Receiving Water Impacts of Sediment Literature Review and Summary Prepared by Tetra Tech, Inc. for USEPA, Office of Science and Technology March 31, 2004 Receiving Water Impacts of Sediment A large body of scientific literature addresses the question of how the health of aquatic resources is impacted by excess sediment loading in waterbodies. At least partly on the basis of the research findings, states across the country have already set sediment targets for receiving waters to protect aquatic resources, and are continually developing and refining targets for geographically specific watersheds.

Demarcation by waterbody type provides context and is an important theme in the literature for purposes of setting sediment targets. For example, conditions in the deltaic lower Mississippi are different from those in montane streams in Idaho. Differences exist not only in the aquatic species,

but also in sediment behavior within the waterbody and in threshold levels of impacts. The biota or aquatic species that are the focus of the literature include aquatic vegetation, macroinvertebrates,

eggs, fry, juvenile and adult fish, shellfish, and corals. Identified waterbody types in the literature include:

° lakes, reservoirs, ponds, and impoundments ° rivers and streams ° wetlands ° oceans, estuaries, and other coastal water ecosystems, including coral reefs Construction Industry Impacts on Sediment Loads Storm water discharges generated during construction activities cause a wide variety of physical,

chemical, and biological water quality impacts. The interconnected process of erosion, sediment transport, and delivery is the primary pathway for introducing pollutants such as excess sedimentation, total suspended solids, nutrients, metals, and organic compounds to aquatic systems ( Novotny and Chesters 1989) in USEPA ( 1999). USDA ( 1989) estimated that 80 percent of phosphorus and 73 percent of the Kjeldahl nitrogen are directly associated with eroded sediment ( cited in Fennessey and Jarrett ( 1994), in USEPA 1999). The 2000 National Water Quality Inventory 3 ( USEPA) states that siltation is one of the top causes of impairment of waters across the United States. The report also states that pollution from urban and agricultural land transported by precipitation and runoff, and which includes pollutants from construction and land development activities, is the leading source of impairment.

Direct and Indirect Impacts The impacts of excess sediment in the water include direct physical effects such as reducing visibility and light in the water column, physical abrasion of plant surfaces, clogging of gill openings, and entombing of eggs and fry in redds. Impacts may also be indirect, as in changes to the chemical composition of the water, light penetration or turbidity, and/ or temperature profile, which in turn affect primary productivity with repercussions in terms of fish behavior and overall community profiles and trophic structure. Thus aquatic resources may be directly affected in terms of aesthetics,

physiology, and mortality, or indirectly affected via changes in the habitat structure of the waterbody.

Suspended vs. Deposited Sediment In this section consideration is given to the impacts of suspended sediment. Bedded sediments,

though they directly affect the survival of fish eggs and fry and other organisms, do so because they alter the habitat structure and are dealt with in Section 8.2.3 under Physical Impacts of Construction and Land Development Activities.

Literature This literature review focuses on study methodologies that describe quantitative effects of sediment imbalance in aquatic systems in a basic dose response relationship and where aquatic organisms are exposed to suspended and/ or bedded sediments. Additionally, although sediment entering a particular aquatic system may be contaminated and generate additional effects through toxic/ non toxic pollutants that are associated with it, this review focuses on the impacts from uncontaminated 4 sediments.

Cold water salmonid fish, predominantly in a stream setting, dominate the literature on the sediment dose response relationship. The literature is neither as extensive nor as rich on estuaries, lakes, and coastal ecosystems or on invertebrates, fish other than salmonids, and aquatic plants.

The aquatic variables impacted are presented below. The review considers literature on several types of aquatic resource: aquatic vegetation and primary production, invertebrates, juvenile fish, fry, and eggs, and adult fish. These aquatic biota are considered within their geographical setting and waterbody type: rivers/ streams, ponds/ lakes, estuaries/ coastal environments. Topics that are covered more extensively in the literature are the impacts of suspended sediment on adult fish and impacts of deposited or substrate sediment on juvenile fish, fry, and eggs.

Measures of suspended sediment In this section, the effects of excessive suspended sediment on aquatic resources are considered.

Measures of suspended sediment include turbidity and total suspended solids. With respect to reviewing these dose response studies authors typically consider how either turbidity or TSS affects biota. However, the relationship between the two measures is often unclear and not explicitly defined. Turbidity is a measure of light dispersion whereas TSS measures the mass of particles in the water column. Larger particles contribute mass to a TSS measurement, but do not scatter light as much as a similar weight of smaller particles. Usually when sediment particles are smaller, turbidity levels are higher.

Studies involving an analysis of the relationship between the two measures of suspended sediment include Packman, et al. ( 1999) who showed that TSS and turbidity have a strong positive relationship in nine urban/ suburban Puget lowland streams. New Mexico TMDLs ( NMED, 2002) converted a turbidity standard to TSS by calibrating with local data, so that the TSS values in units of mg/ L could be converted to sediment loads in lbs/ day. Keyes and Radcliff ( 2002) calibrated turbidity units ( NTU)

to approximate TSS measures using 40 mg/ L kaolin clay set to a standard of 40 NTU. However, in 5 natural streams the composition of suspended particles is not uniformly like that of kaolin clay.

The impact of suspended sediment depends on the type of particle sizes to some extent, and therefore TSS and turbidity measures should be considered together where the information is available. For example, Servizi and Martens ( 1992) reported that salmonids were relatively tolerant of elevated TSS levels when the particle sizes were larger. When the particles are smaller, turbidity is higher, which appears to make conditions more difficult for the salmonids.

Sediment Deposition Rates The effects of sediment deposition from construction activities are known to affect streams far downstream of construction sites. For example, Fox ( 1974), in USEPA ( 1999), found that streams between 4.8 and 5.6 miles downstream of construction sites in the Patuxent River watershed were impacted by sediment inputs.

Erosion from construction sites can also generate the transport of pollutants associated with onsite wastes. The Storm Water Quality Task Force ( 1993), in USEPA ( 1999), states that rain splash, rills,

and sheetwash encourage the detachment and transport of pollutants ( including both sediments and pollutants associated with sediments) to waterbodies. Erosion from construction sites and runoff in developed areas can elevate pollutant loads well above those in undisturbed watersheds. Novotny and Olem ( 1994), in USEPA ( 1999), state that erosion rates from construction sites are much greater than from any other land use. The results from field studies and erosion models conducted by USDA ( 1970), in USEPA ( 1999), found that erosion rates from construction sites are usually an order or magnitude higher than row crops and several orders of magnitude higher than rates from wellvegetated areas such as forests or pastures. A review of the efficiency of sediment basins conducted by Brown ( 1997), in USEPA ( 1999), found that inflows from 12 construction sites had a mean TSS concentration of about 4,500 mg/ L. Kuo ( 1976), in USEPA ( 1999), found that suspended sediment concentrations from housing construction sites in Virginia were measured at 500 3,000 mg/ L, or about 40 times larger than the concentrations in runoff from already developed urban areas. In Wisconsin, Daniel et al. ( 1979) ( in USEPA 1999) monitored storm water runoff from three residential 6 construction sites and found that annual sediment yields were more than 19 times the yields from agricultural areas. Daniel et al. identified total storm water runoff followed by peak storm water runoff as the most influential factors controlling the sediment loadings from residential construction sites, and also found that suspended sediment concentrations were 15,000 20,000 mg/ L in moderate storm events and up to 60,000mg/ L in larger events. Lastly, Wolman and Schick ( 1967), in USEPA ( 1999), studied impacts of development on fluvial systems in Maryland, and found that sediment yields in areas undergoing construction were 1.5 to as much as 75 times greater than detected in natural or agricultural catchments. Lastly, Yorke and Herb ( 1978), in a long term study of subbasins in Maryland portions of the Anacostia River, found that average annual suspended sediment yields for construction sites ranged from 7 to 100 tons per acre.

