gully, trench cut into land by the erosion of an accelerated stream of water. Various conditions make such erosion possible: the natural vegetation securing the soil may have been destroyed by human action, by fire, or by a climatic change; or an exceptional storm may send in torrents of water down the streambed. Gully erosion is closely related to intense local thunderstorms and not to widespread winter precipitation. Gullies in soft rock enlarge rapidly by headward erosion and may destroy much arable land if preventive measures are not taken.