Tributary

A tributary, or an affluent, is a stream that flows into a larger stream (main stem or "parent") or lake. A tributary does not flow directly into a sea or ocean. Tributaries and the main stem collect water from the surrounding drainage basin and groundwater.

A confluence, where two or more bodies of water meet, usually means tributaries are joining together. The opposite of a tributary is a distributary, a river or stream that flows away from the main stream. Distributaries are often found in river deltas.

The Irtysh, a tributary of the Ob river, is the longest tributary river in the world. The Madeira River is the largest tributary river by the amount of water it carries.

Terminology

A right tributary or right-bank tributary and a left tributary or left-bank tributary tell us where a smaller stream joins a bigger one. These words are used based on the way the big stream is moving. If you were floating down the big stream, the smaller one would join from your right or left side.

In the United States, tributaries that share the same name as the river they join are called forks. They are often named using compass directions. For example, the American River in California has North, Middle, and South forks. These naming rules help people talk about and understand the many streams and rivers.

Ordering and enumeration

Tributaries can be listed starting from the ones closest to where the river begins and moving to those closest to where it ends. One way to organize them is called the Strahler stream order. It groups tributaries into levels, like first, second, third, and so on, with the smallest streams being first-order.

Another way is to list tributaries from where the river ends back to where it starts, using a tree-like structure to show how they connect.