The impact of Mount St. Helens ash deposition on snowmelt

The effect of Mt. St. Helens ash deposition on snowmelt was evaluated by comparing runoff from four affected (target) drainage basins and three unaffected (control) basins. The target basins (Nisqually, Cowlitz, Cispus, and American Rivers) received from 5 to over 25 mm of ash, primarily from the major May 18 eruption, while the control basins (Cedar and Stehekin Rivers and Thunder Creek) received insignificant ashfall. Five day mean runoff ratios for target versus control basins were computed for the base period 1951-79 and compared with the same ratios for March-August, 1980. Temperature range was used as a surrogate for cloud cover. The results showed an identifiable increase in runoff from the Nisqually drainage following the May 18 eruption and less apparent but perceptible increases following the minor, initial March 27 eruption from all four target basins. Runoff from the Cispus, Cowlitz, and American basins showed a decrease in runoff following the May 18 eruption, possibly due to the insulating effect of heavy ashfall.