The area drained by a river is called the river basin or drainage basin. This is the region where the waters of the river and its tributaries collect before draining into the sea or a large lake at its mouth.
The drainage basin is always located at a lower altitude as the river and its tributaries meander their way towards the mouth. It serves as a catchment area for the water of the river. The edge of the drainage area is often referred to as a watershed. The drainage area forms a floodplain with rich alluvial deposits from the river. This causes the soil around this region to be very fertile.