The rivers I fish often vary in water height by several feet and in places have very unstable banks, regularly washing them away on one side and depositing stones on the other as well as pushing tons of stone downriver.

The size of the stones on the river bed is usually governed by the speed of the water, the faster the speed the bigger the stones, slack areas will have finer gravel or coarse silt.

If the water suddenly stopped, the different gradients of that river bed would be very obvious.