A tributary is a smaller river or stream that flows into a larger river. Think of it like branches on a tree: the main river is the trunk, and tributaries are the branches feeding water into it. The Missouri River is a major tributary of the Mississippi River, joining it near St. Louis and making the Mississippi much bigger and more powerful.

Tributaries form when rainfall and melting snow create streams in hills and mountains. These streams flow downhill, eventually merging with bigger waterways. A single large river might have dozens or even hundreds of tributaries. The Amazon River, for instance, has over 1,000 tributaries, some of which are enormous rivers themselves.

Understanding tributaries helps explain why rivers grow as they flow: each tributary adds more water, making the main river wider and stronger. This is why small streams near a river's source can become mighty rivers by the time they reach the ocean. Geographers use tributaries to map entire river systems, showing how water travels across landscapes.

The word can also describe something that contributes to or feeds into something larger, like how small ideas can be tributaries flowing into a bigger theory or movement.