The effects of road construction on erosion rates and sediment yields were also examined. In West Virginia, a road construction project studied by Downs and Appel ( 1986) disturbed only 4.2 percent of a 4.72 square mile basin, but it resulted in a three fold increase in suspended sediment yields.

During the largest storm event, it was estimated that 80 percent of the sediment in the stream was attributed to the construction site. Hainly ( 1980) evaluated the effect of 290 acres of highway construction on watersheds which ranged in size from 5 to 38 square miles. He found that even in the smallest watershed, the estimated sediment yield from the construction area was 37 tons per acre during a two year period. In Hawaii, Hill ( 1996) found that highway construction increased suspended sediment loads by 56 to 76 percent in basins of 1 to 4 square miles. The National Association of Counties Research Foundation ( 1970) determined that sediment yields from construction sites could be as much as 500 times the levels in rural areas.

Small vs. Large Construction Site Impacts Studies have indicated that the water quality impact from small construction sites may be the same or greater than large construction sites on a per acre basis. The concentration of pollutants in runoff from small sites is similar to that from large sites. In urban areas the proportion of sediment that makes it to surface waters may be the same because the runoff is delivered directly to storm drain networks, with no opportunity for pollutants to be filtered out ( USEPA, 1999). MacDonald ( 1997), 7 in USEPA ( 1999), states that storm water regulations are more likely to require controls for large sites than smaller sites. The smaller sites that lack sediment and erosion controls would then contribute a disproportionate amount of total sediment from construction activities. Brown and Caraco ( 1997), in USEPA ( 1999) also state that smaller construction sites are less likely to have an effective plan for controlling erosion, are less likely to implement and maintain their plans, and are less likely to be inspected.

To test the theory that small sites have sediment loads on a per acre basis similar to large sites, the EPA gave a grant to Dane County, Wisconsin Land Conservation Department, in cooperation with USGS, to evaluate sediment runoff. In this study by Owens et al ( 1999), in USEPA ( 1999), a 0.34 acre residential development and a 1.72 acre commercial office development were evaluated. At the residential site, total solids concentrations were 642 mg/ L, 2,788 mg/ L, and 132mg/ L for preconstruction, active construction, and post construction, respectively. This equaled 7.4 lbs preconstruction, 35 lbs during construction, and 0.6 lbs post construction on a pollutant load basis At the commercial site, Owens et al found that total solids during preconstruction were 138 mg/ L and 200mg/ L during post construction, but was 15,000 mg/ l during the active construction period. This equaled 0.3 lbs preconstruction, 490 lbs during construction, and 13.4 lbs after construction on a pollutant load basis. The total solids from the commercial site were similar to those in a study by Downs and Appel ( 1986), who evaluated the effects of highway construction in West Virginia. They found that a small storm event yielded a sediment concentration of 7,520 mg/ L.

Several studies have also evaluated the total amount of disturbed land for small and large construction sites. Brown and Caraco surveyed 219 jurisdictions to assess sediment and erosion control programs.

They found that of the 70 respondents, in 27 cases more than three fourths of the permits were for sites less than 5 acres, and in another 18 cases, more than half of the permits were for sites less than 5 acres. MacDonald ( 1997), in USEPA ( 1999), evaluated data on the 3,831 construction site permits for North Carolina from 1994 through 1996. He found that nearly 61 percent of the sites 1.0 acre or larger were between 1.0 and 4.9 acres in size. EPA estimates that construction sites disturbing more than 5 acres is 2.1 million acres ( 78.1 percent of total), while the sites disturbing between 1 and 5 acres totaled 0.5 million acres ( 19.4 percent). Given their high erosion rates, small construction 8 sites can contribute significantly to water quality impairment, particularly in small watersheds.

Paterson ( 1994), in USEPA ( 1999), summarized that, given the critical importance of field implementation of erosion and sediment control programs, much more focus should be given to plan implementation.

I. Effects of Sediment on Primary Production Aquatic plants generate oxygen in the water through the process of photosynthesis, but they provide more benefits than just photosynthesis. They comprise a large part of the biomass available to nourish grazer organisms in the water, which then serve as prey for other organisms. Macrophyte plants also provide cover and niches for fauna, attenuate flow velocity, and generally improve physical habitat quality. Excessive sediments in aquatic system contribute to turbidity which alters the light regime in the water column. This suppresses the photosynthesis process, which directly impacts primary production, submerged aquatic vegetation, and the zooxanthellae in corals. Other effects include coating and physical abrasion of plants.

Best et al ( 2001) described the effect of light changes on aquatic vegetation. Batiuk et al. ( 1992),

Dennison et al ( 1993), and USEPA ( 2000) documented declines of submerged aquatic vegetation ( SAV) in the Chesapeake Bay from suspended sediment. USEPA derived Chesapeake Bay water clarity criteria based on the light requirements for SAV growth and survival. The criteria take total suspended solids, chlorophyll a, epiphytic growth, and salinity profiles into account.

Van Nieuwenhuyse and LaPerrier ( 1986) found that primary production was reduced by 50 percent of background levels when there was turbidity of 170 NTU and TSS 201 mg/ L. Hart and Fuller ( 1972) found that in the Patuxent River Maryland, high turbidity levels limited the development of macrophytes. Reed ( 1983) found that macrophytes colonized deeper parts of the water column in a North Carolina pond when turbidity was decreased. Buck ( 1956) in Vohs et al. ( 1993) found that in clear ponds in Oklahoma primary production was 12.8 times higher than in ponds with TSS of 100 mg/ L. USDA National Sedimentation Laboratory, Knight et al. ( 1998) found that suspended sediment concentrations as low as 100 mg/ L reduced primary production. Westlake ( 1975) found 9 that the gradual reduction of light by turbidity raises the compensation point ( the depth at which photosynthesis equals respiration) below which plants cannot grow.

