Final answer:
A tributary is a smaller stream or river that flows into a larger one, contributing to its volume and influencing its navigability, ecosystem, and flooding potential.
Step-by-step explanation:
The definition of a tributary is a stream or river that flows into a larger stream, river, or lake. These smaller watercourses contribute to the flow of the larger one, which is often referred to as the mainstem or parent river. Tributaries do not flow directly into the sea. They provide additional water volume to their parent river, which can then affect the river's ability to navigate, its ecosystem, and potential for flooding. For example, the Missouri River is a tributary of the Mississippi River because it flows into it.