Cross profiles of a river
Near the source of a river there is more vertical erosion as the river flows downhill, using its energy to overcome friction (A). As a result the channels are narrow and shallow and may contain large boulders and angular fragments eroded and weathered from the steep valley sides. The sediment in the river creates turbulence and friction.
As the river approaches the mouth, velocity and energy increase due to increased discharge. The river performs more lateral erosion making the channel wider, and smoother (B) and (C). As a result there is less turbulence and friction, making the flow of water more efficient.