While turbidity and suspended sediment blocks the penetration of light into a water body and therefore affects photosynthesis, this effect is also influenced by factors such as temperature, current velocities and distribution, flow regimes, and taxon specific response by the biota. U. S EPA ( 1986),

Lloyd et al. ( 1987), Kiffney and Bull ( 2000), and Rosemond et al. ( 2000) found generally that increased suspended sediment reduces primary production, which limits the abundance of primary feeders that form the basis of the food chain for fish. Lloyd ( 1987) found that in Alaska, turbidities of 25 NTU or more could cause light extinction at shallower depths in the water than under normal conditions. This type of impact is typically associated with a decrease in plant production. Lewis ( 1973) found that higher levels of TSS affect primary production not only by reducing light penetration but also through abrasion. Severe abrasive damage was observed after 3 weeks of exposure to 100 mg/ L of coal dust on the leaves of the aquatic moss Eurhynchium riparioides.

Decreased light penetration also often causes aquatic macrophytes to be replaced with algal communities, which in turn alters invertebrate and fish communities ( USEPA, 2003). Kiffney and Bull ( 2000) found that as suspended sediment accumulates in algal mats, the abundance of benthic herbivores are reduced. Davies Colley et al. ( 1992) found that restriction in light penetration into water is an important mechanism by which fine inorganic solids damage streams.

Dissolved Oxygen Primary production is also affected by changes in the dissolved oxygen ( DO) composition in the water. There are several mechanisms through which increased sediment turbidity can reduce dissolved oxygen in the water. Lower DO may have direct physiological effects on organisms at many trophic levels in the waterbody. Organic particles bound to suspended solids remove oxygen from the water column as they decompose. Fish may increase their activity levels in response to turbidity, thereby increasing their respiration rates and removal of oxygen from the water column.

Du Preez et al. ( 1996) found that fish assemblages increase O 2 consumption in the presence of 10 10,0000 mg/ L of suspended silt. Over time the fish became acclimated to the turbid conditions and slowly reduced their respiration rate, allowing ambient DO concentrations to return to normal. Reed et al. ( 1983) found that increased turbidity traps heat in ponds in North Carolina. With increased heat, oxidation rates in the ponds may increase and lower the DO equilibrium concentration in the water.

NAS and NAE ( 1973) found that suspended materials decrease the light penetration through the water column, but they increase the absorption of solar energy near the surface. The heated upper layers tend to stratify the water column. With increased stratification, there is reduced dispersion of DO and nutrients to the lower depths of the waterbody.

II. Effects of Sediment on Macroinvertebrates In general, information is not as abundant on the effects of suspended sediment on macroinvertebrates as on fish.

Drift in normal aquatic ecology is a natural, " continuous redistribution mechanism." In the course of a normal life cycle, benthic invertebrates periodically release from bottom substrates and become mobilized in downstream flow. When there is excess sediment it affects the macroinvertebrate population by causing significant increases in drift through scour or increased turbidity.

Direct Impacts Culp et al. ( 1986) experimentally added sand particles to a stream segment, increasing that segment's bed load. More than 50% of benthic animals in the stream segment were scoured from the bed.

Herbert and Merkens ( 1961) found that populations of macroinvertebrates decreased when suspended solid concentrations rose from 40 to 120 mg/ L. Drift rate increased 90 percent and reductions in macroinvertebrates occurred even in the absence of visible accumulation of sediment in the substrate.

Griffiths and Walton ( 1978) in USEPA( 2003), found that the upper severity of ill effect tolerance level for suspended sediment for bottom invertebrates could be as low as 10 15 mg/ L. 11 Gammon ( 1970) found declines in populations of macroinvertebrates when more than 80mg/ L inert solids were added to the normal suspended solids concentrations. A decline in abundance was attributed to the increased drift out of riffles. The drift rate was directly proportional to increases in suspended sediment concentration up to 160 mg/ L.

Contributors to the literature are still debating the exact effects of suspended sediment on macroinvertebrates, however. Waters ( 1995) found " on the basis of current knowledge, the direct effect of suspended sediment upon benthic invertebrates does not appear to be a significant influence upon stream invertebrate communities." Chapman and Mcleod ( 1987) found that sufficient data are not available to establish reliable thresholds of sediment concentration that cause damage to aquatic invertebrates. Fairchild et al ( 1987) pulsed discharges of large concentrations of suspended sediment to a stream for two hours each week for six weeks. The additional sediment caused no alteration in abundance or diversity of benthos or insect emergence.

NAHB ( 2000) summarized some points from the literature by pointing out that collisions with moving bed load material can physically scour benthic organisms from the stream bottom in rapidly moving streams, and high concentrations of suspended sediment ( e. g., 120 mg/ L) may cause significant displacement of invertebrates by drift, but short term pulses of higher concentrations may not disturb the invertebrate community. Rosenberg and Wiens ( 1978) found that benthic invertebrates exposed to 8 mg/ L of suspended sediment for five hours exhibited increased rates of drift. Invertebrates most sensitive to sediment, i. e, those that drifted almost immediately after sediment addition, included important salmonid prey, indicating the repercussions of sediments effects on invertebrates all the way up the aquatic food chain. Shaw and Richardson ( 2001) found that macroinvertebrate drift tends to increase with longer repeated pulses of sediment influx. Runde and Hellenthal ( 2000) found that macroinvertebrate drift tends to increase with smaller particle sizes. McCabe and O'Brien ( 1983)

found that the filter feeding zooplankton Daphnia pulex displayed a reduced capacity to assimilate food when exposed to 24 mg/ L of suspended sediment for only 15 minutes. 12 Indirect Impacts Indirect impacts of suspended sediment on aquatic insect populations are documented in the literature but have not been widely studied. Wagner ( 1959) found that suspended solids eliminated food and cover and therefore insects, as a consequence, were eliminated in the Wynooche River in western Washington. Arruda et al. ( 1983) considered the effects of suspended sediment on zooplankton in reservoirs and found that concentrations of 50 100 mg/ L reduced daphnia grazing on algae to the point of daphnia starvation. Increased sediment concentrations, they concluded, ( up to 2,451 mg/ L)

can reduce grazing of daphnia on algae by 95%. Wolman and Schick ( 1967), in USEPA ( 1999),

found that benthic organisms in streambeds can be smothered by sediment deposits, consequently causing changes in aquatic flora and fauna, such as fish species composition.

Waters ( 1995) found that there are three major relationships between benthic invertebrate communities and sediment deposition in streams, i. e., there is a correlation between their abundance and 1) substrate particle size, 2) embeddedness of substrate and loss of interstitial space and 3)

change in species composition. Cordone and Kelly ( 1961), Peddicord ( 1980), Wilbur and Clarke ( 2001), and Zweig and Rabeni ( 2001) found a high correlation between deposited sediments and the effects on five biomonitoring metrics in four Missouri streams that the authors examined for benthic fauna tolerance levels to sediment.

Aldridge et al. ( 1987) found that increased levels of suspended sediment were shown to impair ingestion rates of freshwater mussels in laboratory studies. Tester and Turner ( 1988) and Sherk et al. ( 1976) found that copepods reduced feeding activity as a response to increased levels of suspended sediments. Herbert and Merkens ( 1961) found that increases in suspended sediment ( e. g., to 120 mg/ L) can result in increased drift, significantly altering the distribution of benthic invertebrates.

