1. When the river has a high discharge it uses it's heavier load to erode down (vertically) into the bedrock. The discharge is usually big because:
-It just rained/there was a storm
-Meltwater flowing into the river (eg from glaciers)
Because the river is in it's upper course and has high discharge it can carry boulders (by traction) which have lots of energy, and erode the bedrock through abrasion. It usually happens in a particular place because of a fault or weakness in the rock.
2. Loosened materials from erosion are washed into the river, increasing it's load and increasing the amount of abrasion and the process of sub aerial weakening. The slopes of the valley are weakened by weathering, as material (pulled by gravity) rolls into the river where it is transported and keeps on eroding.
3. As time goes on the river uses more of it's energy to erode laterally, which is aided by meanders migrating across the floodplain.
Shape is Dependent on:
1. Climate Because of the high discharge level needed to start the vertical erosion andaid mass movement on valley sides, sufficient water is needed.
2. Geology Rocks and there structure make certain characteristics more likely.
Limestone=steep sides Clays=gently sloping valley sides
3. Vegetation The vegetation binds the soil together and makes the valley sides more stable.