The volume of water in a river is at its greatest in the lower course. This is due to the contribution of water from tributaries. The river channel is deep and wide and the land around the river is flat. Energy in the river is at its lowest and deposition occurs. .
Deltas are often found at the mouth of large rivers. An example is the Nile Delta. Deltas are formed when a river deposits material faster than the sea can erode it.
The Nile Delta
The Nile Delta – source