1. river 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, drainage 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.atchment area, catchment basin, drainage area, drainage 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.