Tributary

What is a tributary? A tributary is a branch of a river (not its main channel, which is called the trunk).

Diagram of a drainage basin showing the main channel. All of the other channels are tributaries. Only one has been marked.

A river is made up of a main channel and many branches, or tributaries that come in to it. Any river or stream that comes into the main channel is a tributary, even if it is very large. For example the Missouri River is bigger than many of the world's rivers, but is still only a tributary of the even bigger Mississippi.

Where a tributary meets a main river, the place is called a confluence. Many towns and cities have been formed at confluences, especially if the main channel and tributary are navigable because the spot is good for exchanging goods and making a market.

The only time rivers are not tributaries is when two headwater rivers meet to make the main channel. Here you can see the Allegheny River from the northeast and Monongahela River from the southeast forming the Ohio River, at Pittsburgh, USA. Pittsburgh was founded as a British fort in colonial times because it allowed troops to go in three directions by river. The Ohio is the biggest tributary of the Mississippi River.

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