What shape does the valley typically have due to vertical erosion in the upper course?

In the upper course of a river, the landscape is primarily shaped by vertical erosion, which occurs as the water flows rapidly over steeper slopes. This process primarily carves out a narrow and steep-sided valley which takes on a V shape. This is because as the river cuts downward into the rock and soil, it removes material directly beneath it, leading to steep valley sides that converge at a point, resembling the letter "V".

Additionally, the flow of water, especially in its youthful stage, is fast and energetic, contributing to this shape as it erodes the valley in a downward direction rather than laterally. The presence of sediment and the nature of the riverbed can also encourage this vertical erosion, reinforcing the formation of the V-shaped valley.

The other shapes, such as U-shaped or circular, are typically associated with glacial activity or other geological processes rather than the energetic erosion seen in the upper course of a river. Similarly, a flat shape is indicative of more mature river stages or floodplains rather than the steep topography created by active vertical erosion.