Spring Flooding

Peak flooding generated by snowpack depends on the amount of area covered by snow, temperature, and lateness of melt (sun angle). Heavy snow years, like 2011, can result in spring floods. The year 2010 also had a high snowpack; a cool May helped maintain extensive snow cover into June followed by rapid warming that produced a 5-year flood on the Merced River. Heavy snow years have generated similar magnitude floods in the past. Larger spring floods occurred in 1983–-the largest snowpack year on record-–and when rain occurs on a rapidly melting snowpack, such as in 1996 and 2005.

Graph showing spring runoff in Yosemite Valley, with a mostly steady increase until peak runoff (early June in 2010 and late June in 2011)
2010 & 2011 had similar peak runoff levels, although 2011 had significantly more snowpack. A warm spring in 2011 helped melt the snow over several weeks (note the multiple peaks in early May and late June), while a cool spring in 2010 preserved the snowpack until a major early-June meltwater spike.
Many people expect daily peak flows to occur in the late afternoon, soon after the hottest part of the day; this is generally true for high-elevation streams, but melting snow takes a long time to reach the Merced River in Yosemite Valley. Peak flows occur many hours after the afternoon sun melts snow, often after midnight. In addition, peak flows occur later each day as the melt season progresses. For example, daily peak flows at the Happy Isles gage occurred at midnight on May 14, 2011, but not until 3 am. on June 23, 2011. This occurs because snow tends to melt first in low-lying areas, such as valley bottoms, then off higher elevation slopes and upper peaks. The farther meltwater has to travel from the edge of the snowpack, across and through soils and into streams, the longer it takes for the snowmelt to reach Yosemite Valley. Read a detailed account of this stream flow phenomena in a Sierra Nature Notes article.
Graph showing years and amount of peak spring flows. Most flows greater than 10 feet deep were between 10 and 11.5 ft with a few much higher flows (up to nearly 14 feet in 1996).
Peak instantaneous spring flood flows recorded at the USGS Pohono Bridge gage (Gage No. 11266500) on the Merced River. Flood stage is defined as a gage height of 10 feet at this location, which is the outlet of Yosemite Valley.

Real-time River Data: Any time of year, view a five-day outlook of the Merced River gage at Pohono Bridge from the California-Nevada River Forecast Center.

Largest Flood on Record: Floods can happen due to peak spring melt or due to rain-on-snow events any time of the year. Yosemite's flood of record is 23.45 feet at Pohono Bridge, occurring Jan. 2, 1997, as a rain-on-snow event. Yosemite's records at Pohono Bridge and Happy Isles gaging stations date back to 1916.

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