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flood plain

or flood·plain

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noun
  1. a nearly flat plain along the course of a stream or river that is naturally subject to flooding.
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Origin of flood plain

First recorded in 1870–75
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for floodplain

flood plain

noun
  1. the flat area bordering a river, composed of sediment deposited during flooding
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Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for floodplain

n.

also flood plain, 1873, from flood (n.) + plain (n.).

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Online Etymology Dictionary, © 2010 Douglas Harper
floodplain in Science

floodplain

[flŭd′plān′]
  1. Flat land bordering a river and made up of alluvium (sand, silt, and clay) deposited during floods. When a river overflows, the floodplain is covered with water.
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The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.