The answer is Abrasion. Smooth, rounded cobbles and pebbles are found in streams a result of Abrasion. This is the processes by which a stream erodes and deepens its channel which is caused by sand, pebbles and cobbles in the bed load when they roll, tumble and bounce along the floor of the channel, thereby grinding and wearing away the underlying bedrock. It also causes the sand grains, pebbles and cobbles to become rounded during transport. Abrasion is the erosional process by which the sediment load of a river grinds away and erodes the rocks on the floor of the channel. The process also rounds the sand and pebbles in the bed load.
