San Francisco’s Hetch Hetchy water system survived a near collapse of a dam, but another storm and spiking temperatures could create a bigger risk of collapse next week.
Sheriff Jim Mele warning at 6 p.m. on March 22 that flooding from torrential Pineapple Express rains moving through the Sierras created an “imminent” risk of the collapse of Moccasin Dam on the Tuolumne River. The 60 feet high and 720-feet wide earthen dam holds 554 acre-feet of water, but it is just downstream of the 910-foot high O’Shaughnessy Dam that created Hetch Hetchy Reservoir that holds up to 360,000 acre-feet of water.
The rain squalls were so intense that evacuation roads, including State Route 132 and State Route 120, were already impassable when the Tuolumne County Sheriff’s Department was alerted by the San Francisco Public Utilities Commission that the overflowing Moccasin Dam was at high risk of failure.
With no exit for the District’s buses, students and teachers at Tenaya Elementary School and Tioga High School were forced to shelter in place to ride out the storm. Fortunately, the area got a reprieve on Friday when the storm moved through and the sun came out.
Preliminary data indicates that the 3-day storm dropped about 10 inches of rain on the Central Coast and lower Sierra Nevada elevations, but Yosemite’s Tuolumne Meadows and higher Sierra Nevada elevations received about 31 inches of wet snow.
But a new cold winter storm rolled into the Sierra Nevada Mountains at about 11 PM on Friday night and is expected to drop 12 to 18 inches of snow through Sunday on lower elevations and up to 30 inches in Tuolumne Meadows above the Hetch Hetchy dams. Temperatures are expected to plunge into the teens overnight and the 30s in the daytime.
Tuolumne County Sheriff Jim Mele warned that Moccasin Dam could be at even greater risk beginning on Thursday when daily mountain temperatures rise by about 30 degrees. The rapid temperature spike could kick-off a catastrophic snowmelt that could swell river flows to over 4,000 cubic feet per second.
Sierra Club founder John Muir in great admiration said, “Hetch Hetchy is a grand landscape garden, one of nature’s rarest and most precious mountain temples. As in Yosemite, the sublime rocks of its walls seem to glow with life . . . while birds, bees, and butterflies help the river and waterfalls to stir all the air into music.”
But the ultra-liberal President Woodrow Wilson signed off in 1913 on the multi-decade construction of a series of dams within Yosemite National Park that flooded Hetch Hetchy Valley to create a massive reservoir, hydroelectric plants, and a 167-mile aqueduct for the sole benefit of the City of San Francisco.
Muir lamented, “These temple destroyers, devotees of ravaging commercialism, seem to have a perfect contempt for Nature, and, instead of lifting their eyes to the God of the mountains, lift them to the Almighty Dollar.”
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