RILL

A 'rill' is a narrow and shallow incision into soil resulting from erosion by overland flow that has been focused into a thin thread by soil surface roughness. 'Rilling', the process of rill formation, is common on agricultural land and unvegetated ground.
Some rills will continue to grow, as they are widened and/or deepened by the runoff which flows through them. Other rills may decline in importance as sediment is deposited within them. Eventually, a hydrologically efficient rill network will be formed. The development of such a network is an example of self-organisation.
In a few cases, rills may continue to grow and become gullies or rivulets then stream and so on to a river.