The predominant action at the upper course of a river is

The predominant action at the upper course of a river is corrosion. This is because the upper course of a river is characterized by steep gradients and high velocity, which leads to the erosion of the riverbed and banks. The force of the water cuts into the rock and soil, wearing it away over time. This process is known as corrosion or hydraulic action. The erosion caused by corrosion leads to the formation of steep-sided V-shaped valleys and gorges in the upper course of a river. The erosion also causes the river to deepen and widen its channel, increasing its capacity to carry water and sediment downstream. As the river flows downstream, transportation and deposition become more dominant processes.