A steep, V-shaped cross-profile is typical in the upper course. This is because of vertical erosion by the river combined with weathering and mass movement of the valley slopes.
In the middle course, the river is flowing through lower-lying land. The gradient is gentler, so the river begins to meander (bend). As it does this the dominant type of erosion becomes lateral, eroding the valley sides. The makes the valley broader. Also, the rate of weathering increases on the softer rocks of the valley sides.
In the lower course, the river is passing through the low-lying country. Flooding results in deposition building up the flood plain and this, along with migrating meanders builds up and widens the valley further.
Find out more about the cross profile of a river.