A river and its tributaries are?

The main channel and all of its tributaries are collectively known as a river system. These systems drain a particular area that is identified as a watershed, basin, or drainage area. Precipitation falls within a watershed and either runs off into the waterways or seeps into the ground to reappear later as springs or groundwater. When the water enters the main river, it is a tributary. As it flows downstream, it may encounter more tributaries. Any body of water that flows and ends up in an ocean, sea, or lake is part of a river system.