What is the primary role of tributaries in a river system?

The primary role of tributaries in a river system is to join and feed the main river. Tributaries are smaller streams or rivers that flow into a larger river, contributing additional water, nutrients, and sediments. This connection enhances the river's flow and can significantly impact its size and capacity, helping to sustain ecosystems along its course. The merging of tributaries with the main river allows for a complex system where water from various sources combines, which is crucial for supporting agriculture, wildlife, and human settlements that depend on the waterway.

While irrigation, draining water, and dispersing sediments are important in the context of rivers, they are more specific functions that can occur within various parts of a river system or due to the actions of the river itself, rather than defining the primary role of tributaries.