It is an area of land adjacent to a stream or river that stretches from the banks of its channel to the base of the enclosing valley walls and experiences flooding during periods of high discharge.
Floodplains. The river now has a wide floodplain. A floodplain is the area around a river that is covered in times of flood. A floodplain is a very fertile area due to the rich alluvium [alluvium: The sediment deposited by rivers. ] deposited by floodwaters. This makes floodplains a good place for agriculture.  The 100-year floodplain is the land adjacent to a river, lake or stream that will be inundated by water during a flood which has a 1% chance of occurring of being exceeded in any given year. Every river, lake or stream has a floodplain associated with it. Floodways are the channel of a river or stream and those portions of the floodplain adjoining the channel which are reasonably required to carry and discharge the 100 year flood; these are areas of moving water during times of flood. New residential construction is specifically prohibited in the floodway. Noun. 1. floodplain-a low plain adjacent to a river that is formed chiefly of river sediment and is subject to flooding. flood plain. champaign, plain, field-extensive tract of level open land; they emerged from the woods onto a vast open plain; he longed for the fields of his youth.  Aggradation and plantation: The Laramie River meanders across its floodplain in Albany County, Wyoming, 1949. Aggradational floodplain: This floodplain of a small meandering stream in La Plata County, Colorado, is underlain by silt deposited above a dam formed by a terminal moraine left by the Wisconsin Glacier. Riparian vegetation on the floodplain of the Lynches River close to Johnsonville, South Carolina. Flood plains are made by a meander eroding sideways as it travels downstream. When a river breaks its banks and floods, it leaves behind layers of alluvium (silt). These gradually build up to create the floor of the flood plain. A floodplain or flood plain is an area of land adjacent to a stream or river that stretches from the banks of its channel to the base of the enclosing valley walls and experiences flooding during periods of high discharge. Flood plains are made by a meander eroding sideways as it travels downstream. When a river breaks its banks and floods, it leaves behind layers of alluvium (silt). These gradually build up to create the floor of the flood plain. The standard flood for floodplain management is the 1% chance (100-year) flood. It is possible to go a very long period of time without experiencing the 1% chance flood. It is also possible to have 1% chance floods two years in a row. Floodways are the channel of a river or stream and those portions of the floodplain adjoining the channel which are reasonably required to carry and discharge the 100 year flood; these are areas of moving water during times of flood. New residential construction is specifically prohibited in the floodway. Under state law, the floodplain is considered to be the land adjoining lakes and rivers that is covered by the 100-year or regional flood. This flood is considered to be a flood that has a 1 percent chance of occurring in any given year. If a person is buying or already owns property on a river shoreline in a community that has adopted floodplain zoning, they should consider the following points: floodway location, flood fringe location, flood protection elevation, flood proofing, and flood insurance. Floodplain Management. Main Content. This is the homepage for FEMA’s Floodplain Management Branch. It contains information on how to join the National Flood Insurance Program, participating in the Community Rating System and other topics related to the adoption and enforcement of floodplain management standards. FEMA’s Floodplain Management Branch provides tools and resources to help communities navigate NFIP requirements and implement higher standards of floodplain management. State and federal agencies, local communities and property owners have a role in reducing flood risk and helping communities become more resilient. Go to freeflood.net and enter your address in the search box. This site will tell you if your property lies within a 100- to 500-year floodplain and whether it is likely to be damaged by a flood. The site cautions that this information is advisory in nature only, and should not be used to determine flood hazard determination for insurance or safety purposes. 
http://www.bbc.co.uk/schools/gcsebitesize/geography/water_rivers/river_landforms_rev3.shtml http://www.michigan.gov/deq/0,4561,7-135-3313_3684_3725-11281--,00.html http://www.thefreedictionary.com/floodplain https://en.wikipedia.org/wiki/Floodplain https://en.wikipedia.org/wiki/Floodplain http://www.michigan.gov/deq/0,4561,7-135-3313_3684_3725-11281--,00.html http://www.dnr.state.mn.us/waters/watermgmt_section/floodplain/faqs_answers.html http://www.fema.gov/floodplain-management http://homeguides.sfgate.com/out-property-flood-plain-6604.html
where is a floodplain