A tributary is a stream or river that flows into a larger stream, river, or lake but does not flow directly into a sea or ocean. It contributes water to the main water body into which it flows, helping drain the surrounding drainage basin. The place where a tributary meets the main river is called a confluence. Tributaries can vary in size and may be seasonal, drying up in certain months. They play an important role in the watershed system, and large rivers often have many tributaries feeding into them, increasing their volume and size.