For a century, the Yosemite Valley's beautiful "twin" has been drowned under 90 metres of water. A controversial project dammed the river that flows through the iconic Yosemite National Park, turning the 1180 square kilometre Hetch Hetchy valley into a giant reservoir. It now supplies most of the San Francisco Bay Area's water and some of its hydropower.
This week, San Francisco will vote on plans to drain the reservoir and restore the valley to its original majesty. New Scientist 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 ? the O'Shaughnessy Dam ? 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 Restore Hetch Hetchy, 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's water. It is so pure, the city does not even filter it.
Wasn't 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 The Yosemite, Muir wrote: "Dam Hetch Hetchy! As well dam for water-tanks the people's cathedrals and churches, for no holier temple has ever been consecrated by the heart of man."
The Sierra Club, which Muir co-founded, has campaigned to drain Hetch Hetchy over the past century, but chose not to involve itself in this year's vote.
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 oppose the bill, 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 Jeffrey Mount 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't this all a bit short-sighted, what with climate change causing droughts?
It's 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't make things much worse for California, says Sarah Null of Utah State University in Logan. She has modelled California's 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(Journal of the American Water Resources Association, doi.org/bpjs6k).
So the reservoir is drained, what next?
At first, the Hetch Hetchy valley would look like a sodden mess: a far cry from the "temple" 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 "bathtub ring". It could take a century for this to disappear.
How would this project work?
The best approach would be to drain the valley a few metres at a time, exposing a narrow strip of sediment around the reservoir, says Joy Zedler 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 (River Research and Applications, doi.org/cqcsts).
"If you were to remove a dam purely for ecological reasons, Hetch Hetchy wouldn't be the one you'd remove," Null says. Removing one of the dams that block Pacific salmon from migrating into California's rivers would have a much broader positive impact on the environment, she says.
For San Francisco, it's purely a value judgement, Mount adds. "There would be no effects other than restoring a pretty place."
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