Why is downcutting the major erosional force in mountain streams?

Downcutting is the major erosional force in mountain streams because the steep gradient and high energy of these streams allow them to rapidly erode rock and sediment, carving deep channels into the landscape. Additionally, the force of gravity pulls water downhill, constantly working to cut deeper into the bedrock. This process is further accelerated by the presence of sediment and boulders carried by the fast-moving water, which act like abrasives to erode the streambed.