What type of erosion is the most important in the upper course of a river?

Vertical or fluvial erosion

Explanation:

Fluvial or river erosion is a geomorphological process of constructed forms in the relief of the river flow.

Flowing over the topographic surface in the direction of the greatest fall, and under the influence of the earth's gravity, river flows create their own forms of relief. These are fluvial forms.

Fluvial forms can be: erosive, if they are created by cutting the river flow, accumulative if they are built by accumulating eroded fluvial material in places where the erosive and transport power of river water is weak.