River Erosion

Rocks being transported are eroded. They collide with each other, as well as the river bed and river banks. Sections of the river bank break away and rocks break apart, becoming more and more smaller and rounder as they travel further downstream. This is why you see more smaller rocks in the river's lower course, and larger ones in the upper course.

Corrasion is the wearing away of the river bed and banks by the load carried by the river. Rocks often leave striations in the river bed. Striations are 'scratches' which show the path that the rock took when it marked the river bed. Small depressions are created in the river bed, but usually turn to pot holes due to rocks falling into them and swirling around until the depression gets deeper.

Rocks dissolve in the water and are carried away. Limestone and chalk are examples of the type of rocks that will dissolve. Chemical ions are removed.

Hydraulic action is the force of air and water in cracks and on the sides of the river. If air gets trapped in the rock, the rock is forced apart, splitting into two pieces. Hydraulic action can lead to undercutting.