Question

The water flowing over stones in a stream or river makes them round and smooth mainly duo to

Answer 1

Mainly when a boulder primarily comes out of a cliff or bigger stone, it flows with the current. In that time rolling friction gives it a round shape and makes its surface smoother than initial condition. Fluid friction also works in that time when the stone surface has no attachment with any other solid particle but only water current.

But when that stone gets settled underwater, fluid friction acts as the current flows over it continuously, the effects of fluid friction is much less than the effects of rolling friction though, it mainly makes the surface smoother but the size reduction happens mostly by the rolling friction in the first place.