Hydrology of the Castle Lake blockage, Mount St. Helens, Washington

The debris avalanche that occurred during the May 18, 1980, eruption of Mount St. Helens blocked South Fork Castle Creek and created Castle Lake. Stability of the blockage was of concern, and a digital model that simulates three-dimensional ground-water movement
in the blockage was constructed as part of the analysis used in a follow-up study that assessed the blockage's stability. The model simulates seasonally high water levels, recharge and discharge, and provides a means to estimate hydraulic gradients in the blockage. This report discusses the construction and calibration of the model as well as the geohydrologic information necessary for this study. Recharge from precipitation accounts for approximately 81 percent of the total recharge to the blockage during the calibration period of the model and 81 percent of discharge from the blockage occurs as seeps. Ground-water movement in the blockage is downward and horizontal under the blockage crest and upward under Castle Lake and the blockage toe.