Update: On 6 November 2012, San Francisco voters rejected Proposition F, which would have spent $8 million drafting a plan to restore the Hetch Hetchy valley.
Original article, posted 5 November 2012
For a century, the Yosemite Valley鈥檚 beautiful 鈥渢win鈥� has been drowned under 90 metres of water. A controversial project dammed the river that flows through the iconic , turning the 1180 square kilometre Hetch Hetchy valley into a giant reservoir. It now supplies most of the San Francisco Bay Area鈥檚 water and some of its hydropower.
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This week, San Francisco will vote on and restore the valley to its original majesty. 快猫短视频 looks at what might be achieved.
Why was Hetch Hetchy flooded?
In 1913, despite strident opposition from the nascent environmental movement, the US Congress voted to build a hydropower dam 鈥� the O鈥橲haughnessy Dam 鈥� across the Tuolomne River, which flows through what is now Yosemite National Park, and pipe its water to San Francisco 258 kilometres away. Spreck Rosekrans, policy director of , suspects that the bill was passed partly out of sympathy for San Francisco after its 1906 earthquake.
Hetch Hetchy has been flooded ever since, and now supplies 85 per cent of the city鈥檚 water. It is so pure, .
Wasn鈥檛 that rather destructive?
A project like this could never happen today, as Yosemite National Park is now protected from development. But the bill was controversial at the time, most famously opposed by naturalist John Muir. In his 1912 book , Muir wrote: 鈥淒am Hetch Hetchy! As well dam for water-tanks the people鈥檚 cathedrals and churches, for no holier temple has ever been consecrated by the heart of man.鈥�
The , which Muir co-founded, has campaigned to drain Hetch Hetchy over the past century, but .
What does the ballot propose?
Proposition F, which the city of San Francisco will vote on tomorrow, would budget $8 million to draft a plan for the draining of Hetch Hetchy. Then a 2016 ballot would allow voters to decide whether to actually begin the project. Most city and state politicians , arguing that draining the reservoir would be too expensive and leave the Bay Area without a steady water supply.
Would San Francisco really lose its water supply?
Not at all, says hydrologist of the University of California at Davis. Another reservoir, called Don Pedro, near Hetch Hetchy holds more than twice as much water. It would be expensive, but not complicated, to move the pipes to take the water from this reservoir instead, Mount says.
Isn鈥檛 this all a bit short-sighted, what with climate change causing droughts?
It鈥檚 true that climate change could hurt water supply generally. Rainfall could decrease, there will be less snow on the Sierra Nevada mountains where the Tuolomne River begins, and evaporation will increase, depleting surface water.
But undamming Hetch Hetchy wouldn鈥檛 make things much worse for California, says of Utah State University in Logan. She has modelled California鈥檚 future water supply, and says that even in a worst case climate scenario, losing the reservoir would only cause the San Francisco Bay Area minor additional shortages().
So the reservoir is drained, what next?
At first, the Hetch Hetchy valley would look like a sodden mess: a far cry from the 鈥渢emple鈥� that Muir loved. But studies have concluded that with decades of careful management, ecosystems could be restored.
Grassy meadows would return first, along with some animals and fish. After 20 years, coniferous forests would be well on their way, and after 50 years, deciduous oak forests would have taken hold. Because lichen only grows on the rocks that sat above the water line, the cliffs would probably have a 鈥渂athtub ring鈥�. .
How would this project work?
The would be to drain the valley a few metres at a time, exposing a narrow strip of sediment around the reservoir, says of the University of Wisconsin in Madison.
Researchers will need to experiment on this sediment to determine what to plant, she says. Nearby Yosemite Valley could serve as a model, but the altered climate and poor soil conditions might make it hard for new species to establish themselves in Hetch Hetchy.
In the meantime, keeping most of the valley flooded would deter invasive species, which thrive in poor environments.
Would it boost wildlife beyond the valley?
Probably not. Land species are confined to the valley, and a recent study suggests that undamming the Tuolomne would have little effect on downstream species ().
鈥淚f you were to remove a dam purely for ecological reasons, Hetch Hetchy wouldn鈥檛 be the one you鈥檇 remove,鈥� Null says. Removing one of the dams that block Pacific salmon from migrating into California鈥檚 rivers would have a much broader positive impact on the environment, she says.
For San Francisco, it鈥檚 purely a value judgement, Mount adds. 鈥淭here would be no effects other than restoring a pretty place.鈥�
