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Elwha dam-removal project held back as silt estimate too low

It turns out the world's largest dam- removal project in history is even bigger than originally thought, as scientists discover an early mapping error of Lake Mills and learn the dams impounded even more sediment than first projected.

It turns out the dam-removal project on the Elwha River already the biggest anywhere in the world is even larger than originally thought.

In the project, long predicted to affect more than 24 million cubic yards of sediment, the amount of sediment once impounded by the dams is actually about 34 million cubic yards, said Barb Maynes, spokeswoman for the National Park Service.

The park service is in charge of the $325 million project, which began in September 2011.

The new estimate is partly the result of discovering a long-standing error in mapping Lake Mills, with elevations recorded 20 feet higher than they really were back in 1917, Maynes said.

Those incorrect measures were passed along in a 1976 map used by engineers in their sediment estimates for the dam removal project, because the 1976 map had been marked as corrected when it wasn't.

The error eventually was detected when it became clear the elevations recorded on the two maps were the same. Further, as scientists walked the landscape in surveys since dam removal started, the emerging landscape wasn't matching the map.

"The mistake has finally been located and corrected," Maynes said.

Scientists have long understood that management of the immense amount of sediment stuck behind the dams would be the trickiest part of the project, driving everything from the pace of dam removal to mitigation needed to accommodate it.

The revised sediment estimates are still within the capacities of mitigation projects built to manage sediment released by the dam removal project, Maynes said, from higher levees to water-treatment plants.

But there already have been surprises, and there are likely to be more, as the unprecedented project takes its course.

Dam removal has been put on hold for at least a month, beginning in January, as contractors work to modify an industrial water treatment plant built as part of the dam removal project to protect the quality of water used in a new tribal hatchery, a state salmon rearing channel, and water used by a Port Angeles paper plant.

Intake screens at the plant became clogged with leaves and sticks transported by the river after winter storms, leading to a cascade of malfunctions, including settling tanks that had to be repeatedly cleaned out, Maynes said.

None of the malfunctions was serious enough to harm the facilities the water-treatment plant protects, or to disrupt their function, Maynes said, although they had to operate with less than the promised quality of water the plant is supposed to deliver.

Work on the plant has meant shutting down removal of Glines Canyon Dam for a month, to avoid disturbing more sediment until the plant is back in satisfactory shape, Maynes said. She did not have an estimate on the cost of the work, or know who would pay for it.

Elwha Dam was built in 1910 in the lower river without fish passage, and was removed last March. Glines Canyon Dam, built about eight miles upriver, is about two-thirds gone.

Contractors should be able to stay on schedule even with the delay this month and get that dam out by summer, Maynes predicted.

The dam-removal project had no peer in terms of the amount of sediment that scientists are managing as the dams come down even before the estimate got bigger.

At 210 feet tall, Glines Canyon Dam also is the tallest dam ever removed.

Already the project has opened about 25 miles of spawning habitat for fish. Pinks, chinook, steelhead and coho have all been spotted utilizing every portion of the new habitat, from side channels to the main stem.

Biologists counted 203 redds dug above the former Elwha Dam site after more than 500 adult chinook surged into upriver with the removal of Elwha Dam. And while scientists saw salmon carcasses along the river last year, this year they are only seeing pieces.

Animals are eating the fish coming back to the river, from otters to bears to water ouzels, in the beginning of a broader, ecosystem recovery, said John McMillan, a biologist frequently surveying the river for NOAA fisheries.

The survival of thousands of new plantings in the emerging landscape of the former lake beds has also been much better than predicted, with only about 8 percent of the plants dying off in the first year, despite a very dry summer, said Joshua Chenoweth, who is helping to lead the replanting effort.

Sediment the plants are growing in retained moisture from the former lakes well into September, Chenoweth said, giving new plants a good start.

Lynda V. Mapes: 206-464-2736 or lmapes@seattletimes.com. On Twitter @lyndavmapes.



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