a nearly flat plain along the course of a stream or river that is naturally subject to flooding.
the flat area bordering a river, composed of sediment deposited during flooding
Collins English Dictionary - Complete & Unabridged 10th Edition
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If a river floods repeatedly, over time this sediment will build up into a flood plain.
Each year, the river overflowed, spreading rich sediment across its broad flood plain.
If the water spreads out over a flood plain, it is much more shallow than it would be if still contained in the river.
Historically, such settlements have clung to the higher ground on the flood plain.
Permissible uses within flood plain but outside floodway.
Amends various provisions relating to flood plain management.