Riverbed, or simply bed.
a dry river bed
channel
The path that a stream follows is a channel.
Benthos
the flow of the river.
velocity
Delta
A channel is a passage connecting two bodies of water. The term is also used to indicate a navigable passage, as in a river or harbor.
Tangent plane "is the floor". I never heard that the touching point has a specific name.
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In geography, a channel refers to a narrow waterway that connects two larger bodies of water, such as a river or a strait. It can also refer to a man-made waterway constructed for navigation or irrigation purposes.
Sea Floor Spreading