Rains close Yosemite: A tale of two storms, and two rivers

Yosemite Valley Closed by Warm April Rain

A tale of two storms, and two rivers

After a mostly dry winter, two early spring storms drenched much of California and especially Yosemite. 

Heavy rain in the Sierra foothills on March 22-23 threatened San Francisco's Moccasin dam, but it was the smaller storm on April 6-7 that flooded Yosemite Valley. Park officials anticipated the April flood and "closed" Yosemite Valley. 

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For more pictures and video of the flood, see The National Park Service's Facebook page. Visitors who skirted the closure rules were treated to a spectacle, but may have tested the patience of Rangers who take their oath to protect public safety seriously.

Photo: National Park Service

Yosemite Valley's flood was caused by three inches of rain over April 6-7. Three inches of rain over two days is a decent amount, but it's less than the five inches that fell March 21-22 when no flooding occurred. The principal reason for this difference is that the April storm was 10 degrees warmer. In the higher elevations, most of the precipitation from the March storm fell as snow; in April it fell as rain.

Figure 1 shows two-day average flows on the Tuolumne and Merced Rivers, Yosemite's principal streams, for each storm. Each bar shows the extent to which the flow into Yosemite Valley and Hetch Hetchy Reservoir compares to the river's total flow measured downstream, at the foot of the Sierra. (These total flows are calculated as "Full Natural Flows" - as if no dams existed.)  Figure 1 shows that the flows upstream at Hetch Hetchy and Yosemite Valley were far higher in April, even though the total flow for each river was lower.

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Figure 1 also Illustrates one other interesting fact. The Tuolumne River is much larger than the Merced River, carrying almost twice as much water on average. Note also that Hetch Hetchy Valley is only about half as wide as Yosemite Valley (but about the same length). So a flood in Hetch Hetchy would squeeze twice as much water in half the space - a truly amazing spectacle when viewed from a safe place.

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Hetch Hetchy Valley, with habitat types as described in our Fall 2017 Newsletter, is shown transposed over Yosemite Valley. The valleys are roughly equal in length, but Hetch Hetchy is on average narrower. A free flowing Tuolumne River will be a beautiful sight at any time, and will dominate the valley at high water. 

Our first step is to empty the reservoir so Hetch Hetchy can be restored as a valley that park visitors can enjoy. Once those plans are underway, we can start to think about how to best appreciate Hetch Hetchy under a variety of conditions - whether it's the power of a flood or "standing waist deep in wildflowers on a sunny day in June", as John Muir often stood.