Grand Coulee Dam managers are spilling so much water to make room for massive late-season snowmelt that they're unleashing deadly gas bubbles that are killing hundreds of thousands of fish in the Columbia River.

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Grand Coulee Dam managers are spilling so much water to make room for massive late-season snowmelt that they’re unleashing deadly gas bubbles that are killing hundreds of thousands of fish in the Columbia River.

Virtually all of those fish have been farmed steelhead living in net pens owned by Pacific Aquaculture, a fish farm just outside Nespelem in Okanogan County. The company says it may lose $30 million worth in the next few weeks if dam managers don’t find a new way to handle spring runoff.

But wild river life, too, may be at risk, as dissolved nitrogen and other gases in the water rise to levels that can essentially cause aquatic creatures to get the bends.

“They’re basically sterilizing this entire stretch of river,” said John Bielka, Pacific Aquaculture manager. “That’s going to wipe out not only the fish in our farm, but also the bull trout, the lamprey, the sturgeon and every other wild thing.”

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The U.S. Bureau of Reclamation, which runs the dam, said it had no choice but to release all that water, and deadly gases are the result. So much snow is still sitting high in the mountains that workers must drain reservoirs now or risk dangerous flooding later as far downstream as Portland.

Agency officials maintain most wild fish will survive because they can swim deep to escape danger. Farmed fish are trapped in nets at the surface.

But bureau officials concede the gas buildup in coming days likely will get worse before it gets better.

“Everybody’s doing the best they can,” said Charles Hudson, with the Columbia River Intertribal Fish Commission. “In a normal year, this wouldn’t be a big problem. But Mother Nature is running the Columbia River right now.”

The root of the problem: There’s just too much water. With snowpack in many places nearly double normal levels, the bureau is trying to keep reservoirs low so they have storage capacity when it all starts to melt.

“As fast as it’s coming in, we’re letting the water out,” said Scott Ross, deputy power manager for Grand Coulee.

Normally, Ross could release water through turbines or through gates near the top of the dam.

But one turbine is shut down for a major maintenance overhaul, and he’s keeping the reservoir so low that water levels don’t reach the dam’s gates.

That means massive amounts of water are being spilled through tubes near the dam’s bottom, which causes deadly pockets of dissolved gas to build up near the water’s surface. It hasn’t been this bad, Ross said, since at least 1997.

In fact, dam operators are prohibited from allowing dissolved gases to get above 120 percent of normal — “except in a case like this when we are trying to prevent flooding or an emergency,” said Brian Gorman, spokesman for the National Marine Fisheries Service. In recent days those gas levels have hovered around 130 percent of normal and at times have topped 135 percent.

That can cause trauma in fish and other creatures that sometimes proves deadly. But like scuba divers, some can swim deeper to compensate.

“It is clearly worrisome when we have a situation like this,” Gorman said, “but it is the equivalent of a flood that we are trying to control. We are really sort of unable to do much more about it.”

So far this week, scientists have seen only a few instances of trauma or death in wild fish as a result of the dam spill. But in 1997, the Confederated Tribes of the Colville Reservation saw daily fish kills of walleye, sculpin, trout and kokanee when dissolved gas was lower than it’s been this year.

To Hudson, with the tribal fish commission, the message is clear: The greatest harm is to fish farms, which perhaps should not be there at all.

“The Columbia River is not the greatest place to do net-pen rearing of fish, for a variety of reasons. This is just one.”

Bielka and his colleagues couldn’t disagree more.

They have 2.7 million steelhead swimming in 40 net pens, the fish ranging in size from less than a pound to more than 7 pounds. They sell to seafood customers year-round. But the dead fish will probably become fertilizer, Bielka said.

When his staff saw dissolved gas levels rising earlier this week, they sent scuba divers out to start inspecting their net pens.

“We’ve got at least 10,000 dead fish in one, and 25,000 in another,” said Bill Clark, site manager for Pacific Aquaculture. “We’ve easily got hundreds of thousands of dead fish. They get stuck in pockets in the back of the net.”

As flow from the dam increased, Clark had to yank divers because the river was moving so fast — about 2.5 knots — it was unsafe to keep inspecting the damage. But every hour that gas levels remain high, he suspects they lose thousands more fish.

The company has lobbied lawmakers and federal agencies to find other ways to release water, perhaps by asking Canada to hold more back as storage or release it in another way. In one of its last acts before ending its special session, the state Senate on Wednesday unanimously passed a resolution calling on the federal government to find a new spill approach.

“The policy they’re choosing is economically devastating to us, and ecologically devastating to the river,” said Craig Urness, attorney for the fish farm’s parent company, Pacific Seafood.

He and others complain that Grand Coulee managers picked the worst possible time to take a turbine offline.

But Ross said that decision was made months ago precisely because spring runoff is usually over by now.

He and Gorman said it’s indicative of the problems to come, as other dams in the system are forced to start releasing water, too.

“If we were convinced it would provide great benefits without causing undue harm, we would gladly alter our plans,” Ross said. “But in the next few weeks we’ll be spilling so much more it will make this look small.”

Craig Welch: 206-464-2093 or cwelch@seattletimes.com. Staff reporter Lynda V. Mapes contributed to this report.