
chemical, physical, or biological integrity of paragraph (a)(1) waters
using the tools and approaches described in sections IV.C.4.c.iii and
IV.C.5.c.iii of this preamble. As part of that analysis, the agencies
will determine if there is a surface or subsurface hydrologic
connection downstream that is maintained over, through, around, or
underneath the structure that creates the impoundment. Such a
hydrologic connection can occur in a variety of ways, such as
overtopping of the structure or through features like dam spillways,
drainage and other galleries, sluiceways, culverts, pipes, diversion
tunnels, or conduits that are built to maintain a hydrologic connection
through the dam or levee. Subsurface hydrologic connectivity can also
occur via seepage through or underneath the dam or similar structure.
Field staff can document that surface or subsurface hydrologic
connectivity occurs using direct observation of overtopping or a
feature that is constructed to maintain a hydrologic connection,
through review of construction plans for the structure, through other
field observations (e.g., dye tests or tracer studies, or observations
of flow within the spillway such as bent over vegetation or water
staining where the spillway is concrete, soil saturation, changes in
vegetation above and below the structure), or through remote tools
(e.g., aerial photography interpretation that provides indications of
wetter signatures below the dam). As stated in section IV.C.9 of this
preamble, a hydrologic connection to a paragraph (a)(1) water is not
necessary to determine that the water being evaluated significantly
affects the integrity of paragraph (a)(1) waters, though it is one of
the factors that is considered. Where such a hydrologic connection
exists at the surface or subsurface, it can help to facilitate the
functions that the tributary of the paragraph (a)(2) impoundment
performs that impact the downstream paragraph (a)(1) water, such as
contribution of flow, pollutants, sediment, and organic material. In
the rare circumstances where such a hydrologic connection does not
exist, the lack of such a connection can facilitate other functions,
such as holding back floodwaters that could otherwise harm paragraph
(a)(1) waters. See preamble section IV.C.9 for additional information
on implementing the significant nexus standard more generally.
4. Tributaries
a. This Rule
    Consistent with the proposal, this rule retains the tributary
provision of the 1986 regulations, updated to reflect consideration of
the law, the science, and agency expertise. The 1986 regulations
defined ``waters of the United States'' to include tributaries of
traditional navigable waters, interstate waters, paragraph (a)(3)
``other waters''

(a category that has been modified and codified in this rule as
paragraph (a)(5) waters) and impoundments. With this rule, the agencies
are adding the territorial seas to the list of waters to which
tributaries may connect to constitute a jurisdictional tributary and
removing paragraph (a)(3) waters from the list. This rule defines
``waters of the United States'' to include tributaries of traditional
navigable waters, the territorial seas, interstate waters, or paragraph
(a)(2) impoundments if the tributaries meet either the relatively
permanent standard or the significant nexus standard.
    The 1986 regulations do not contain a definition of ``tributary,''
and the agencies similarly are not including a definition in this rule.
However, for more than 45 years, the agencies have recognized the need
to protect ``the many tributary streams that feed into the tidal and
commercially navigable waters . . . since the destruction and/or
degradation of the physical, chemical, and biological integrity of each
of these waters is threatened by the unregulated discharge of dredged
or fill material.'' 42 FR 37121, 37123 (July 19, 1977). Accordingly,
the agencies are maintaining their interpretation of tributary for
purposes of the definition of ``waters of the United States.'' See
Rapanos Guidance at 6 n.24. A tributary for purposes of this rule
includes rivers, streams, lakes, ponds, and impoundments, regardless of
their flow regime, that flow directly or indirectly through another
water or waters to a traditional navigable water, the territorial seas,
or an interstate water. Waters through which a tributary may flow
indirectly include, for example, impoundments, wetlands, lakes, ponds,
and streams. A tributary may flow through a number of downstream
waters, including a non-jurisdictional tributary or non-jurisdictional
features, such as a ditch excluded under paragraph (b) of this rule or
an excluded waste treatment system, and jurisdictional waters that are
not tributaries, such as an adjacent wetland. But to be jurisdictional,
the tributary must be part of a tributary system that eventually flows
to a traditional navigable water, the territorial seas, or an
interstate water. The agencies will utilize the Corps' well-established
definition of an ordinary high water mark (OHWM) to assist in
identifying tributaries for purposes of this rule. See section
IV.C.4.c.i of this preamble for information on using the OHWM to assist
in identifying a water as a tributary for purposes of this rule. To be
a jurisdictional tributary under this provision of the rule, the
tributary must meet either the relatively permanent standard or the
significant nexus standard.