Erman and Erman ( 1984) found that as substrate embeddedness increases and the substrate particle size distribution reflects greater deposited sediments, it affects the structure and function of benthic macrofaunal communities. 13 III. Effects of Sediment on Fish This section is organized along three main categories of effects on fish:

° Direct physiological effect, e. g., asphyxiation and suffocation, gill clogging ° Behavioral stresses and effects on population survival due to increased turbidity, lower light levels, and reduced water clarity ° Substrate and deposited sediment effects Sediment Dose and Exposure The type of effect that excess sediment exerts on the fish is loosely correlated with the sediment dose:

high concentrations exact direct acute physiological and mortality effects on fish while lower concentrations can elicit behavioral responses, and, if exposure is persistent or frequent, stunted growth and reproduction. Newcombe and MacDonald ( 1991) established that it is not only the sediment concentration, but also the duration of exposure that is critical. Shaw and Richardson ( 2001) found that the frequency of exposure is important to consider as well as concentration ( measures of concentration over time).

Direct Physiological Effect There is a body of literature that documents the process of direct physiological damage to fish,

including Wallen ( 1951), Alabaster ( 1972), Ritchies ( 1972), Stroud ( 1967), and Trautman ( 1981),

that documents the clogging of fish gills by sediment particles. USEPA ( 2003) stated that stress hormones are released as a response to gill function, and that gill clogging causes asphyxiation and traumatization of gill tissue. NAHB ( 2000) surveyed several studies that describe salmonid fish survival despite exposure to suspended sediment in various concentrations over various durations.

Their point is to expose the large variability in data published on both the survival period of fish and the extent of damage to fish gills by suspended sediment. For example, they described the contrast between Herbert and Merkens ( 1961) study of increased fin rot and observed thickened gill 14 epithelium in rainbow trout in TSS of 270mg/ L and Redding et al. ( 1987), who found that salmon exposed to 2000 mg/ L of suspended solids for seven days did not show degradation of the gills.

NAHB ( 2000) also reports few corroborated estimates of threshold lethal levels of suspended sediment in the literature, noting Mccleay at al. ( 1987) in contrast to Lloyd ( 1987). They also point out that mortality as a direct outcome of the physiological process of gill damage through suspended sediment is not conclusive in the literature, citing Stroud ( 1967), and that gill damage is only apparent after a long period of exposure to high levels of TSS in Wallen ( 1951). Farnworth et al, ( 1971) found that experimental fish survived for a week or longer in 100,000 NTU turbidity conditions Additionally, NAHB ( 2000) mentions that climate variability influences the effects of TSS in fish,

citing Noggle ( 1978). They note that the summer threshold for salmon survival in waters with elevated suspended solids is much lower than the autumn threshold. They concur with the understanding of duration of exposure as a key factor of consideration along with concentration of suspended solids in determining fish effects.

Waters ( 1995), Everest et al. ( 1987), Newport and Mayer ( 1974), Wallen ( 1951), and Lake and Hinch ( 1999) all found that direct acute effects of suspended sediments on adult fish may not be observed until concentrations reach thousands to tens of thousands of mg/ L.

Anderson et al. ( 1996) and Newcombe and MacDonald ( 1991) found greater sensitivity in younger fish, particularly sac fry, with increased mortality evident at concentrations on the order of 1000 mg/ L or less. Slaney et al. ( 1977) found a significant relationship between suspended sediment duration ( concentration x days) and percent egg to fry survival of rainbow trout. Survival dropped below 30% at about 1000 mg/ L day and approached zero at about 2000 mg/ L day. Reynolds et al. ( 1989)

found that Arctic grayling sac fry mortality was higher than a control group that was not exposed to the suspended sediment duration.

Indirect Effects: Behavioral Changes and Physiological Stress 15 Elevated suspended sediment may not always be directly lethal to fish, but it can have sublethal impacts, causing adverse behavior in fish populations. Deprivation, displacement, and changes in trophic dominance can limit species' normal functions of feeding, reproduction, and predation to the point at which healthy populations are not sustained. Quote Berg & Northcote ( 1985) found that salmon dominance was not sustained, territories were not defended, and gill flaring occurred more frequently at elevated sediment concentrations. These authors also noted that normal behaviors were restored when the water in the experiment were restored to normal NTU ( turbidity) levels.

Effects of stress on morbidity, growth, and mortality patterns in fish include increased blood sugar levels, coughing, altered behavior, emigration and avoidance, reduced feeding rates, reduced reaction distances, reduced growth, reduced survival, reduced primary production, reduced density and reduced feeding rate, reduced food assimilation, and reduced reproductive potential. For each type of behavior summarized in the literature, authors indicate the range of turbidity at which the behavior is observed. For example, altered behavior, emigration and avoidance, and reduced feeding rates effects are noticed in a range between 3 and 30 NTUs. At even lower thresholds in turbidities reduced reaction distances are observed. A decrease in growth has been found in turbidities of 22 NTUs and reduced survival rates were seen in turbidities as low as 15 NTUs.

Servizi and Martens ( 1992) found that blood sugar levels ( a secondary indicator of stress) increased with turbidity at all levels tested ( juvenile coho), and coughing increased significantly between 3 and 30 NTUs. Many of these studies were conducted in laboratory settings and/ or with artificially induced turbidity andmostly represent chronic, continuous exposure.

Lloyd ( 1987), Reed et al. ( 1983), McCleay ( 1987), Scannell ( 1988), and Bisson and Bilby ( 1982)

found that salmon avoid turbid waters and have a low tolerance for them. Servizi and Martens ( 1992)

and Bjornn and Reisser ( 1991) found that salmon migration is delayed or impaired in turbid waters or ceases in highly turbid waters.

Lloyd ( 1987) and Birtwell et al. ( 1984) found that fish are displaced as result of turbidity. Lloyd et al. ( 1987), Alabaster and Lloyd ( 1982), Berg and Northcote ( 1985), and Breitburg ( 1988) found that 16 there is a reduction in predator efficiency due to turbidity clouding the water and impairing the ability of sight dependent hunting.

McCabe and O'Brien ( 1983) found that filter feeding by fish on zooplankton declined in a turbid pond. NAHB ( 2000) stated that the evidence about turbidity's effects on fish may be uncertain.

Bisson and Bilby ( 1982), showed that moderately turbid waters are tolerated by fish in their study.

Gradall and Swenson ( 1982) found that Creek chubs preferred turbid waters over moderately turbid waters. Herbert and Richards ( 1963) stated that reasonable fish populations were surviving in ` many rivers' containing industrial waste solids. Gregory and Levings ( 1988) found that because larger fish strike at prey less in turbid waters, younger salmon were actually surviving at higher rates in turbid waters. Sweka and Hartman ( 2001) found that turbidity decreases predator efficiency.

