Plug Pulled on Dam

Yosemite visitors may not even know of hydroelectric project.

by Mark Grossi
Fresno Bee - November 13, 2003

Eighty-five years ago, the National Park Service dammed the Merced River with timbers, boulders and debris for a hydroelectric project that brought hot water and lights at the touch of a switch.

When the hydroelectric project was finished, 35,000 annual visitors were coming to Yosemite National Park, and such modern comforts outweighed consequences to nature.

Deb Schweizer, Yosemite National Park spokeswoman, walks across the top of Cascades Diversion Dam while giving a tour during the start of demolition. The dam will be gone next month, but crews will work through April to complete restoration and roadside work on the project. (Mark Crosse / The Fresno Bee)

On Wednesday, the roles reversed. Heavy equipment began pounding the small hydro dam to pieces. The only dam between the Sierra crest and foothills on the Merced should be gone by December, officials said.

"This will allow the river to return to its natural flow," said project manager
Mike Pieper, looking at the buildup of silt and sediment behind the dam. "We'll use some of the sediment in the restoration of the banks, but we'll let the river move a lot of it out naturally."

Yosemite visitors may not even know Cascades Diversion Dam and Hydroelectric Powerhouse exists. The low-profile dam can be seen in the river next to the junction of Highway 140 and Big Oak Flat Road on the far west end of Yosemite Valley.

The dam, which often has water flowing over its top, is easily seen now with the Merced River's puny November flow channeled around the structure. Officials want to get the dam out of the river before winter rain and snow begin filling the channel again.

Though the dam will be gone next month, crews will work through April to complete restoration and roadside work on the $2.8 million project. The work is part of the $150 million of Yosemite Valley Plan projects scheduled for the next three years.

Like many projects in the often-delayed valley plan, this one has been waiting for a few years. A lawsuit over the Merced River temporarily delayed the Cascades dam removal. Though the same lawsuit is still active after an appeals court ruling last month, officials say they do not believe it will interfere with the removal.

The dam looked like it was well on its way to being demolished Wednesday. A concrete wall that was added to the dam in the early 1980s was cracked on one end, and timber beams with rusty spikes were bared. The timbers had formed "cribs" for boulders and rocky debris that blocked the river flow.

The dam was used from 1918 to 1986 to hold water that was sent downstream more than a mile through a pipeline into the powerhouse, which made about two megawatts of power. A megawatt is enough to light 750 to 1,000 homes.

"That was the source of electricity for Yosemite Valley," said spokeswoman Deb Schweizer. "But the valley has been on the state's central electricity grid for many years now."

The turbines have been removed from the old powerhouse about a mile downstream of the dam. The powerhouse remains as a power transfer substation that helps get electricity into the valley.

In 1997, the 184-foot-wide Cascades dam was damaged in a large flood. It was deteriorating, but officials knew at that point that they needed to remove it. Their analysis showed the dam could be a hazard if another flood broke it apart.

Pieper said he consulted historic photographs of the area to understand what the area looked like in a natural state. He found what he called a "witness tree," an old incense cedar tree that existed at the time of the dam's construction.

"You can see they probably picked this area for the dam because of a big rock slide that opened up this area," Pieper said. "There are not too many of these dams left. The construction with timber makes them susceptible to rot and decay. It's a pretty unusual dam."