How the 2004 Onset of Snowmelt and Streamflow Varied with Elevation

In 2004, spring snowmelt began anomalously early across the Western United States. USGS-gaged streams draining the Sierra Nevada, Cascades, and Rocky Mountains all recorded a spring pulse of meltwater during the second week of March. However, data from streamgages monitoring nested streamgages along the Tuolumne and Merced Rivers in Yosemite National Park suggest that this early onset of melt did not occur uniformly at all basin scales and elevations. The Yosemite monitoring network has been operational since summer 2001, and gages are located at elevations from 1200 m to 3300 m in basins with various slopes and aspects. In 2002, spring melt began uniformly at all monitored elevations, and in 2003 spring melt began uniformly at all but the highest gage. However, in 2004, the onset of spring snowmelt varied widely. For example, streamflow on the South Fork of the Tuolumne River at 2040 m started flowing 7 March, and flows did not decline until the snowpack was depleted. In contrast, a gage monitoring inflow to Fletcher Lake at 3109 m recorded no flow prior to 30 April. Many gages at elevations between these extremes recorded a small flux of water in mid-March but no strong increase in streamflow until mid- to late-April. These differences in snowmelt onset dates were supported by not only in streamflow records but also by observation of vegetation, as botanists noticed flowers blooming anomalously early below 2800 m but at their average times in higher regions. This paper seeks to answer the following questions: What were the primary factors controlling the observed differences in snowmelt onset dates in 2004? How did solar radiation and temperature inputs differ between 2002, which had a uniform melt onset, and 2004? Was melt dominated by different processes at different locations? What are the implications for climate forecasts, which predict earlier spring onsets in a warmer climate? How can snowmelt and streamflow models better capture the different behavior observed at the highest altitudes?