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 into which they flow drain the surrounding drainage basin of its surface water and groundwater, leading the water out into an ocean, another stream, or into an endorheic basin.

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

The Irtysh, a tributary of the Ob river, is the longest tributary river in the world with a length of 4,248 km (2,640 mi). The Madeira River is the largest tributary river by volume in the world with an average discharge of 31,200 m³/s (1.1 million cu ft/s).

Terminology

A right tributary or right-bank tributary and a left tributary or left-bank tributary describe where a smaller stream flows into a larger one. These terms are used based on the direction the larger stream is flowing. If you were floating down the larger stream, the tributary would join from either your right or left side.

In the United States, tributaries that have the same name as the river they flow into are called forks. These are often named using compass directions. For example, the American River in California has North, Middle, and South forks. These naming rules help people understand and describe the different streams and rivers they encounter.

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.