What are the main process of erosion that take place in rivers?

River erosion by four main processes - hydraulic action, abrasion, solution, and attrition - wears away the river bed and banks and breaks down carried material.

Through hydraulic action, the force of the water traps air in cracks in the riverbank. The pressure (as the air is compressed) weakens the banks, wearing away the rock and soil.

Abrasion is the main process of vertical and horizontal river erosion. The river bed is worn away by transported material that scrapes along the surface with a sandpaper-like action, as it is carried downstream.

Solution involves the dissolving of soluble particles and rock (such as limestone and chalk) by slightly acidic river water, breaking down the bed and banks chemically.

Finally, attrition breaks down river material which collides during transportation. Due to the impact, they fragment and chip into smaller pieces, becoming more rounded and smooth.