A flood plain is a low, flat stretch of land adjacent to a body of water that is flooded when water levels are high. The rivers in the Midwestern United States are referred to as being in the 100-year floodplain because they have a history of regular flooding.
Alluvial land is types of land formed by the regular flooding from bodies of water where rainfall, storm surges and melting snow and ice regularly raise the water level. A combination of tidal action and river sedimentation work together to form coastal floodplains.
River floodplains are alluvial land formations that often contain rich, fertile soil. River valleys like the land along the banks of Egypt's Nile River have been farmed for thousands of years because the flooding action of the river keeps supplying the land with rich sediments.
In the United States, the rivers often have levees and dykes to manage the damage from periodic flooding. The Tennessee River and the Mississippi River have dams and floodways to divert the surging flood waters. Natural floodplains connect with swampy areas which hold the excess water like a sponge and release it slowly into the surrounding earth.
According to Michael E. Ritter in "The Physical Environment: an Introduction to Physical Geography," when flood water flows onto the floodplain, the heaviest sediments drop out first with the waters carrying finer silts farther away from the river to cause a back swamp. These back swamps help protect downstream areas from damaging flooding.
Lakes are formed by the action of moving glaciers. The ice digs into the ground as it advances and scoops out large basins. Other lakes are formed in the calderas of volcanoes or by meandering rivers. Lakes like the Great Lakes in North America are subject to seasonal variations in their water level, which can cause flooding on low-lying shorelines.
Storms on large lakes can raise heavy waves and flooding on the shoreline. The storm surge floods the low-lying shoreline with water and deposits sediment from the lake bottom.
Coastal Flood Plains
Areas along the Atlantic coastline are a large type of floodplain. For thousands of years, the Atlantic waves have crashed upon the eastern continental shelf. According to the National Oceanic and Atmospheric Administration, the shelf slows the water down, allowing it to deposit the sediment it carries along the shoreline.
At the same time, river deltas form as rivers enter the sea and sediment from the river water builds up, adding to the floodplain. The combination of sedimentation from both sources builds up the coastal floodplain. This type of flat, low-lying land form is called a submergent/depositional coastline.
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