Tributary

A tributary is a stream or river flowing into a larger river. The water being drained from two areas now flows in one main channel. A river grows as more tributaries flow into it.

There is no special size that a tributary has to be. Some of the World's longest rivers are joined by tributaries that themselves are major rivers. One example is the Missouri River, a major tributary of the Mississippi River.

A river in Costa Rica. The yellow coloured river is the tributary.