Tributary

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The thick blue line is the river. The thin blue line is its tributary.

A tributary of a river is another river that flows into it. If one river flows into a second river, then the first river is a tributary of the second river.

For example, in the United States the Missouri River flows into the Mississippi River. That means the Missouri River is a tributary of the Mississippi River.

A river and all its tributaries form the catchment of that river.

A watershed separates the catchments of separate rivers.

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