    Like the 1986 regulations, this rule includes tributaries of
interstate waters since interstate waters, like traditional navigable
waters and the territorial seas, are waters clearly protected by the
Clean Water Act. In this rule, the agencies are adding the territorial
seas to the list of waters to which tributaries may connect to
constitute a jurisdictional tributary because the territorial seas are
explicitly protected by the Clean Water Act. Because the territorial
seas are explicitly covered by the Clean Water Act, it is reasonable
and appropriate to protect tributaries to the territorial seas that
meet either the relatively permanent standard or the significant nexus
standard for the same reasons that tributaries to traditional navigable
waters are protected. In practice, the agencies recognize that most
tributaries will reach a traditional navigable water before they reach
the territorial seas. Finally, consistent with the 1986 regulations,
this rule includes tributaries that flow directly or indirectly through
another water or waters to paragraph (a)(2) impoundments.\97\
---------------------------------------------------------------------------

    \97\ See discussion of tributaries to paragraph (a)(2)
impoundments in section IV.C.3 of this preamble.
---------------------------------------------------------------------------

    The agencies' longstanding interpretation of the Clean Water Act
includes tributaries that are natural, modified, or constructed waters.
The Clean Water Act, in defining ``navigable waters,'' does not turn on
any such distinctions, which have no bearing on a tributary's capacity
to carry water (and pollutants) to paragraph (a)(1) waters. See, e.g.,
Technical Support Document section II.B.iv.3 (explaining that human-
made ditches ``perform many of the same functions as natural
tributaries,'' including ``convey[ing] water that carries nutrients,
pollutants, and other constituents, both good and bad, to downstream
traditional navigable waters''). Given the extensive human modification
of watercourses and hydrologic systems throughout the country, it is
often difficult to distinguish, as a practical or scientific matter,
between natural watercourses and watercourses that are wholly or partly
modified or constructed. For example, tributaries that have been
channelized in concrete or otherwise have been modified would still be
tributaries for purposes of this rule so long as they contribute flow
to a traditional navigable water, the territorial seas, or an
interstate water, and so long as they are not excluded under paragraph
(b) of this rule. Thus, tributaries can include ditches and canals.
    Under this rule, swales and erosional features (e.g., gullies,
small washes) characterized by low volume, infrequent, or short
duration flow are not tributaries and are not jurisdictional. See
section IV.C.7 of this preamble.
    Once a water is determined to be a tributary, under this rule, the
tributary must meet either the relatively permanent or significant
nexus standard to be jurisdictional. The relatively permanent standard
encompasses tributaries that have flowing or standing water year-round
or continuously during certain times of the year. Relatively permanent
waters do not include tributaries with flowing or standing water for
only a short duration in direct response to precipitation. In
evaluating tributaries under the significant nexus standard, the
agencies will determine whether the tributaries, either alone or in
combination with similarly situated waters in the region, significantly
affect the chemical, physical, or biological integrity of paragraph
(a)(1) waters. Implementation of each of those standards for purposes
of determining jurisdiction over tributaries is discussed below in
section IV.C.4.c of this preamble.
b. Summary of the Agencies' Consideration of Public Comments and
Rationale for This Rule
    Commenters expressed a range of views on the agencies' proposed
treatment of tributaries. This section of the preamble provides a
summary of the major comments received on the regulatory text and the
agencies' consideration of the comments. The preamble to the proposed
rule also provided information about the agencies' longstanding
interpretation of practice for identifying tributaries for purposes of
the definition of ``waters of the United States,'' and this section
also summarizes and addresses major comments received on those topics.
