define alluvial plain
alluvial plain. (geology). A plain formed from the deposition of alluvium usually adjacent to a river that periodically overflows. Also known as aggraded valley plain; river plain; wash plain; waste plain. 1  Facebook.
Noun. 1. alluvial plain-a flat resulting from repeated deposits of alluvial material by running water. alluvial flat. flat-a level tract of land; the salt flats of Utah.