Dam repairs mean it's time for trade-offs

Managing Dworshak Dam to strike a balance between flood control, salmon and steelhead and water quality is always tricky but will become even more so in the coming months because of work on a hydroelectric turbine there.

Contractors will spend the winter and a good chunk of the spring refurbishing the No. 3 turbine, the largest of three at the dam, making it unavailable during the time period that the U.S. Army Corps of Engineers usually releases water to make room for spring runoff.

That means the agency will have to release more water from the dam's spillways, a process that raises levels of total dissolved gas in the North Fork of the Clearwater River below the dam and also in the Clearwater River that the North Fork joins about a mile below the dam.

If gas levels become too high, it can harm fish and other aquatic organisms and also exceed state water quality standards. Measures designed to ensure gas levels aren't exceeded, such as drawing the reservoir down sooner or lower than normal, could threaten the chances it will refill, presenting the corps and its partners with difficult choices.

"I wouldn't be surprised if it doesn't come down to trade-offs, decisions that will be made in real time as we go through the season," said Stephen Hall, water manager for the corps' Walla Walla District.

Normal procedure

As snow piles up in the mountains above the North Fork Clearwater River, the corps calculates how much space will be needed in Dworshak Reservoir to capture spring flows and ensure flooding doesn't occur in places like Portland and Vancouver, Wash. When it determines that more space is needed, the corps releases water from the reservoir. The preferred method of doing so is to run it through turbines at the dam.

The powerhouse has a capacity of just more than 10,000 cubic feet per second. If the corps needs to release water at a higher rate, as it sometimes does, it also uses the dam's spillways. When necessary to control flooding, the corps has a waiver from the Idaho Department of Environmental Quality, allowing it to exceed gas standards.

Once enough space is made, the corps begins to refill the reservoir while releasing enough to provide migrating juvenile salmon and steelhead sufficient flows to reach the ocean.

The corps has a target to reach full pool at the reservoir by July 1. Soon after that date, the agency again starts to release water, this time to cool the lower Snake River, an operation critical to threatened and endangered salmon and steelhead.

Because of the work on Unit 3, the dam will only be able to release about 4,800 cfs from the powerhouse as it makes room for flood water, meaning it will have to spill more, which increases the likelihood that gas levels will exceed state standards and perhaps harm fish in the river and those in Dworshak National Fish Hatchery.

Priorities

Idaho is urging the corps to do its best to ensure the reservoir refills so the normal volume of cooling is again available in the summer of 2017. But the state also wants the corps to do its best to not exceed total dissolved gas standards.

Balancing those competing priorities is difficult. To ensure the reservoir has its best chance of refilling, the corps would try to only release as much water as needed to control downstream flooding. However, that comes with risk. If weather such as a rain on snow event were to occur, the agency would have to quickly make room in the reservoir by spilling water. Spilling a large volume of water would cause high levels of dissolved gas.

To prevent exceeding water quality standards, the corps could slowly release more water during the winter than it believes might be held in the mountain snowpack. But that would jeopardize the odds of refilling or leave less water available during spring runoff when some juvenile fish need it to push them toward the Pacific Ocean.

"You could end up digging a hole deeper than you need to there, if you don't get all the snowpack you are forecasting," said Pete Hassemer of the Idaho Department of Fish and Game.

The Nez Perce Tribe is also worried that a rapid release of a high volume of water could result in gas levels that would harm juvenile fall chinook below the dam, juvenile salmon and steelhead in Dworshak Hatchery and potentially other juvenile fish at the Nez Perce Tribal Hatchery at Cherrylane or at an acclimation site near Peck.

"We would be concerned of over exposing those fish to chronic elevated total dissolved gas concentrations," said David Johnson, director of Nez Perce Tribal Fisheries.

Testing gas levels

Last spring, the corps ran two days of tests. It spilled enough water during the first day to produce gas levels at 115 percent of the state standard. On the second day, it took gas levels up to 120 percent. During both operations, the agency measured the ability of degassers in Dworshak Hatchery and natural processes in the river, to mitigate for the elevated gas levels.

"It helped us identify what was going on in the system and what was happening in the river and that will help us make informed decision as we move forward," Hall said.

Johnson said communication between the corps and regional fish managers will be key to making sure the agencies strike the right balance.

"We have asked the corps to meet with fish managers on a regular basis and determine what modeling is telling us and try to balance all of those concerns - not gassing up the river too much, providing flow for spring migrants and making sure we refill the reservoir," he said.

Hall said that is what the agency plans to do.

"As we move through the season, we are going to be bending over backwards to try to do the communication as best as possible."

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Barker may be contacted at ebarker@lmtribune.com or at (208) 848-2273. Follow him on Twitter @ezebarker.

Ken Meyers