When a stream's gradicent steep and the channel is well above base level downcutting is the dommant activity. Abrasion caused by bed load sliding and rolling along the bottom and the hydraulic power fast- moving water slowly lowers the streambed. The result is usually a V-shaped valley with steep sides. A classic example of a V-shaped valley is located in the section of Yellowstone.
The most prominent features of a V-shaped valley are rapids and waterfalls. Both occur where the stream's gradient increases significantly, a situation usually caused by variations in the erodability of the bedrock into which a stream channel is cutting. Resistant beds create rapids by acting as a temporary base level upstream while allowing downcutting to continue downstream. In time erosion usually eliminates the resistant rock. Waterfalls are places where the stream makes an abrupt vertical drop.