silt
A stream lengthens when downcutting occurs, downcutting is when the bottom of the stream is slowly dug out, this process will dig at the end of the river until it becomes part of the river.
Silt is one type of material found on the bottom of a stream. Silt is fine and can be carried along in moving water, but is heavyier than water, so settles out of the water as it slows. It can be organic or mineral.
Fast moving stream
A stream bed is the bottom (floor) of the stream.
It is a convection current.
Most likely a fast moving stream.
A glacial stream is a stream of liquid water on, in, or under a glacier, moving under the influence of gravity.
Bed
A drainage basin/ watershed.
Sediment that is carried by a stream along the bottom of its channel.
Potholes form in the stream bottom through erosion caused by water and rocks and wildlife.
drift