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 freshwater 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.

watershed
There are many governmental and nonprofit groups organized to protect the health and safety of local watersheds.

Dead Watershed
At 420 meters (1,378 feet) below sea level, the Dead Sea is the lowest point on Earth. It serves as a watershed for much of the Jordan Rift Valley. Because it is an endorheic basin, water that flows into it evaporates, leaving behind any salt it contains. The Dead Sea is one of the saltiest bodies of water on Earth, with water that is 10 times as salty as seawater.

What's Your Watershed?
Find out here.

agricultural development
Noun

modern farming methods that include mechanical, chemical, engineering and technological methods. Also called industrial agriculture.

algae
Plural Noun

(singular: alga) diverse group of aquatic organisms, the largest of which are seaweeds.

algal bloom
Noun

the rapid increase of algae in an aquatic environment.

Plural Noun

(singular: bacterium) single-celled organisms found in every ecosystem on Earth.

Noun

one of the seven main land masses on Earth.

Noun

point or area that separates which directions a continent's river systems flow.

Noun

area of low oxygen in a body of water.

deplete
Verb

to use up.

dissolve
Verb

to break up or disintegrate.

drainage basin
Noun

an entire river system or an area drained by a river and its tributaries. Also called a watershed.

Noun

community and interactions of living and nonliving things in an area.

endorheic basin
Noun

watershed that empties into an internal body of water, not the ocean.

evaporate
Verb

to change from a liquid to a gas or vapor.

extensive
Adjective

very large.

fertilizer
Noun

nutrient-rich chemical substance (natural or manmade) applied to soil to encourage plant growth.

flourish
Verb

to thrive or be successful.

freshwater
Noun

water that is not salty.

Gulf Coast
Noun

land in the United States surrounding the Gulf of Mexico.

marine
Adjective

having to do with the ocean.

nitrogen
Noun

chemical element with the symbol N, whose gas form is 78% of the Earth's atmosphere.

Noun

large body of salt water that covers most of the Earth.

oxygen
Noun

chemical element with the symbol O, whose gas form is 21% of the Earth's atmosphere.

phosphorus
Noun

chemical element with the symbol P.

Noun

all forms in which water falls to Earth from the atmosphere.

prevent
Verb

to keep something from happening.

ridge
Noun

long, narrow elevation of earth.

Noun

large stream of flowing fresh water.

river system
Noun

tributaries, mouth, source, delta, and flood plain of a river.

Noun

overflow of fluid from a farm or industrial factory.

seep
Verb

to slowly flow through a border.

soil
Noun

top layer of the Earth's surface where plants can grow.

Noun

body of flowing water.

Noun

stream that feeds, or flows, into a larger stream.

United Nations
Noun

international organization that works for peace, security and cooperation.

Noun

entire river system or an area drained by a river and its tributaries.