The water behind the Newport Avenue Dam should start dropping Monday, allowing crews from PacifiCorp to inspect the leak that's turned much of Mirror Pond into a mudflat.
The company that owns and operates the downtown Bend dam discovered the leak Oct. 2. Water levels in Mirror Pond dropped by roughly 2 feet in the days that followed as water drained through the leak. The pond then rebounded briefly as water managers upstream adjusted flows in the Deschutes River to prepare for the end of irrigation season. The water level has since receded a second time, stabilizing at around 2 feet below its typical winter level. PacifiCorp spokesman Bob Gravely said the company now plans to draw down the water even further in order to get a better look at the damaged area starting Monday.
Gravely said it's unclear how far the water will have to come down to allow crews to inspect the leak.
“We don't know exactly. What's going to happen is, our folks will be at the dam monitoring it, and there will come a time when they say, 'We can do this now,'” he said.
Under state law, if a water release from a dam is likely to create excessive turbidity — suspended silt — downstream, the dam operator must seek permission from the Oregon Department of Environmental Quality.
Eric Nigg, water quality manager for the DEQ Bend office, said his office granted PacifiCorp permission to lower water levels under a provision that allows violations of the state's turbidity standards under narrowly defined standards.
In consultation with the Oregon Department of Fish & Wildlife, the DEQ settled on allowing PacifiCorp to drain the pond at a rate of 2 inches per hour, and refill it at a rate of 4 inches per hour. Nigg said the “ramp rate,” as it's called, should minimize downstream turbidity and the risk that fish could be stranded by rapidly falling water. The utility had originally requested a ramp rate of 6 inches per hour both for lowering and raising the water level, Nigg said.
Under the conditions set by DEQ, PacifiCorp will be monitoring turbidity levels and any fish stranding during its operations.
Gravely said PacifiCorp suspects the risk to fish and other wildlife is minimal.
“We don't think it's very likely here at all,” he said. “The pond, it's already down, and this will be done so gradually we don't think it's very likely.”
Gravely said the draw down of water is likely to take two to three days, while actual inspection is expected to last about eight hours. Any repairs to the dam are likely to occur at a later date, which would necessitate another draw down of water levels.