The river bed
the bottom of a river
A river channel
The "Main Channel".
the bottom of the river is called a bed
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 the river bed.
river bank.
the river Oise and the river Marne are important tributaries to the Seine river.
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