watershed, elevation or divide separating the catchment area, or drainage basin, of one river system or group of river systems from another system or group of systems. The term is also often used synonymously with drainage basin. The continental divide in the US along the Rocky Mts. forms the watershed division between westward-flowing and eastward-flowing streams; the Mississippi watershed extends from the Rockies to the Appalachians. Often the watershed is a range of mountains, the two sides of which have widely different climatic characteristics, as in the Americas and Australia. The low Valdai Hills are an important watershed of Russia.