i. Comments on the Tributaries Provision of This Rule
    Some commenters requested that the agencies include a definition of
``tributary'' in this rule. A subset of these commenters stated that
the definition should include waters with a bed, bank, or other
evidence of flow that contribute flow directly or indirectly to
downstream paragraph (a)(1) waters. Other commenters maintained that
the lack of a formal definition makes it unclear which features are
tributaries and which are not. Some of these commenters stated that the
lack of a

definition left too much discretion to the agencies to identify
tributaries based on physical features, which they asserted would lead
to confusion. Some commenters supported the proposed approach for
assessing tributaries, stating that the longstanding interpretation and
practice would allow for regionalized implementation. Although the
agencies are not promulgating a new definition of ``tributary'' the
agencies have decades of experience implementing the 1986 regulations
(which also did not include a definition of ``tributary'') and have
concluded that a new regulatory definition of tributary is not
required. To provide further clarity, the agencies have been careful in
this preamble to articulate and explain the agencies' well-established
interpretation and practices for identifying tributaries. In addition,
the agencies note that while the first step under this provision of the
regulation is to identify whether a water is a tributary under
longstanding practice, that is not the end of the inquiry under this
rule, in contrast to the 1986 regulations. A water must not only be a
tributary but must also meet either the relatively permanent standard
or the significant nexus standard to be jurisdictional under this
provision. These standards provide important limitations that also help
define the scope of the tributaries that are jurisdictional under the
rule.
    Commenters on the proposed rule expressed a variety of perspectives
on the appropriate scope of jurisdiction for tributaries. Some
commenters supported the proposal that tributaries are jurisdictional
if they meet either the relatively permanent or significant nexus
standard. Other commenters asserted that tributaries should meet both
standards. Some commenters stated that this rule should include
categorical protections for all tributaries (e.g., features with an
OHWM), rather than requiring case-by-case analysis, asserting that such
an interpretation is supported by the science and Supreme Court case
law. For the reasons described in section IV.A of this preamble, this
rule defines ``waters of the United States'' to include tributaries
that meet either the relatively permanent standard or the significant
nexus standard on a case-specific basis.
    Some commenters criticized the definition of ``tributary'' from the
2020 NWPR, while others supported that definition, stating that it was
clear and logical. The 2020 NWPR defined ``tributary'' as a river,
stream, or similar naturally occurring surface water channel that
contributes surface water flow to the territorial seas or a traditional
navigable water in a typical year either directly or indirectly through
other tributaries, jurisdictional lakes, ponds, or impoundments, or
adjacent wetlands. A tributary was required to be perennial or
intermittent in a typical year. 85 FR 22251 (April 21, 2020). The
definition of ``tributary'' in the 2020 NWPR failed to advance the
objective of the Clean Water Act and was inconsistent with scientific
information about the important effects of many types of tributaries on
the integrity of downstream paragraph (a)(1) waters.
    The key limitations that the 2020 NWPR created in its definition of
``tributary,'' which this rule does not adopt, are the categorical
exclusion of ephemeral streams and the requirement that streams
contribute flow to a traditional navigable water or territorial sea in
a ``typical year.'' With respect to ephemeral streams, commenters
provided a wide variety of perspectives on whether they should be
jurisdictional under this rule. Some commenters asserted that the
agencies' interpretation of tributary should exclude ephemeral streams.
Some commenters asserted that ephemeral streams should be categorically
jurisdictional under this rule. These commenters referenced the
importance of ephemeral streams for providing functions like nutrient
and materials transport, erosion and flood control, water quality
maintenance downstream, drinking water and irrigation provisioning,
groundwater recharge, and wildlife habitat. Other commenters asserted
that ephemeral streams are important for buffering against the impacts
of climate change, supporting Tribal communities, and providing
functions in specific regions like arid areas. Another group of
commenters stated that all ephemeral streams should be non-
jurisdictional across the country, or non-jurisdictional in certain
regions such as the arid West. These commenters asserted that ephemeral
streams do not flow frequently enough or provide sufficiently important
functions to impact the integrity of downstream paragraph (a)(1)
waters. As discussed further in section IV.A of this preamble, the
agencies are not categorically including or excluding streams as
jurisdictional based on their flow regime in this rule. The agencies
agree that ephemeral streams can provide many important functions for
paragraph (a)(1) waters.
