Small channels, between 5 and 2  000 mm in width, and very closely spaced. They develop well in areas with heavy rainfall, especially upon weaker rocks, such as volcanic ash. Sediment yields increase with slope gradients and flow rates (the former having more effect than the latter) and greater flow rates cause more rill erosion and soil loss under the same slope gradient (Lei et al. (2006) Eur. J. Soil Sci. 57, 3). Torri et al. (2006) Geomorph. 76, 3–4 find a power relationship exists between width and total flow discharge in rills. Rills may widen and deepen to form gullies.