Erosion starts when the flow energy of the water exceeds the resistance of the material of the river bed and banks. Flow energy depends on depth of water and gradient and thus of stream velocity. The point in time when material is set in motion is regarded as critical state. The corresponding stress is termed ‘critical shear stress’ (Mangelsdorf and Scheurmann 1980; Robert 2003). This varies for different particles. Hjulström already recognised 1935 the relation between flow velocity and particle size of the eroded, i.e. the moved material (Press and Siever 2003). The Hjulström-Diagram you can see if you follow this hyperlink (in German).
Fluvial erosion proceeds in two ways:
The eroded material is transported by the river.