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A century-old power plant was idle Thursday after an old water flume in Big Cottonwood Canyon ruptured, sending a mudslide of rocks and silt down the canyon wall and flooding the highway.

The nearly 70-year-old line likely split about half a mile up the canyon around 2 p.m. Wednesday, when the wooden waterline was hit by a huge boulder tumbling down the mountain, according to Salt Lake County sheriff's spokesman Rod Norton."The water that came pouring from the break started an additional slide and dumped rocks and debris on the street," said Norton. "In all, traffic traveling up and down the canyon was backed up for about two hours."

Water and power crews used bulldozers and snowplows to clear the mess before much debris could reach the neighboring Big Cottonwood Purification Plant. The facility was closed for a few hours but was back in operation Wednesday evening.

A steady flow of sediment also poured into Big Cottonwood Creek during cleanup efforts.

The flume, the property of Utah Power, connects two tiny power plants built in 1897. They are the Granite Plant, near the mouth of the canyon, and the Stairs Plant, which is about 2 miles up-canyon.

The lower Granite Plant was shut down after the waterline break. The Stair Plant continues to generate power.

The flume was built in 1927 and was "steadily built and repaired because, naturally, the wooden flume is subject to a lot more wear than a metal one would be," said Utah Power spokesman David Es-kel-sen.

"The Granite Plant will have to remain shut down until the flume is repaired," he said. It's possible that the U.S. Forest Service may ask Utah Power to replace it with a metal flume.

"The company will be cooperative," if such a request is made, Eskelsen added. "The reason that it's still a wooden flume and hasn't been replaced is because it's relatively expensive to do so."

The output of the ancient power plants - one of which was the first in the country to generate alternating power - is pitiful by modern standards. Granite generates 1.1 megawatts, while Stairs is all of 1.2 megawatts.

By comparison, a coal-fired generating station can put out 1,100 megatwatts.

The capacity of Stairs and Granite is even less than usual just now because Big Cottonwood Creek slows during the winter. The flume has a capacity of 45 cubic feet per second, but when the break occured, it was running at 16 cfs, Eskelsen said.