Lloyd et al. ( 1987) found that turbidity decreases the abundance of food organisms ( secondary production), thereby affecting the abundance of fish. Servizi and Martens ( 1992) found that predatory salmonids avoid highly turbid waters, and Vogel and Beauchamp ( 1999) found that they do not benefit from increased macroinvertebrate drift associated with turbidity because sight distances and capture rates are reduced. Sigler et al. ( 1984) found that avoidance behaviors lead to decreased growth of fish. Bachmann ( 1958) found cessation in feeding in cutthroat trout exposed to a suspended sediment concentration of 35 mg/ L over a 2 hour period. Gammon ( 1970) found that loss of fisheries can occur due to avoidance or failed reproduction.

Tables, Thresholds, and Targets A landmark in the literature on the effects of sediment on fish is the tabular summaries of modeled effects based on exposure, duration, and type of fish organism. These tables summarize the available data on the effects of suspended and bedded sediment on fish and invertebrates. Newcombe and Jensen ( 1996) presented an extensive data table on the effects of suspended and bedded sediments on fish In their tables they categorized effects called " Severity of Ill Effects" as behavioral, sublethal,

para lethal, and lethal. Based on the concentration of sediment, the duration that the fish organism could tolerate for each Effect category ( e. g., behavioral effect or sublethal effect) was calculated 17 using the model formulas. The type of fish organism is another dimension in the tables. For example,

the different categories of organism on which effects are noted include: salmonid adults, salmonid juveniles, eggs and larvae, and non salmonid adults.

Wilbur and Clarke ( 2001) modified the dose response graphs that Newcome and Jensen ( 1996)

created to provide an easy reference for estimating biological responses of estuarine aquatic organisms to suspended sediment. They also related their findings to sediment conditions associated with dredging projects. Other groups have also categorized TSS concentrations based on their effects on the aquatic environment, primarily fish. The European Inland Fisheries Advisory Commission ( 1964) concluded ranges of concentration tolerance as follows:

° If conc < 25 ppm = no harmful effect on fisheries ° 25   80 ppm = possible to maintain good to moderate fisheries ° 80 400 ppm = unlikely to support good fisheries ° 400 ppm = poor fisheries The classification has been the basis of other authors who have developed recommendations of TSS ranges for fish protection. Mills et al. ( 1985) for USEPA, classified impairment of aquatic habitat or organism by TSS in terms of " probable impairment" as follows:

° If conc < 10 mg/ L = improbable impairment ° If conc > 10mg/ L and < 100 mg/ L = potential impairment ° > 100 mg/ L = probable impairment Effects of Sediment on Fish in Lakes Whittier and Hughes ( 1998) described a northeastern U. S. study that concluded that fish that could tolerate suspended sediment disturbances in streams were intolerant or only moderately tolerant of disturbances in lakes. 18 Effects on Sediment on Warmwater Fish Reports of direct and indirect impacts of suspended solids in warmwater environments are less readily available than reports from salmon research. Waters ( 1995) found a great variation in tolerance to suspended sediment among warmwater fish. Reed et al. ( 1983) reviewed early papers by Wallen ( 1951) and concluded that lethal suspended solids levels ranged from 38,250mg/ L to 222, 000 mg/ L for fourteen warmwater species of fish.

Waters ( 1995) also found that fish have disappeared over a long time from warmwater systems.

Trautman ( 1981) found that siltation caused gill asphyxiation is responsible for the extinction of Harelip. Heimstra et al. ( 1969) in Vohs et al. ( 1993) found that bass attacking behavior, efficiency,

and social order were disrupted in turbid waters. Edwards et al. ( 1983) found that smallmouth bass fry are displaced at high turbidity. McMahon et al. ( 1984) found that spotted bass in the southeastern U. S, are intolerant of turbid water. Gardner ( 1981) found lower bluegill feeding rates in higher turbid waters. Vinyard and O'Briend ( 1976) found that reaction distances of bluegill were shorter at higher turbidity ( though there is overlap in the reaction distance data among the experimental levels of turbidity). Heimstra et al. ( 1969), in Vohs et al. ( 1993) observed greenfish and showed that their activity was altered and their social order was noticeably altered. In contrast to normal conditions where an individual fish made all the attacks in a group, in turbid waters, many individuals in the groups made random attacks.

Decreased DO induced by excess sediment may have an effect on warm water fish. Doudoroff and Shumway ( 1970), in Reed et al. ( 1983) did a review of literature of DO requirements of warmwater fish. The researchers reported that largemouth bass avoided waters where DO was only 3 to 4.5 mg/ L. In opposition, a lab study in Horkel and Pearson ( 1976) reported that oxygen consumption rates of green sunfish are not affected by turbid suspensions of up to 3500 FTU. NAHB ( 2000)

equated this amount of turbidity with approximately 13,000 mg/ L, roughly equivalent to the experimental concentration of DuPreez et al. ( 1996).

Heimstra et al. ( 1969) found that the altered behavior in both largemouth bass and green sunfish was 19 apparent at 14 16 JTU. Walters et al. ( 2001) found that the Index of Biotic Integrity ( IBI) values in Georgia streams were found to be consistently lower where low flow turbidity values exceeded 8 NTU. Wallen ( 1951) in Reed et al. ( 1983) showed that stressed behavior such as visible confusion and agitated behavior was observed in green sunfish in turbid conditions. Hamilton and Nelson ( 1984) found that certain fish accommodate and do well in turbid conditions by demonstrating that the spawning, feeding and growth of white bass is not acutely affected by turbidity. McMahon et al.

( 1984) found that warmouth do fine in turbid waters up to 100 NTUs and that walleye prefer turbid waters to clear, well lit zones in the water column. Johnson and Hines ( 1999) showed in a lab study that high turbidity ( 250 mg/ L of suspended sediments) can improve the survival rate of young razorback suckers.

PHYSICAL EFFECTS Embedded Sediment Sediment embeddedness measures the degree to which cobbles and large gravels are buried and their interstitial spaces filled because of fine sediment deposition. In a study of habitat restoration in a highly sedimented Idaho stream, Hillman et al. ( 1987) found that interstitial spaces among cobbles may be essential winter habitat for juvenile chinook salmon. When large cobble was added to an otherwise embedded stream, juvenile populations increased. When that same cobble became embedded, the population decreased.

Embeddedness blocks passages and removes small cover spaces for eggs, fry, and juvenile fish.

USEPA ( 2003) summarized that sediment deposition has caused a 94% reduction in numbers and standing crop biomass in large game fish due to increased vulnerability of their eggs to predation in gravel and small rubble, reductions in oxygen supply to eggs, and increased embryo mortality.

Weaver and Fraley ( 1993) ( in USEPA 2003) reported that emergence success of cutthroat trout was reduced from 76% to 4% when fine sediment was added to redds. NAHB ( 2000) reported that as fry grow into juvenile fish they seek out the slow moving water at the channel edges for cover. These areas also are favored for deposition of suspended sediment. When these areas are filled with excess 20 sediment, sheltered space is lost, and the juveniles are forced out into the channel to compete at a disadvantage with the adult fish. Waters ( 1995) also found that juveniles face habitat degradation from the sedimentation of pools. Information quantitatively relating embeddedness levels to effects on aquatic fauna is limited.

