The flowing consonants of rill affirm its traditional English meaning of “small brook,” but the term also denotes a still smaller, shiftier, and more transient stream. Rainwater flowing as sheetwash down an erodable slope, as steepness and its accumulating depth enable it to move material, will often break midslope into incipient channels. These closely spaced, roughly parallel rills, or shoestring rills, an inch or two wide and deep, may enlarge and interjoin to form gullies lower on the slope. A primary sculptor of badlands and many desert landscapes, rilling is also the initial conduit system by which the soil of sloping farmland is lost to river drainages. Wave or tidal backwash on beaches makes rilled patterns on saturated sand, and the term has been extended by astronomers—usually as rille—to denote long, narrow trenches on the surface of the moon.

John Daniel