DECONSTRUCTING WATER-SYSTEM TRIGGERING HYPOTHESES FOR ROCK FALLS FROM GLACIER POINT, YOSEMITE NATIONAL PARK

More than 100 rock falls have been documented from Glacier Point, Yosemite National Park since 1870. Fatalities resulted from rock falls above Happy Isles (July 10, 1996) and Curry Village (June 13, 1999). A lawsuit, dismissed in 2010, claimed that septic leachate and water-tank overflows increased hydraulic pressures above stability thresholds in key fractures. Although evidence shows that the rock falls were not triggered hydrologically, in order to address lawsuit claims, we present hydrologic data showing that the water systems are unlikely triggers for rock falls from Glacier Point.

During 1996, precipitation, which, as usual, was mostly snow, provided 30 million gallons of water upslope from the Happy Isles rock-fall source area; septic leachate provided 30 times less water (0.9 million gallons). It is unlikely that small amounts of leachate raised hydraulic pressures above those that occurred during infiltration of spring snowmelt. The variability of annual precipitation in the watershed during 1988-1997 is 10 times greater than the average annual volume of leachate from the septic system. Groundwater-levels measured in shallow monitoring wells at Glacier Point are highest during peak snow melt; it is likely that hydraulic pressures in cliff fractures also are highest then. Leachate is disposed only during the dry season when evapotranspiration rates are high and groundwater levels naturally decline – precluding elevated hydraulic pressures at Happy Isles.

Similar disproportionate volumetric relations (between precipitation and overflowing water tanks) occur upslope from the Curry Village rock-fall source area. Field calibrated analyses of satellite imagery indicate that melting snow was present in Staircase Creek watershed during the late spring, but seepage observations show that melting snow or tank overflow in the Staircase Creek watershed does not reach the rock-fall source area. Because of its proximity, precipitation on the upper cliffs moving down slope through unsaturated fractures is the likely source of most seepage at the rock-fall source area. Purported correlations between tank overflows and rock falls disappear when all data are evaluated. We conclude that these Glacier Point rock falls, like the hundreds of documented rock falls in Yosemite Valley, were naturally occurring events.