Embeddedness in the range of 67 percent caused changes in the macroinvertebrate fauna. Increased embeddedness of cobbles ( fines filling spaces around the rocks) led to lower density of fry found by Bjorn et al. ( 1977). Nelson et al. ( 1997) found an average embeddedness of 35 percent in natural streams in granitic watersheds ( i. e., South Fork Salmon River, Idaho). Based on their review of existing data, Chapman and McLeod ( 1987) were unwilling to generalize on the effects of embeddedness level of surface fines and salmonid rearing densities. They did conclude that abundance of insects declines at an embeddedness level of about two thirds to three quarters. They also found that embeddedness levels this high would probably violate spatial needs of overwintering fish for sediment free interstices.

NAHB ( 2000) found that invertebrate study results are often complicated by the fact that the various invertebrate species in a community responds very differently to increased sediment levels. Aquatic insect densities may decline at embeddedness levels of approximately two thirds to three quarters.

In the Payette and Boise Forest Plan thresholds for streams in the South Fork Salmon River watershed were set contingent on 1988 sediment conditions, however, Nelson et al. ( 1997) found them too restrictive and came up with embeddedness targets and free matrix percentage appropriate for their findings of 35 percent average natural embeddedness conditions in the river.

Levels of embeddedness linked with high, moderate, or low habitat conditions for endangered species were determined for Clearwater and Nez Perce National Forests and Cottonwood ( Idaho) area BLM/ USDA FS et al. ( 1998). High levels of habitat conditions were associated with embeddedness < 20 percent. At > 30 percent, habitat conditions were considered low. Intermediate embeddedness was considered a moderate habitat condition. 21 State Targets Several approved Total Maximum Daily Load ( TMDL) studies in California have a target for riffle embeddedness that is <= 25 percent or a decreasing trend toward 25 percent. While the 25 percent figure is universal in the TMDLs that consider embeddedness, there is little supporting evidence for this threshold. The fact that an improving trend is also acceptable shows that the threshold was loosely interpreted. New Mexico has established embeddedness thresholds for aquatic life use support. Streambeds that are less than 33 percent embedded represent fully supporting sediment conditions and are not compared to reference conditions. For streams with greater than 33 percent embeddedness, support is defined in comparison to reference conditions. NMED ( 2002) found that embeddedness values less than 27 percent greater than reference values are supporting and embeddedness values more than 40 percent greater than reference conditions are non supporting.

Surface Sediment Surface sediment describes the percentage of streambed area with exposed fine sediments. Targets are developed to describe thresholds of suitability of stream substrates for invertebrate and salmonid habitation. Using the Wolman pebble count method, percent surface fines may be calculated. The same method is also used to determine the median substrates size ( d50). This is used as a sediment target. The percentage of area is one measure, but particle size distribution, geometric mean particle size, median particle size, or other indices like fredle index may be used to describe the streambed's exposed fine sediment area.

Salmonids prefer mid sized substrates with interstitial cover to either fine sediment or boulders and bedrock. Ephemeroptera, Plecoptera, and Trichoptera ( important fish food organisms) also respond positively to gravel and cobble substrates ( Waters 1995). However, the percent coverage of fine sediments by area and the effects on salmonids and invertebrates have not been extensively investigated. Wessche ( 1985) found that the diversity of the available cover for bottom fauna appears to decrease as the mean particle size decreases. This study found that substrate dominated by rubble is the most productive. Substrate particle size is important because it affects permeability, which 22 ensures an adequate flow of water through the redd. Wessche also found that if the bottom materials contain <= 5 percent by volume of sands and silts that pass through the 0.833 mm sieve, the substrate is considered to have high permeability; when the materials contain >= 15 percent, its permeability is considered low.

Richards and Bacon ( 1994) in their longitudinal study of Bear Valley Creek, Idaho, found stream size influenced macroinvertebrate colonization of the streambed surface more than fine sediment accumulation. Surface fines may be most useful in trend analysis. Hill et al. ( 2000) found that percent fines (< 2 mm) negatively correlated with periphyton biomass in mid Atlantic streams. In a study of 562 streams in four northwestern states, Raylea et al. ( 2000) found that changes in invertebrate communities ( especially Ephemeroptera, Plecoptera, Trichoptera [ EPT]) occur as fine sediments (<=

2 mm) increase above 20 percent coverage by area.

In an analysis of data from 279 stream sites in Idaho, Mebane ( 2001) found that higher levels of surface sediment less than 6.0 mm negatively affected EPT taxa and salmonid and sculpin fish species.

Significant ( p < 0.05) inverse relationships between number of EPT taxa and percentage of fine sediment measured across both bankfull and instream channel widths were found. More age classes of salmonids and sculpins were significantly ( p < 0.05) associated with less instream fine sediments.

Multiple age classes of both salmonids and sculpins were uncommon where average instream surface fines were greater than 30 percent, and nearly absent above 40 percent. Zweig et al. ( 2001), in their work on four Missouri streams, determined that taxa richness significantly decreased in a linear manner with increasing deposited sediment in 3 of 4 streams ( over a range of 0 to 100 percent deposited sediments). Density, Ephemeroptera, Plecoptera, Trichoptera ( EPT) richness, and EPT density were significantly negatively correlated with deposited sediment across all four streams. Taxa richness and EPT/ Chironomidae richness were significantly negatively correlated in three streams.

Hale et al. ( 1985) investigated the preferred particle size for the Habitat Suitability Index Model for chum Salmon and assumed that a substrate composition of gravel 10 to 100 mm ( and < 10 percent fines) is excellent. As the amount of overlying sediment and the percentage of fines becomes too great, damage to the eggs and blockage of the emergence of the fry is possible. Hassler ( 1970) 23 showed that silt deposition of 1.0 mm per day was associated with 97 percent mortality or higher in northern pike. When silt covered eggs for a period greater than six days, mortalities were lower.

Hassler also found that after hatching food availability was a more important factor than temperature change or silt deposition on the survival of yolk sac larvae.

A relationship exists between channel morphology and the expected sediment composition in a well adjusted or dynamically equilibrated channel. Overton et al. ( 1995) summarized sediment monitoring in the Salmon River basin, Idaho, and found that natural conditions for surface sediment averaged 25 percent in A channels ( SD = 23), 23 percent in B channels ( SD = 21), and 34 percent in C channels SD = 25). Overall mean for all reaches equaled 26 percent with a standard deviation of 22. Mebane ( 2001) agreed with Overton et al. regarding natural surface sediment coverage.

