banks
A V-shaped valley
The sides are known as the river bank or river banks.
The "Main Channel".
A river channel
the bottom of a river
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 river bed
The Seine River discharges into the English Channel.
If a river channel has a steep gradient, the river flows faster and has more erosive energy.
channel