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More than 10 years after they could not be found in the White River, tens of thousands of pink salmon are making their way upstream near Buckley, Pierce County, as part of an eight-week spawning run.

The fish, which return every other year in increasingly large numbers, swim to a diversion dam that guides them into a trap, from which they’re hauled by truck about 12 miles around the Mud Mountain Dam to continue their seasonal migration.

But once they arrive at the century-old diversion structure, the fish face a life-threatening obstacle. Officials from the Puyallup Indian Tribe predict more than 100,000 salmon will die this year trying to make the journey. They say a new diversion dam is needed to protect the fish from injury and death.

Replacement also would increase efficiency and lower maintenance costs for the Army Corps of Engineers, which partners with Cascade Water Alliance to operate the dam.

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The $38 million project, proposed by the corps, is inching its way through the federal budget-review process. Dan Johnson, operations project manager for the corps, said even if everything moves forward smoothly, it could be about eight years before a new diversion dam is fully operational.

“It’s taxpayer dollars, and we want to make sure what we do is closest to the right solution as we can possibly get,” he said.

Fred Goetz, a fish biologist with the corps, said the longer it takes to replace the mostly wooden dam with a concrete structure, the more likely it could fail altogether. That would disrupt the natural migration of the fish, he said.

“We could see a much bigger problem than we have now,” Goetz said.

The pinks have been returning to the White River in growing numbers. In 2011, about 622,000 were trapped and transported — up from 540,000 in 2009 and 130,000 in 2007, Goetz said. Officials are planning for even more salmon this year than they saw two years ago.

The diversion dam lies downstream from the Mud Mountain Dam. Built in the 1940s, Mud Mountain provides flood control for about 400,000 residents from Tacoma to Enumclaw, as well as local tribes and major arterials such as Interstate 5 and Highway 410.

Mud Mountain is an earthen structure, constructed with rock and soil, and has two tunnels that allow river water to pass through.

Johnson said the tunnels are not suitable for fish to travel through because the water moves too fast. Salmon typically swim in flows of about 5 feet per second; the water under Mud Mountain flows about 100 feet per second, he said.

The trapping done at the diversion dam gets salmon safely to the other side of Mud Mountain Dam. But the diversion dam, built in 1912, poses two problems: the inability to control currents, and unsafe surfaces for the salmon near the trap.

Crews cannot easily modify or regulate water flows to guide the salmon into the trap for transport, Johnson said.

Meanwhile, the White River carries debris from mountain runoff to the dam, damaging the once smooth wooden apron that the salmon hit while making their way to the trap.

“It makes for a condition that’s much more challenging for the fish than it needs to be,” Johnson said.

Building a new diversion dam with the ability to control currents would attract more fish into the trap, Johnson said.

“(The salmon) will find the flow that looks the most attractive,” he said.

Goetz said it’s hard to pinpoint exactly how many fish are injured or killed now.

Tribal officials have estimated 100,000 to 200,000 salmon will die this year before they can be captured and transported. But corps officials say it’s hard to know the accuracy of that assessment, in part because of poor visibility into the river.

“We are working with (the Puyallup Tribe) to conduct helicopter surveys of the White River where we may get an estimate of the number of salmon that are in the river and near the trap over the course of the migration season,” Goetz said.

Debris also necessitates frequent repairs to the diversion dam. Goetz said it would be more cost-effective to replace the entire structure soon rather than continue to repair the old one every other year.

Tanya King, corps spokeswoman, said the agency continues to find new ways to use the decades-old equipment.

“We’ve made a lot of innovations,” she said.

But demands have risen with the increase in pink salmon, and Goetz said delaying replacement of the diversion dam risks failure of an important structure that aids the region environmentally and commercially.

“(The fish) regulate our river system in the state,” he said. “The whole ecosystem benefits from a healthy salmon population.”

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