Percent surface fines ( particles < 6 mm) were interpreted as indicating high, moderate, or low habitat conditions with respect to endangered species determinations in the Clearwater and Nez Perce National Forests and Cottonwood ( Idaho) area BLM lands ( USDA FS et al. 1998). High levels of habitat conditions were associated with surface fines <= 10 percent in A and B channels and <= 20 percent in C and E channels. At >= 21 percent in A and B channels or >= 31 percent in C and Echannels habitat conditions were considered low. Intermediate sediment coverages were considered moderate habitat conditions. Surface fine sediment levels have been recommended by the Forest Service and Bureau of Land Management in their draft Environmental Impact Statement for the Upper Columbia River Basin ( Interior Columbia Basin Ecosystem Management Project 1997). Their recommendations are stratified by channel type and watershed geology.

In chinook salmon and steelhead trout spawning areas of the South Fork Salmon River ( Idaho),

surface and subsurface fine sediment (< 4.75 mm) accumulations were monitored for a 20 year period ( Platts et al. ( 1989). The period began with a logging moratorium imposed because of detrimental logging activity, followed by streambed recovery, and then a resumption of limited logging activity.

In the worst condition ( 1966), surface sediments covered as much as 46 percent of the stream area.

By 1985, surface sediments averaged 19.7 percent of the spawning area and further recovery seemed possible. 24 NAHB ( 2000) found a notable absence of data regarding effects of suspended sediments on warmwater fish. They also found evidence that some warmwater fish may be able to spawn on muddy substrate.

Studies on the effects of surface sediment from construction activities are limited. However, one study by Reed ( 1997) did reveal that sediment from road construction in Northern Virginia reduced aquatic insect and fish communities by up to 85 percent and 40 percent, respectively.

Subsurface Sediment Surface fines and embeddedness are apparent to the human observer, and are thus relatively easy to measure, but subsurface or depth fines also have a major effect on the suitability of spawning habitats.

The amount of subsurface fine sediments as measured at the head of riffles in likely spawning areas can be an indication of redd site suitability, conditions for egg survival, and alevin emergence in the constructed redd, as well as habitat quality for fry and prey.

Information on the biological effects of subsurface sediment varies according to the size of sediment and geographic area of concern. Some of the variability is reduced by standardizing the habitat and stream types ( e. g., Rosgen [ 1994] level II) sampled.

Subsurface sediment targets can serve as a measure of suitability for fish spawning grounds, and they are most applicable in riffles and spawning areas in streams with gravel/ cobble/ boulder streambeds.

If there are excessive subsurface fines they can have detrimental effects on salmonid and invertebrate habitat suitability and redd conditions. In the western U. S. redd construction is often upstream from riffles or at the tail end of pools where there is a net flow of stream water downward into the substrate. Where upwelling groundwater rather than surface irrigates the substrate, the fines are no longer in the position to block the flow of water into the redd, and therefore are a less important threat ( Waters, 1995).

The target for subsurface sediments is supported by studies of salmonid embryo survival rates in redds 25 with varying fine sediment composition. Sediment particle size must be suitable to allow fry to migrate to the sediment surface. NAHB ( 2000) found that in the cases they reviewed, the fine sediment fraction has the greatest impact on fry emergence by " cementing" spaces between gravels and boulders. McMahon ( 1983) reported that in the Habitat Suitability Index report model for coho salmon, survival to emergence of coho salmon fry was high at <= 5 percent fines, but dropped sharply at >= 15 percent fines. Kioski ( 1966) in Raleigh ( 1986) reported that pre emergent salmonid eggs were entombed ( could not emerge) with >= 15 percent fines on the stream substrate.

A comparison of ambient streambed subsurface fines to substrate composition in adjacent redds was made by Kondolf ( 2000), who found that redds typically had one third less fine sediment than the adjacent streambed throughout the incubation period. This is partly caused by the fact that spawning involves some gravel cleaning actions. However, redd sediment compositions can still be used to detect trends or ranks of condition ( not numerically absolute conditions).

Other studies on sediment and salmonid survival abound. Hall ( 1986) found survival ( eyed egg to emergence) of coho, chinook, and chum salmon to be only 7 10 percent in gravel mixtures made up of 10 percent fines < 0.85 mm as compared to 50 75 percent survival in gravel mixtures with no fines < 0.85 mm. Reiser and White ( 1988) observed little survival of steelhead and chinook salmon eggs beyond 10 20 percent fines < 0.84 mm. In a laboratory study, fry survival declined significantly when fines < 0.25 mm in diameter approached 5 percent of the substrate in the egg pocket of artificial trout redds ( Bjornn et al., 1998). In the Kootenai National Forest ( MT), numbers of bull trout redds were compared to percent subsurface fines ( Wegner 1998, 2003a).

The numbers of redds were apparently negatively related to percent subsurface fines in spawning areas, though the comparisons were not statistically rigorous and another report showed ambiguous response to slight changes ( Wegner, 2003b). Raleigh et al. ( 1986) assumed that optimal spawning gravel conditions for brown trout are less than or equal to 5 percent fines, whereas the proportion of fines greater than 30 percent is assumed to result in low survival of embryos and emerging fry.

According to Lisle and Eads ( 1991), the " threshold of concern" for fine sediment content varies among experiments, species and grain size, but most commonly falls around 20 percent. As an 26 indication of the cause of this relationship, dissolved oxygen in the redds was inversely related to the percentage of fines under 0.85 mm.

In a study of Yellowstone cutthroat trout, Thurow and King ( 1994) described redd siting and substrate characteristics, and tested the effect of habitat conditions on the completed redds in Pine Creek, Idaho. They found that the spawned sites contained particles up to 100 mm, though most were less than 32 mm, 20 percent were less than 6.35 mm, and 5 percent were less than 0.85 mm.

Results from Nelson et al., ( 2002b) showed that in important spawning areas of the Payette and Boise National Forests, smaller fines (< 0.85 mm) consistently represented less than 10 percent of the core samples. With the exception of one site that had been severely degraded by historic mining activities,

the percentage of smaller fines averaged approximately 5 percent over a 25 year monitoring period.

However, in these regions of restricted logging, the percentages of larger fines (< 6.3 mm) from the same sample locations were routinely found to be near 30 percent. While these are not pristine watersheds, they have been managed for sediment reduction since the 1960s ( with a 20 year logging moratorium followed by limited logging).

Upon testing a fisheries sediment response model in the Clearwater River drainage, Nelson and Platts ( 1988) recommended that three tiers of subsurface sediment conditions be delineated. At < 20 percent subsurface fines (< 6.3 mm), the conditions are considered good for embryo incubation and survival.

From 20  to 27 percent, conditions are marginal and influences of other environmental factors cause variable survivability. Above 27 percent, subsurface fines, survivability was considered improbable.

NAHB ( 2000) cites NCASI ( 1984) laboratory studies of coarse sediment deposition indicating that coarser sediment that blocks smaller fines from entering the egg pocked while allowing sufficient water flow can help egg to survive. ( Nearly 90 percent salmonid eggs survived with 20 percent fines when the fines were defines as less than 6.4 mm   considered larger fines.)

