Erosion of tephra on hillslopes north of Mount St. Helens was measured at intervals of several months between 1980 and 1983 using large arrays of stakes. Rill erosion was initially greater than sheetwash, but its importance decreased comparatively quickly. Both rill erosion and sheetwash decreased by one to two orders of magnitude over the three-year period, because a stable rill network developed and more permeable and less erodible substrates were created or exposed. The decrease occurred prior to recovery of plants. The causes for the rapid decline in erosion at Mount St. Helens explain patterns of erosion following several other recent tephra eruptions.
Basin-wide annual rates of erosion were computed by mapping the surface cover, the hillslope gradient, and the texture and thickness of tephra and by using statistical relations developed between them and erosion rates. Annual erosion decreased from 26 mm between May 1980 and May 1981 to 1.8 mm between May 1982 and May 1983. Projecting this rate of decline into the future indicated that only about one-sixth of the tephra on hillslopes will be removed by water erosion before soil creep and other forms of mass wasting again dominate hillslope evolution.