Note that this process happens more vigorously during a flood, when the volume, velocity, and turbulence of the water is much greater. Rocks on Canyon walls break free and tumble downslope and eventually into the river. Then, they are washed away. Note that erosion of slopes is joint controlled. Rock breaks free along joints.
Joints are natural cracks in rocks.
The spacing between joints depends on the thickness of beds. Not all units erode at the same rate. That's because the joints in these layers are far apart. In these intervals, beds are very thin, and joints are very closely spaced. Thus, you get tiny little steps that average into a gentle slope.