Federal land management agencies ( Forest Service and BLM) have developed guidelines specific to their local conditions. Evaluation of the effects of subsurface sediment on habitat conditions on Clearwater and Nez Perce National Forests and Cottonwood ( Idaho) area BLM lands 27 showed high levels of habitat conditions associated with < 20 percent fines (<= 6 mm) at depth, while at > 25 percent fines, habitat conditions were considered low ( USDA FS et al., 1998).

On the Salmon Challis National Forest, the Forest Plan for the Challis Zone sets a threshold of 30 percent fines < 6.3 mm such that activities which would result in the exceedance of the threshold are not allowed ( Challis National Forest, 1987). The Forest Plan for the Salmon Zone has standards of 20 percent fines by depth for streams supporting anadromous fish and 28.7 percent fines by depth for streams supporting only resident salmonid populations ( Salmon National Forest, 1987). Recent thinking on the Salmon and Challis National Forest bases subsurface sediment standards on watershed geology ( Betsy Rieffenberger, Salmon and Challis National Forest, personal communication). In quartzite drainages, the Forest classifies streams in good condition as having subsurface sediment <

20 percent, streams in fair condition have 20 25 percent fines, and streams in poor condition will have over 25 percent fines. In granitic, volcanic, and sedimentary drainages, streams in good, fair,

and poor condition will have < 25 percent, 25  30 percent, and > 30 percent fines, respectively.

Studies documenting effects of fine sediment on macroinvertebrates are limited. A field study of benthic invertebrate colonization of trays with varying percentages of fine sediments showed significant ( though weak) responses to increases in sediment from 0 to 30 percent ( Angradi, 1999).

Riffle Stability The Riffle Stability Index ( RSI) indicates the relative percentage of the streambed that is mobile during channel forming flows. Bed mobility is related to pool quality and abundance. With lower RSI values, there is overall greater residual pool volume, because less of the streambed is susceptible to moving. Pool habitat provides critical refuge for juvenile and adult salmonids.

The RSI has been used as an indicator of beneficial use, especially as related to cold water biota. The RSI is measured as the percentage of the substrate particles ( from a Wolman pebble count) that are smaller than the largest particles that are moved in channel forming flows. Particles on point bars are measured to determine the largest mobile particles. 28 The substrate mobility expressed by RSI may be related to the density and species composition of stream insects ( Kappesser, 1993). Cobb, Galloway, and Flannagan ( 1992) reported a decrease in insect density up to 94 percent in an unstable riffle compared to no reduction in a stable riffle. In Colorado, von Guerard ( 1991) concluded that as the grain size of streambed material approaches that of bedload, benthic invertebrate populations might be adversely affected. Kappesser ( 1993) looked at RSIs from B channel streams in northern Idaho. He reported an RSI range from 29 riffles in unentered ( e. g., relatively undisturbed) watersheds of 33 to 74 ( mean 50.8) while RSIs from 286 riffles in entered watersheds ranged from 38 to 100 ( mean 79.5). In a survey of B channel streams of the St. Joe River drainage ( Idaho), bull trout redds were consistently found in reaches with RSI values less than 65 and were missing from reaches with higher RSI values ( Cross and Everest, 1992).

Pools are critical habitat for salmonids ( Spangler, 1997; Saffel, 1994; Stichert et al., 2001; Harwood et al., 2002; Kruzic et al., 2001; Jakober et al., 2000; Solazzi et al., 2000). As riffle stability degrades, pool habitat decreases, reducing daytime and winter refugia. Destabilized stream reaches may contain lengthened riffles and shallow pools ( Lisle, 1982). In the St. Joe River drainage ( Idaho),

reaches with lower RSI values had greater residual pool volume ( Cross and Everest, 1992).

Riffle stability may be a factor effecting redd scour if bankfull flows occur during the incubation period. The likelihood of mortality from scour increases for stocks of fish incubating during seasons when peak flows commonly occur ( Seegrist and Gard, 1972). To avoid scouring flows that would disturb deposited eggs, salmonids either bury their eggs below the annual scour depth or avoid egg burial during times of likely bed mobility. Such protective patterns were noted in west slope Pacific Northwest watersheds ( Montgomery et al., 1999), and are likely to be prevalent throughout Idaho.

Intergravel Dissolved Oxygen One effect of the accumulation of fine sediment in the aquatic environment is reduced permeability of the substrate resulting in less oxygen exchange to support fish embryos and macroinvertebrates. Salmonids excavate streambed substrate to deposit eggs then backfill the " egg 29 pocket" to protect the eggs during the incubation period. The eggs are dependent on the flow of oxygen rich water through the substrate to survive. The accumulation of fines in the redd restricts water flow and reduces oxygen to the eggs which results in decreasing survival ( Shapovalov and Berrian, 1939; Wickett, 1954; Shelton and Pollock, 1966). Intergravel dissolved oxygen is more of a concern in areas outside the Idaho batholith. Fines in the batholith are mostly in the sand to fine gravel range and permeability associated with these textures is not restrictive to the transport of dissolved oxygen ( Burton et al., 1990).

Dissolved oxygen ( DO) in intergravel flow is a more direct measure of streambed suitability for salmonid egg development than subsurface sediments. In the substrate, an increase in deposited fines restricts the flow of well oxygenated water into the substrate to replace the DO consumed by normal biodegradation. Intergravel flow may be more or less dependent on ambient streambed sediment conditions, depending on local hyporheic conditions. If water flows into the redd from the overlying water column then there is the chance of the flow being choked by the intrusion of fine sediments in the bedload. If, however, redds are located in areas of hyporheic discharge, then the surface sediment conditions and delivery during incubation may be less important because the oxygenated water source is from below the redd. Fall chinook salmon and bull trout select spawning sites based at least in part on influences of hyporheic flow ( Spangler, 1997, Geist, 1998). Bull trout embryo survival was found to be significantly higher and less variable in areas with groundwater discharge and higher water temperatures over the incubation period ( Baxter and McPhail, 1999).

Several studies have related intergravel dissolved oxygen to egg/ fry survival. Survival of embryos has been positively correlated with intergravel dissolved oxygen in the redds for steelhead ( Coble, 1961) and brown trout ( Maret et al., 2003). Silver et al. ( 1963) found that embryos incubated at low and intermediate DO concentrations produced smaller and weaker alevins than embryos incubated at higher concentrations. Weak sac fry cannot be expected to survive rigorous natural conditions. In a review of embryo development studies, Chapman ( 1988) noted several examples of developmental impairment at lower DO concentrations, but did not recommend a single threshold. Bjornn and Reiser ( 1991) recommended that intergravel DO concentrations should be at or near saturation, and that temporary reductions should drop to no lower that 5.0 mg/ L. 30 Observations of the effects of intergravel flow on macroinvertebrates are much less extensive than those for fish. Excessive sediment affects macroinvertebrates by accumulating on the body surfaces and reducing the effective area of the respiratory structures ( Lemly, 1982) or by covering pupae cases and reducing the flow of oxygenated water to the metamorphosing insect ( Rutherford and Mackay,

1986).

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