When slopes become 25 degrees or greater, almost all the redistribution occurs in a downslope direction. Considerable transport of surface sediments on slopes occurs by rainwash and surface runoff . On relatively flat surfaces, runoff occurs as a continuous layer of water commonly called sheetwash . The erosive potential of sheetwash is usually quite limited because this type of flow is shallow and non-turbulent and cannot readily entrain surface particles. However, topographic irregularities can quickly transform sheetwash into small channels called rills .
Rills then coalesce into larger stream channels and so on.
Rills and large stream channels concentrate the movement of water causing an increase in flow velocity and turbulence. Higher flow velocities and turbulence lead to a greater potential for entrainment and subsequent transport of hillslope materials. This type of slope is common to landform features like alluvial fans , screes , talus cones , sand dunes , and glacial outwash deposits. In some cases, these movements can organize themselves into larger avalanches through a domino effect. Mass movement on non-cohesive materials can also occur by way of shallow sliding.