The National Soil Erosion Research Laboratory
Rill erosion is the removal of soil by concentrated water running through little streamlets, or headcuts. Detachment in a rill occurs if the sediment in the flow is below the amount the load can transport and if the flow exceeds the soil's resistance to detachment. As detachment continues or flow increases, rills will become wider and deeper.
Rilling is one of the most common forms of erosion. The rill channels can temporarily be obliterated by tillage. Tillage loosens the soil making it more susceptible to rill erosion. Thus, every time they are destroyed - the rills can reform, resulting in much more soil lost.