Obszar przylegający do rzeki głównej nieob-jęty zlewniami cząstkowymi tworzy tzw. przyrzecze. Inaczej rzecz ujmując, jest to obszar, z którego woda odprowadzana jest bezpośrednio do rzeki głównej.

http://imig.pl/pliki/artykuly/2014-1/2014-1_003-010_Kolerski...

Watersheds

New York's waters (lakes, rivers and streams) fall within one of 17 major watersheds, or drainage basins. A watershed is an area of land that drains water into a specific body of water. Watersheds include networks of rivers, streams, and lakes and the land area surrounding them. Watersheds are separated by high elevation geographic features (mountains, hills, ridges).

https://www.dec.ny.gov/lands/26561.html

watershed

Watershed is a geographical term, originally. The area that drains into a single river is the watershed for that river. Watershed can also mean a ridge, like that formed by a chain of mountains, which sends water to two different rivers on either side.
(...)
the entire geographical area drained by a river and its tributaries; an area characterized by all runoff being conveyed to the same outlet
Synonyms:
basin, catchment area, catchment basin, drainage area, drainage basin, river basin

https://www.vocabulary.com/dictionary/watershed