    With respect to the ``typical year requirement'' in the 2020 NWPR
definition of ``tributary,'' the agencies found it challenging and
sometimes impossible to implement, for the reasons discussed in section
IV.B.3.c of this preamble. The ``typical year'' requirement for
tributaries was also not supported by science. Scientific information
does not demonstrate that only those streams that contribute
intermittent or perennial flow to a traditional navigable water or
territorial sea in a ``typical year'' have significant effects on the
chemical, physical, and biological integrity of larger downstream
waters, including paragraph (a)(1) waters. See sections IV.B.3.a and
IV.B.3.b of this preamble. Because the limitations in the 2020 NWPR's
definition of ``tributary'' are inconsistent with science and created
substantial implementation difficulties, the agencies are not adopting
this definition. See section III.A of the Technical Support Document
for more information on the agencies' rationale for the scope of
tributaries covered by this rule. Streams that are tributaries,
regardless of their flow regime, will be assessed under the relatively
permanent or significant nexus standard per paragraph (a)(3) of this
rule, and streams that are not tributaries will be assessed under the
relatively permanent or significant nexus standard per paragraph (a)(5)
of this rule.
    Some commenters opposed as arbitrary and unsupported by the law or
science the agencies' proposed approach to delete the category for
intrastate lakes and ponds, streams, or wetlands that do not meet
another jurisdictional category (the (a)(3) ``other waters'' provision
from the 1986 regulations) as a category of waters to which tributaries
may connect to be determined ``waters of the United States.'' Some of
these commenters requested clarification as to how tributaries to
intrastate lakes and ponds, streams, or wetlands that do not meet
another jurisdictional category would be assessed. One commenter
asserted that the agencies were ``excluding'' tributaries to paragraph
(a)(5) waters. Streams that flow to paragraph (a)(5) waters are not
excluded in this rule. Deleting the cross reference to the category for
intrastate lakes and ponds, streams, or wetlands that do not meet
another jurisdictional category (the (a)(3) ``other waters'' provision
from the 1986 regulations) as a category of waters to which tributaries
may connect reflects the agencies' consideration of the statute as a
whole and the jurisdictional concerns and limitations of SWANCC and
Rapanos. The agencies have concluded that a provision that authorizes
consideration of jurisdiction over tributaries that meet the relatively
permanent or significant nexus standard when assessed based simply on
connections to such waters would have

too tenuous a connection to paragraph (a)(1) waters. However, in this
rule any such streams that flow to jurisdictional paragraph (a)(5)
waters could be assessed themselves under the paragraph (a)(5) waters
category to determine if they meet the relatively permanent or
significant nexus standard. For example, a stream that flows to a lake
that meets the significant nexus standard under the paragraph (a)(5)
waters provision could itself be assessed under the paragraph (a)(5)
waters provision to determine whether it significantly affects the
chemical, physical, or biological integrity of a paragraph (a)(1)
water.
ii. Comments on the Interpretation and Implementation of the
Tributaries Provision of This Rule
    As discussed further above, the agencies interpret tributary for
purposes of this rule to include rivers, streams, lakes, ponds, and
impoundments that flow directly or indirectly through another water or
waters to a traditional navigable water, the territorial seas, an
interstate water, or a paragraph (a)(2) impoundment. The agencies
received comments on elements of this longstanding interpretation of
tributary for purposes of the ``waters of the United States.''
    Some commenters disagreed with the agencies' interpretation that
tributaries include certain lakes and ponds. Some of these commenters
stated that lakes and ponds should comprise a separate jurisdictional
category. Several commenters asserted that considering certain lakes
and ponds to be tributaries could lead to overly broad jurisdiction,
and one commenter requested clarification in this rule that not every
feature that might be considered a lake or a pond is necessarily
jurisdictional. Other commenters agreed with the agencies' longstanding
approach. Lakes, ponds, and impoundments function as part of the
tributary system where they contribute flow to downstream waters, and
therefore it is reasonable to assess them for jurisdiction as
tributaries under this rule. The agencies will continue to interpret
the regulations to address lakes, ponds, and impoundments with both an
inlet and outlet connected to the tributary network, as well as lakes,
ponds, and impoundments with an outlet connected to the tributary
network as tributaries if they contribute flow directly or indirectly
through one or more waters or features that lie along the flowpath to a
paragraph (a)(1) water. The agencies have extensive experience
implementing this approach under pre-2015 practice. The agencies
disagree that this approach will lead to overly broad jurisdiction, as
these lakes, ponds, and impoundments that are tributaries must meet
either the relatively permanent standard or significant nexus standard
to be jurisdictional. Therefore, not every lake, pond, or impoundment
is jurisdictional as a tributary or under other provisions of this
rule.
