The May 18, 1980, eruption of Mount St. Helens caused an immediate destruction of life and property and profoundly changed the local environment. Hydrologic effects of the event persist to this day and may have additional drastic impacts on property and, perhaps, life in the years ahead. The most serious and potentially persistent hydrologic problem is the sedimentation in the Toutle and Cowlitz river systems initiated by volcanically generated mudflows and aggravated by massive erosion in the devastated area. This sedimentation has drastically reduced the carrying capacity of these streams. Flood flows this winter and spring, whether caused by rainfall, snowmelt, outbreaks from debris-dammed lakes, or by volcanically induced snowmelt, may not be contained within river channels. Ash deposits and developing drainage systems provide a continuing source of sediment flow. Destruction of forests and burial of the forest litter under ash has changed the infiltration characteristics of drainage basins near the mountain.