There are 2.5 million dams blocking U.S. rivers and streams and 85% of them are past their 50 year life expectancy. Popular Mechanics shows how the world’s biggest dam removal will return Washington’s Elwha River to its free-flowing state.
In the fall of 2008, Lake Mills, the 415-acre reservoir above the Glines Canyon Dam, will be lowered 50 ft. Then the dam’s curving center arch, a section of concrete just 4 ft. wide at the top but 21 stories tall, will be cut out using diamond-wire saws. About a year later, the gates atop the Elwha Dam will be cranked open to drain 18 ft. from its reservoir, 267-acre Lake Aldwell. Crews will lower the stepped gravity dam–108 ft. tall and 100 ft. thick at its base–in 7- to 10-ft. increments. All traces of the dam will be removed by the fall of the third year.
Destroying both dams involves breaking up and recycling 35,000 cubic yards of concrete–more than half the amount used to construct the Empire State Building–along with hundreds of tons of metal. "We want to use as much of the material on site as possible," Winter says. Earth fill and crushed bedrock will be used to reshape the slopes around the dams to their original contours. The Elwha hydro plant and penstock tubes, with inside diameters that could comfortably accommodate an elephant, pose thornier problems. Some parts, such as the turbines, might be preserved as historic exhibits.
The two reservoirs must ultimately be drained of 48,600 acre-ft. of water–enough to flood Safeco Ballpark in Seattle, the home of the Mariners, to the height of a 130-story skyscraper. Then there is the reservoirs’ sediment, 18 million cubic yards of the stuff, an amount so large that scientists have studied the debris flows from the explosion of Mount St. Helens to gauge how aquatic life will react to it washing downstream. Reservoir drawdowns will be carefully timed to manage turbidity.
By late 2011, both dams will be history, reservoirs drained and raw banks softened by sprouting trees. It will take three to five years for the river to flush out excess sediment, but the fish, Winter says, will likely reappear in a few months. It will be 30 years before the population is fully restored.