drainage basin - the entire geographical area drained by a river and its tributaries; an area characterized by all runoff being conveyed to the same outlet; flood control in the Missouri basin. catchment area, catchment basin, drainage area, river basin, watershed, basin. detention basin - a storage site (such as a small reservoir) that delays the flow of water downstream. retention basin - a storage site similar to a detention basin but the water in storage is permanently obstructed from flowing downstream.