A river bank is the "edge" of a river where the land begins. Sometimes there is a gradual slope as the land rises above the level of the water. At other times the river has cut material away from its edges and left a fairly straight "side" which is quite vertical. It may be a few inches to many feet in height.
Banks of rivers can range in shape from gradual slopes to vertical walls of earth. By looking at a number of river banks, an investigator can see the variation. There are several links below to images of river banks.