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"  On Election Day, residents of San Francisco turned down a ballot measure that asked if they wanted to drain one of the country&amp;#039;s most historic (and controversial) reservoirs and restore the Hetch Hetchy Valley in Yosemite National Park. The outcome was expected, but it was the latest in a political battle that has been raging for decades. Even as the dam was approved in 1913 to assist earthquake-ravaged San Francisco, environmentalists and nature lovers, who said the valley&amp;#039;s beauty surpassed even Yosemite Valley&amp;#039;s, were fighting its construction.  Unlikely as it may be (San Francisco has to get its water from somewhere), the Hetch Hetchy Reservoir removal is an issue that its proponents promise to keep pressing, so it could continue to show up every voting season. Should the measure ever pass, California would have to figure out how to even do such a thing. How is a dam removed? What happens to the environment of an area, upstream and downstream, when the dam is gone? Can you restore a beautiful valley that&amp;#039;s been underwater for a century? Restoring a Valley The removal of the Hetch Hetchy dam and reservoir isn&amp;#039;t even in the planning stages yet; the measure on November&amp;#039;s ballot simply asked voters to earmark money for research and design of a new water system to take the place of the reservoir. But proponents have already mapped out a plan to show how the valley&amp;#039;s renewal would work. First, the reservoir would have to be drained, a step that isn&amp;#039;t as difficult as it may seem. Because a reservoir&amp;#039;s job is to save fresh water during the rainy season for residents to use during the dry season, there is already a tunnel and series of aqueducts in place to let water flow downstream. In fact, 50 percent of the Tuolumne River (which is the source of the reservoir&amp;#039;s water) is diverted into the tunnel every year, after which the pipes are closed and the reservoir fills back up to capacity. To drain the reservoir, the Hetch Hetchy engineers would need to open the pipes\u2014and leave them open until the water is gone.  Mark Cederborg, an environmental restoration specialist and vice chairman of Restore Hetch Hetchy, the group that sponsored the bill, says the draining process would take about three years. There&amp;#039;s little concern about the downstream effects of releasing all the water, he argues. For one thing, the riverbed is already well-acclimated to the natural flow of water, which vary naturally throughout the year because of melting snowpack. For another, he says, there is a second reservoir called Don Pedro a few miles downstream from Hetch Hetchy that is six times larger and helps to regulate the water flow. If the valley is exposed to world again it will essentially be a blank canvas. &amp;quot;Most of the valley is under 300 feet of water,&amp;quot; Cederborg says. &amp;quot;There&amp;#039;s nothing growing down there. What&amp;#039;s potentially exciting is it would reveal waterfalls people wouldn&amp;#039;t know existed. The landform would get exposed, the river would flow through its historic channel, and you&amp;#039;d have a flat valley bottom that would be waiting to be seeded and begin growing again.&amp;quot; The valley would be covered in about two inches of sediment, which is unusual to Hetch Hetchy; many dams collect large amounts of sediment, however the Tuolumne riverbed is mostly granite and erodes slowly. But the restoration effort would have to begin quickly. There is concern that rainfall could create sediment runoff, as none of the ground would be anchored by the roots of plants and trees. &amp;quot;As you drain the reservoir you need to stabilize the valley floor and the river banks. Control erosion, re-vegetation, and the gradual process of habitat restoration,&amp;quot; Cederborg says.  Some of this erosion control would take place naturally, but humans could speed the process by seeding the ground with local grasses. Within two years, he says, meadows and seedlings would appear. &amp;quot;It seems so final when you flood something,&amp;quot; he says. &amp;quot;But when you uncover it, it&amp;#039;s incredible how fast nature is capable of restoring itself. Some restoration projects that are five to 10 years old you&amp;#039;d never know they were inundated.&amp;quot; . A Dam Removal in Progress The restoration of Hetch Hetchy would be a simple task compared to some dam removal efforts. Consider one project in progress that involves re-routing an entire river. The San Clemente Dam on the Carmel River in Northern California creates the Northern boundary of Big Sur, and its reservoir provides water to Monterey. But since its construction in 1921, the reservoir has filled up with so much sediment that only 10 percent of its volume is water. Plus, modern engineering studies and a look through photos of the near-hundred-year-old construction site have shown that the dam&amp;#039;s builders used just a single layer of reinforcing steel, and that the dam was not exactly earthquake-proof. It was time to replace the dam. Richard Svindland, vice president of engineering for California American Water, which is overseeing the construction project, says that when it became clear the dam was beyond repair, the next step was to figure out what to do with all that sediment. There are 18 miles of potentially floodable riverbed below the dam and 2.5 million cubic yards of sediment. If the dam came down, increasing the flow of the water, and the movement of the sediment wasn&amp;#039;t carefully controlled, it would be pushed downstream and cause problems for communities on the riverbanks.  