The 16 May 2005 Flood in Yosemite National Park--A Glimpse into High-Country Flood Generation in the Sierra Nevada

On 16 May 2005, a Pacific storm drew warm, wet subtropical air into the Sierra Nevada, causing moderate rains and major flooding. The flood raised Hetch Hetchy and Tenaya Lake levels markedly and inundated large parts of Yosemite Valley, requiring evacuations and raising public-safety concerns in Yosemite National Park. This was the first major flood to be recorded by the high-country hydroclimatic network in the Park. Since 2001, scientists from US Geological Survey, Scripps Institution of Oceanography, California Department of Water Resources, National Park Service, and other institutions have developed the network of over 30 streamflow and 50 air-temperature loggers at altitudes ranging from <1500 m to >3000 m above sea level, and 8 snow-instrumentation sites measuring snow-water contents, snow depths, radiation, soil moisture, and temperatures in air, snow, and soil. The network documented flooding that derived its runoff mostly from high-altitude rainfall on soils already wet due to the onset of snowmelt a few days earlier. Air temperatures during the storm were above freezing up to altitudes of nearly 3000 m, so that rain fell to as high as 3000 m, compared with normal winter snowlines nearer 1500 m. Streams flooded below 3000 m, and above that altitude did not flood or contribute much to the flooding below. Meanwhile, no significant snow-water content changes were measured. Thus this flood resulted from rain-through-snow runoff rather than rain-on-snow melting. In the Park as a whole, about five times more catchment area received rain, rather than snow, during this storm than during typical cool winter storms. Because the flood was more a result of the large area that received rainfall than of melting snow, snowpack reductions that are expected if recent warming trends continue would not have reduced the flood. Instead, the opportunity for warm storms may increase if warming continues, in which case the potential for this kind of flooding will increase.