A stream is a body of water that flows on Earth’s surface. The word stream is often used interchangeably with river, though rivers usually describe larger streams.

Streams provide many benefits to humans. Besides providing drinking water and irrigation for crops, streams wash away waste and can provide electricity through hydropower. People often use streams recreationally for activities such as swimming, fishing, and boating. Streams also provide important habitat for wildlife.

Streams need two things to exist: gravity and water. When precipitation falls onto the ground, some water trickles into groundwater, but much of it flows downhill across the surface as runoff and collects into streams. A watershed, or drainage basin, is the area that collects water for a stream. As smaller streams flow downhill, they often merge together to form larger streams. These smaller streams are called tributaries.

Streams create channels by wearing down rock and carrying it and other sediment downstream. This process is called erosion. The forces that make backyard gullies are the same as those that carved out the Grand Canyon.

Depending on the landscape they flow through, streams have different shapes. Meandering streams have one channel that snakes across the landscape. Over time, these curves can become so wide that they meet and cut off the bend from the rest of the stream, creating oxbow lakes. Braided rivers have many channels that separate and merge like a braid of hair. Planetary scientists have found formations that look like stream channels on the surface of Mars, suggesting water flowed there in streams at one point in the planet’s history.

Streams provide an important ecosystem for many plants and animals, including fish, birds, salamanders, snails, and mammals. Different plants and animals have evolved to thrive in streams. Plants often have strong root systems that act as anchors, and long, flexible stems that can move with the current. Fly larvae nibble on leaves that have fallen into the water, and the flies themselves later become fish food. Mussels bury into sediment on the stream’s floor, to stay in place as they filter the water for food. Many fish have streamlined shapes that help them stay in position and keep them from being washed away by strong currents.

Many factors can harm a stream’s health. Dams stop the natural flow of sediment and water, and prevent wildlife, nutrients, and water from flowing freely through the stream system. Urban areas sometimes flush untreated sewage into streams, causing algal blooms that suffocate wildlife. Pollutants can wash into streams from farms and factories, and harm wildlife and humans who depend on the water. 

In response to these threats, scientists and conservationists are taking action. Hydrologists study streams and how water quality and flow change over time. Conservationists can work with communities to revive stream basins. Osvel Hinojosa Huerta is a National Geographic Explorer who worked to improve the Colorado River ecosystem in the United States and Mexico.  Sandra Postel, a past National Geographic Freshwater Fellow, created a program that restored billions of gallons of water to river systems in North America.

 

Stream

This stream in Crested Butte, Colorado, United States is an important water source for many humans and animals. 

braided river
Noun

flowing body of water separated into channels by tiny islands.

conservationist
Noun

person who works to preserve natural habitats.

dam
Noun

structure built across a river or other waterway to control the flow of water.

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.

Noun

act in which earth is worn away, often by water, wind, or ice.

evolve
Verb

to develop new characteristics based on adaptation and natural selection.

gravity
Noun

physical force by which objects attract, or pull toward, each other.

Noun

water found in an aquifer.

Noun

environment where an organism lives throughout the year or for shorter periods of time.

hydrologist
Noun

person who studies the distribution, circulation, and properties of water.

hydropower
Noun

energy generated by moving water converted to electricity. Also known as hydroelectricity or hydroelectric power.

Noun

watering land, usually for agriculture, by artificial means.

larva
Noun

a new or immature insect or other type of invertebrate.

meandering stream
Noun

flowing body of water that curves through land.

Noun

lake formed from an abandoned bend in a river.

Noun

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

Noun

large stream of flowing fresh water.

Noun

overflow of fluid from a farm or industrial factory.

Noun

body of flowing water.

Noun

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

Noun

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