    Some commenters supported the agencies' longstanding interpretation
that tributaries include waterbodies that flow ``directly or
indirectly'' to a paragraph (a)(1) water, while other commenters
asserted that tributaries must flow ``directly'' into a paragraph
(a)(1) water. There is no text in the Clean Water Act supporting this
limitation, and the agencies have never interpreted the Act to cover
only such tributaries. Even the Rapanos plurality opinion did not so
limit the scope of tributaries covered by the Act. 547 U.S. at 742.
Moreover, the science is clear that the chemical, physical, and
biological integrity of paragraph (a)(1) waters depends on the many
tributaries, including headwater streams, that feed such waters. It
would be impossible to restore and maintain the chemical, physical, and
biological integrity as required by the Clean Water Act with a
definition of ``waters of the United States'' that included solely the
last tributary that flows ``directly'' into a paragraph (a)(1) water.
Tributaries upstream provide key functions that support the chemical,
physical, and biological integrity of paragraph (a)(1) waters. If
protections for tributaries ended just above the very last one,
functions like habitat for salmon spawning, baseflow to maintain water
levels, and nutrient replenishment would all be at risk. See Technical
Support Document sections I.A and III.E.ii.
    A tributary may contribute flow through a number of downstream
waters or features, including both non-jurisdictional features, such as
a ditch excluded under paragraph (b) of this rule, and jurisdictional
waters that are not tributaries, such as an adjacent wetland. However,
the tributary must be part of a system that eventually flows to a
paragraph (a)(1) water. Waters that are part of a system that never
reaches a paragraph (a)(1) water, for example, a small system of
streams that ultimately flow to a non-navigable stream in an intrastate
basin with no outlet, are not jurisdictional under this provision of
this rule.
    Some commenters asserted that the agencies' approach to
interpreting ``tributary'' would potentially allow the agencies to
include wetlands as tributaries. The agencies disagree. While wetlands
may be a water through which a tributary flows directly or indirectly
to a paragraph (a)(1) water, the agencies do not consider that wetland
to be a tributary itself. This is consistent with pre-2015 practice.
Only when a wetland lies entirely below the OHWM, will it be identified
as part of the tributary consistent with current practice; even then,
the wetland is not identified as a tributary itself. Otherwise, such
wetlands are considered adjacent wetlands and will be evaluated under
paragraph (a)(4) of this final rule.
    Some commenters supported the agencies' longstanding interpretation
that there is no meaningful distinction among natural, human-altered,
or human-made tributaries in terms of their functions, values, and
influence on the integrity of downstream waters. Some commenters
requested clarification as to whether both human-made and natural
tributaries would be regulated in this rule. Some commenters asserted
that the agencies' proposed approach to interpreting ``tributary'' is
overly broad and expansive because it would potentially allow the
agencies to include ditches and human-made conveyances as tributaries.
The agencies disagree with commenters who asserted that the agencies'
approach to human-made tributaries is overly broad and expansive. The
approach is consistent with the agencies' decades-long practice and the
scientific record, and such tributaries must still meet either the
relatively permanent standard or the significant nexus standard to be
jurisdictional under this rule. As noted above, given the extensive
human modification of watercourses and hydrologic systems throughout
the country, it is often difficult to distinguish between natural
watercourses and watercourses that are wholly or partly human-made or
human-altered. Because natural, human-altered, and human-made
tributaries provide many of the same functions, especially as conduits
for the movement of water and pollutants to other tributaries or
directly to paragraph (a)(1) waters, the agencies have interpreted the
1986 regulations to cover such tributaries. Ditches, for example, are
tributaries under this rule if they flow directly or indirectly to
paragraph (a)(1) waters and they are jurisdictional tributaries if they
also meet the relatively permanent standard or significant nexus
standard and are not excluded from jurisdiction under this rule. See
section IV.C.7 of this preamble for additional discussion on excluded
ditches.

