A watershed is an entire river system—an area drained by a river and its tributaries. It is sometimes called a drainage basin.

Watersheds can cover wide areas. Runoff water from a large watershed in the midcontinental United States drains into the Gulf of Mexico through the Mississippi River system. The Amazon River watershed is huge, draining over a third of the entire South American continent.

Most fresh water in the world flows through watersheds that eventually drain into the ocean. However, sometimes a watershed will not drain into the ocean, but into an internal body of water. Water can only leave these bodies of water, called endorheic basins, by evaporating or seeping through the soil. The Aral Sea, bordering Uzbekistan and Kazakhstan, drains one such endorheic basin in Central Asia.

Some watersheds are sharply defined by the crest of a high ridge, or by a continental divide. When used in this sense, the term “watershed” does not refer to the drainage basin of one river system, but rather to the divide between two or more drainage basins. Precipitation that falls on opposite sides of this type of watershed flows in different directions. Watersheds in low or gently rolling areas may be poorly defined, but can be identified by the flow of the rivers.

The Continental Divide of the Americas is a prominent watershed in North America. The Continental Divide roughly follows the crest of the Rocky Mountain range. Rain, snow and other precipitation falling on the west side of this divide flows into the Pacific Ocean. Precipitation falling on the east side flows into the Atlantic and Arctic Oceans.

One of the reasons watersheds are important to scientists is that they affect the quality and amount of flow through a stream or river at a given point. For example, as the Mississippi River empties into the Gulf of Mexico, it is carrying water from its entire watershed, the second-largest in the world. It includes about 40 percent of the area of the continental United States and provides water for millions of people.

Extensive agricultural development throughout the Mississippi River watershed has led to problems with its water quality. Phosphorus and nitrogen from fertilizers feed bacteria and algae along the Gulf Coast. The resulting algal blooms deplete oxygen dissolved in the water, preventing marine life from flourishing there. So-called “dead zones” like this threaten ecosystems and fishing industries wherever they occur. The United Nations estimates that there are about 150 dead zones in the world as a result of agricultural development.