backswamp

(geology) Swampy depressed area of a floodplain between the natural levees and the edge of the floodplain.

On a flood plain, a marshy area where floodwater may be confined between the river levées and higher ground. When artificially drained, backswamp areas yield fine-grained soils, often rich in organic matter. It is a little lower than the neighbouring alluvial fans which extend from the valley sides, and is a site of slow accumulation of silts and clays.