The ultimate base level of a stream is sea level. A base level is the lowest point to which a stream can erode its channel.
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.
river bank.
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