Peak Spring Flooding
2011 Merced River Spring Peaks: 10.43 feet at 4:15 a.m. June 16 at Pohono Bridge gage & 8.03 feet at 3 a.m. June 23 at Happy Isles gage. View Pohono Bridge Merced River gage, including USGS steamflow data reflecting the 10-foot flood stage.
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.
During the snowmelt season, Yosemite's streams and rivers are raging. Many people expect daily peak flows to occur in the late afternoon, soon after the hottest part of the day; this can be generally true for high-elevation streams, but melting snow takes a long time to reach the Merced River in Yosemite Valley. Peak snowmelt flows occur many hours after the afternoon sun melts snow. 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 a.m. on June 23, 2011. Snow tends to melt first in low-lying areas, such as valley bottoms. The result: Melt water travels further and further from the edge of the snowpack across soil surface to streams. The further this distance, the longer it takes for snowmelt to reach Yosemite Valley. A detailed account of the stream flow phenomena can be read in a Sierra Nature Notes article.
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 gage stations date back to 1916.