Officials considered using trucks or conveyor belts to remove sediment, but the CAW team estimated it would take about 250,000 truckloads to remove it, and the work would be done along narrow roads. They considered controlling the flow of the river to naturally return the sediment down the river. &amp;quot;In the modeling they found that was very unpredictable,&amp;quot; Svindland says. &amp;quot;If all the sediment gets stuck down at the bottom that will create flooding issues in the lower part of the river,&amp;quot; he said. At this point, Svindland says, the team decided there was a much easier solution: &amp;quot;Why don&amp;#039;t we just cut a hole in the ridge and re-route the river around the sediment?&amp;quot;  The challenge is to ensure that the river doesn&amp;#039;t naturally revert to its original bed. So the team is using boulders and rocks from a nearby ridge to build structures that will route the river into a new riverbed. Once it&amp;#039;s done they will create what&amp;#039;s called a diversion dike\u2014basically piling large boulders in front of the river to force the water into its new bed.  &amp;quot;It&amp;#039;s a big plug at the top end of the river,&amp;quot; Svindland says. &amp;quot;We will be building a new earthen rock filled dam, which will withstand an earthquake, and it doesn&amp;#039;t hurt if it shakes and moves around. The slopes of the new bed will be gentle and stable, and that will prevent the river from reverting.&amp;quot; The new river&amp;#039;s gentle slope will make it suitable for fish passage, too. Currently, wild salmon traveling up the Carmel River get past the dam through what&amp;#039;s called a fish ladder, which is basically a series of steps onto which the fish hurl themselves.  &amp;quot;The fish jump from panel to panel. It&amp;#039;s a man-made solution, but it doesn&amp;#039;t work for all fish.&amp;quot; Svindland says.  The new river will have a series of small pools, strong enough that they won&amp;#039;t move around under the force of the water or in the case of an earthquake. The fish will be able to make their way upstream slowly and take a break in the pools if they become tired. The old dam will also be slowly removed throughout the course of the project and replaced by a large boulder-based wedge to hold the sediment in place so it won&amp;#039;t overtop the dam and flow downstream during rainstorms. There was some concern that new sediment running down the river would create a problem downstream, but modeling has suggested the outcome will actually be positive. &amp;quot;There&amp;#039;s still sediment that comes down the river system. It&amp;#039;s been trapped since 1921, and now it will be able to go downstream and help the areas of the river where it has eroded,&amp;quot; he said.  The Largest Dam Removal in American History In September of 2011, Washington state began a three-year, $27 million project to restore the Elwha River in Olympic National Park by removing two dams, the Elwha and the Glines Canyon. The project is the largest dam removal ever to take place in America; its goal is to restore sediment downstream and make salmon breeding grounds more habitable. Each dam requires a different strategy. But in both cases, the first step was to lower water levels in the reservoirs behind the dams. With the Elwha, crews could simply open the spillways and intakes and allow the reservoir to drain. Once the existing spillways lowered the water by about 15 feet, teams dug a diversion channel (essentially making one of the dam&amp;#039;s spillways bigger) and then built temporary dams around it to force the water to drain fully out of the new opening. At that point the sediment was fully exposed and then removed along with the dam itself. Much like the potential plans for Hetch Hetchy, the newly-exposed land will be re-vegetated to resemble its look before being flooded. But in the case of the Glines Canyon dam, the spillways were located high up on the dam. So the only way to empty the water was to tear down the dam. Workers accomplished this by using hydraulic hammers mounted on barges. The hammers pounded away the top 17 feet of the dam&amp;#039;s concrete, lowering it down to the bottom of the spillway gates. Water behind the Elwha dam was much simpler. The reservoir&amp;#039;s spillways and intakes were simply opened and water was allowed to drain out naturally. From there, the Glines Canyon dam is being slowly removed in notches on alternate sides\u2014taken apart bit by bit and creating spillways to release water with each layer. Once one side is lowered and water is draining from there, crews move to the other side and lower it below the first. Once the team reaches the final layer, they&amp;#039;ll use controlled blasts of dynamite to blow up the rest of the dam. "^^<http://www.w3.org/2001/XMLSchema#string>
<http://www.popularmechanics.com/science/environment/a8515/what-happens-when-you-remove-a-dam-14845676/>