a low plain adjacent to a river that is formed chiefly of river sediment and is subject to flooding
"The ancient floodplain, built up over centuries by silt deposited during seasonal floods, now supports lush wetlands along the winding river."
An alluvial plain that may or may not experience occasional or periodic flooding.
"The farmer planted corn on the low-lying floodplain, knowing it might get submerged after heavy rains but usually recovering quickly once the water receded."