A stream or river that flows into a larger river, rather than directly to the sea, is called a tributary. Some large rivers have hundreds of tributaries!
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 snow melt from that area. Each tributary’s watershed makes up the larger watershed of the mainstem.
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 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.
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