c. Implementation
    A tributary for purposes of this rule includes rivers, streams,
lakes, ponds, and impoundments that flow directly or indirectly through
another water or waters to a traditional navigable water, the
territorial seas, an interstate water, or a paragraph (a)(2)
impoundment. A tributary may flow through a number of downstream
waters, including non-jurisdictional features. This section of the
preamble provides additional information on the agencies'
interpretation and implementation of the tributary provision of this
rule. This section first explains how to determine whether a water is a
tributary for purposes of this rule. The section next explains how to
determine whether a tributary is jurisdictional under the relatively
permanent standard or under the significant nexus standard.
i. Determining Whether a Water Is a Tributary for Purposes of This Rule
    This section describes how to (1) identify a tributary for purposes
of this rule and (2) determine whether the tributary is part of the
tributary system of a traditional navigable water, the territorial
seas, an interstate water, or a paragraph (a)(2) impoundment.
(1) Identifying a Water as a Tributary
    In implementing this rule, the agencies are maintaining their
longstanding interpretation that tributaries for purposes of Clean
Water Act jurisdiction include rivers, streams, lakes, ponds, and
impoundments. See 2007 Corps Instructional Guidebook at 8, 9. As
discussed above, although tributaries are required to flow directly or
indirectly through another water or waters to certain downstream
waters, tributaries are not required to have a specific flow regime to
meet the agencies' interpretation of ``tributary.'' However, flow
characteristics like duration and timing of flow will be considered in
determining whether tributaries meet the relatively permanent or
significant nexus standard, as described further below in sections
IV.C.4.c.ii and IV.C.4.c.iii of this preamble. Lakes, ponds, and
impoundments may be at the headwaters of the tributary network (e.g., a
lake with only an outlet to the tributary network) or farther
downstream from the headwaters (e.g., a lake with both an inlet and
outlet connected to the tributary network). Even though such waters are
considered to be lentic or ``still'' systems, such waters still
contribute flow downstream at the point that they outlet to the
tributary network and therefore the agencies have long concluded it is
appropriate to consider such waters to be tributaries.
    As discussed above in this section of the preamble, the agencies'
longstanding interpretation of ``tributary'' for purposes of the
definition of ``waters of the United States'' includes natural, human-
altered, or human-made waterbodies that flow directly or indirectly
through another water or waters to a traditional navigable water, the
territorial seas, or an interstate water. See Rapanos Guidance at 6.
    The agencies will utilize the Corps' well-established definition of
an ordinary high water mark (OHWM) to assist in identifying tributaries
for purposes of this rule. See section IV.C.8 of this preamble (adding
the definition of OHWM to EPA's regulation). Tributaries typically have
at least one indicator of an OHWM and, consistent with pre-2015
practice, physical OHWM characteristics are used to identify
waterbodies including streams, lakes, ponds, and ditches that are
present on the landscape. See, e.g., ``Final Notice of Issuance and
Modification of Nationwide Permits,'' 65 FR 12818, 12823-24 (March 9,
2000); 2007 Corps Instructional Guidebook; RGL 05-05 (December 7,
2005). The OHWM, a term unchanged since 1977, defines the lateral
limits of jurisdiction in non-tidal ``waters of the United States,''
provided the limits of jurisdiction are not extended by adjacent
wetlands. See 42 FR 37144 (July 19, 1977); 33 CFR 323.3(c) (1978). The
regulations at 33 CFR 328.3(e) and 329.11(a)(1) list the factors to be
applied. RGL 05-05 further explains these regulations. Delineation of
an OHWM in tributaries relies on the identification and interpretation
of physical features, including topographic breaks in slope, changes in
vegetation characteristics (e.g., destruction of terrestrial vegetation
and change in plant community), and changes in sediment characteristics
(e.g., sediment sorting and deposition). Field indicators, remote
sensing, and mapping information can also help identify an OHWM. The
Corps continues to improve regulatory practices across the country
through ongoing research and the development of regional and national
OHWM delineation procedures, as described further in section IV.A.ii of
the Technical Support Document. For example, the Corps has developed
field indicators to help field staff identify the OHWM in common stream
types in the arid West. Consistent with longstanding practice, the
agencies will apply the regulations and use RGL 05-05 and applicable
OHWM delineation manuals, as well as take other steps as needed to
ensure that the OHWM identification factors are applied consistently
nationwide. See Rapanos Guidance at 10-11 n.36.
    The agencies will assess any discontinuity in the OHWM and,
consistent with pre-2015 practice, a natural or human-made
discontinuity in the OHWM does not necessarily sever jurisdiction
upstream. A discontinuity may exist where the stream temporarily flows