the bottom of a river
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.
the bottom of the river is called a bed
river bank.
The bottom of the river is called the river bed.
the river Oise and the river Marne are important tributaries to the Seine river.
The "Main Channel".
channel
If a river channel has a steep gradient, the river flows faster and has more erosive energy.
The River's Velocity, Gradient, and Channel shape/roughness