Rills are created when water erodes the topsoil on hillsides, and so are significantly affected by seasonal weather patterns. They tend to appear more often in rainier months.[1] Rills begin to form when the runoff shear stress, the ability of surface runoff to detach soil particles, overcomes the soil’s shear strength, the ability of soil to resist force working parallel to the soil’s surface. This begins the erosion process as water breaks soil particles free and carries them down the slope.[2] These forces explain why sandy, loamy soils are especially susceptible to the formation of rills, whereas dense clays tend to resist rill formation.[3]