Erosion

The rocks and pebbles being carried by the river crash against each other, wearing them down to become smaller, rounded pebbles.

The erosion of the river bottom and the riverbank by material being carried by the river itself.

The chemical erosion of the rocks of the riverbank by the slightly acidic water. This occurs in streams running through rocks such as chalk and limestone.

The water forces air to be trapped and pressured into cracks in the rocks on the bank of the river. This constant pressure eventually causes the rocks to crack and break apart.