River and stream erosion Along with wind, rivers and streams are the primary agents that cause soil erosion. Kinetic energy is the main factor influencing the geological work done by rivers and streams. From the point of view of physics, kinetic energy is proportionate to the mass of the water and square of the stream velocity. Many fast-flowing rivers and streams drain the Himalayas and erode large amounts of soil in their upper course.

Mountains sculpted by the eroding action of rivers. Credit: Karamjeet Singh

Erosion by fast-flowing waters affects the environment in many ways. There is an undercutting of valley sides which causes the mountainsides to become unstable and ultimately ends in landslides. Moreover, unsorted sediments are deposited over agricultural fields and settlements by the rivers, especially during monsoons. During floods, tremendous amounts of erosion occurs on the banks of rivers and streams.

Some rivers tend to attain their peak discharge around mid- afternoon every day from late June to early October. This is due to the faster rate of melting of the snow in the uplands, as the sun's heat becomes intensive. Thus within the span of an hour or so, the water level rises and unwary people and animals get swept away with the on-rushing water. Many human and animal lives are lost every year due to the daily rise in the water level of Himalayan rivers and streams.