Energy of the river is used to overcome the friction between the water and the rock particles. There is little or no erosion that take place. However, when there is a heavy downpour, rain would flow down the steep-sided slope of the valley and enter the river. The rain would erode the steep-sided valley and also enable vertical erosion to occur. The vertical erosion causes the river to have a steep-sided, narrow and deep valley. Headward erosion take place at the head of valley and with mass wasting, it extends the valley upslope