Question

why erosion is the dominant activity of a river in its upper coarse?

Answer 1

In the upper course the dominant activity of a river is erosion. Many land forms are shaped by this activity of the river. Along its course, it erodes bed and banks, and carries large boulders of rock with it. This is due to depsoition of sediments over a large area near the mouth of rivers

Answer 2

When discharge is high vertical erosion erodes the river bed and larger sediments are transported by traction. Mid-course - here the gradient becomes less steep. The river channel gets deeper and wider as the bed and banks are eroded. The sediment load of the river gets smaller in size.