A tributary is a freshwater stream that feeds into a larger stream or river. The larger, or parent, river is called the mainstem. The point where a tributary meets the mainstem is called the confluence. Tributaries, also called affluents, do not flow directly into the ocean.

Most large rivers are formed from many tributaries. Each tributary drains a different watershed, carrying runoff and snowmelt from that area. Each tributary's watershed makes up the larger watershed of the mainstem.

The Missouri River's massive watershed, for example, is created by the watersheds of dozens of tributaries extending from the provinces of Alberta and Saskatchewan, Canada, through seven states in the Upper Midwest of the United States. The Missouri, in turn, is the largest tributary of the Mississippi River, which it meets at a confluence in St. Louis, Missouri, U.S. The Mississippi River watershed is the fourth-largest in the world.

A "left-bank tributary" or "right-bank tributary" indicates the side of the river a tributary enters. When identifying a left-bank or right-bank tributary, a geographer looks downstream (the direction the river is flowing).

The Euphrates River, the longest river in southwestern Asia, stretches 2,700 kilometers (1,678 miles). The tiny streams that feed the Euphrates originate in the mountains of eastern Turkey. These streams become the Balikh and the Sajur Rivers, which join the Euphrates at different confluences in Syria. The Balikh is a left-bank tributary of the Euphrates, while the Sajur is a right-bank tributary.

Sometimes, tributaries have the same name as the river into which they drain. These tributaries are called forks. Different forks are usually identified by the direction in which they flow into the mainstem.

The Shenandoah River, for example, flows through the U.S. states of West Virginia and Virginia. It has two long tributaries, the North Fork and South Fork, which meet at a confluence in Harpers Ferry, West Virginia.

The opposite of a tributary is a distributary. A distributary is a stream that branches off and flows apart from the mainstem of a stream or river. The process is called river bifurcation.

At the Continental Divide in the U.S. state of Wyoming, the small North Two Ocean Creek bifurcates into Pacific Creek and Atlantic Creek. The water from each of these distributaries flows into the ocean for which it is named.

Classifying Tributaries

There are two leading methods geographers and potamologists (people who study rivers) use to classify tributaries.

The first method lists a river's tributaries starting with those closest to the source, or headwaters, of the river. The earliest tributaries of the Rhine River, for example, include the Thur River of Switzerland and the Ill River of Austria. The Rhine, one of the longest rivers in Europe, has its source in the Alps and its mouth in the North Sea.

The second method lists a river's tributaries by their flow. Small streams are identified with low numbers, while larger tributaries have higher numbers. The Tshuapa and Kasai Rivers are both left-bank tributaries of the Congo River, the deepest river in the world. The Tshuapa is a smaller river, and has a lower tributary ranking, than the Kasai. 

People and Tributaries

Human activity in tributaries is often responsible for polluting the mainstem. The river carries all the runoff and pollution from all its tributaries.

Rivers with tributaries that drain land that is not used for agriculture or development are usually less polluted than rivers with tributaries in agricultural or urban areas.

Development, not size, determines the pollution of rivers. The Amazon River, with the largest drainage basin in the world, is much cleaner than the Hudson River, for instance. Tributaries to the Amazon flow through undeveloped regions of the Andes Mountains and rainforests in Brazil, Venezuela, Colombia, Peru, and Bolivia. The Hudson River flows through one of the largest urban areas on Earth, the U.S. city of New York, New York.