This bar graph shows the average annual erosion of cropland in Montana for every five years between 1982 and 2010. The lighter part of each bar represents wind erosion while the darker part represents sheet and rill erosion. Sheet erosion is the erosion of a thin layer of topsoil over an entire area, usually caused by the particle movement and impact of rainfall Rill erosion occurs when runoff water create small concentrated channels as it runs down a slope.