Runoff following rainstorms and spring snowmelt can create small rills in bare, unprotected farmland. Left untended, these rills can quickly deepen into gullies, carrying away tons of valuable topsoil. Conservation-minded farmers often grade these unnatural waterways into broad, shallow channels, then seed them with deep-rooting grasses. The resulting grassed waterways slow the ﬂow of water and direct it to a suitable outlet. The grass cover not only helps protect the drainageway from gully erosion, but it may also ﬁlter out harmful farm chemicals like pesticides and herbicides carried by the runoff water. Small animals and birds may also ﬁnd food and shelter in the grasses.