A catchment.
an area drained by a river is called pooo lol
An area drained by a river or river system is called a drainage basin.
watershed
watershed
a watershed, also called a drainage basin, is an area drained by a river.
The area of land drained by a river is called a "catchment area". It is so-called because is the the area that catches the precipitation (rainfall).Another term is "river basin", but this is less specific.(The geographical term, "catchment area", is often used to describe the area served by, say, a school or hospital).
I believe you might be referring to a "watershed".
polder
Land drained by a river is a watershed. This is an area of land that feeds all the water running under it and draining off of it into a body of water.
It is called a drainage basin or watershed. This area is defined by the topographical boundaries that channel water towards a common outlet.
Basin
This is its catchment - surrounded by its watersheds.