the bottom of a river
The "Main Channel".
A river channel
The River Channel is just before the the river meets the mouth or sea.
The River Severn is the main river that flows into the Bristol Channel.
river bank.
The river bed
the river Oise and the river Marne are important tributaries to the Seine river.
If a river channel has a steep gradient, the river flows faster and has more erosive energy.
channel
The River's Velocity, Gradient, and Channel shape/roughness
delta
Its the Severn river.