At the request of federal, state and tribal salmon managers, the U.S. Army Corps of Engineers this week throttled up discharges from east-central Idaho’s Dworshak Dam as a hedge against what’s to come and to increase flows for juvenile salmon swimming toward the Pacific Ocean.
The Corps’ Dworshak Dam had been flushing water at a high rate by running its powerhouse at full capacity of more than 10,000 cubic feet percent and spilling as much as 14 kcfs since the beginning of month to get the reservoir down to its prescribed flood control elevation of 1,451.4 feet by April 15.
With that accomplished, dam operators settled into a refill mode, dropping total outflows to as low as 5 kcfs during the next five days with a target of reaching 1,471.6 elevation by the end of the month. The pool raised by three feet during those five days. Full pool of 1,600 elevation is desired by July 1 for recreation and to provide a full supply of cold water to augment flows downstream for migrating salmon and steelhead during the late summer.
But on Wednesday NOAA Fisheries Paul Wagner told the Technical Management Team that fish managers, meeting as the Fish Passage Advisory Committee, felt that flows should be increased out of Dworshak to boost flows downstream into Lower Granite Dam’s reservoir to as much as 120 kcfs. Inflows Thursday averaged 109.4 kcfs.
TMT, made up of representatives of federal, state and tribal fish and hydro management entities, discuss dam operational strategies that might benefit fish. Dworshak is on the North Fork of the Clearwater River, which flows into the Clearwater and then the Snake River. Lower Granite is on the lower Snake.
Flows in and out of Lower Granite of up to 120 kcfs would not increase in total dissolved gas to levels that are harmful to fish. And by sending more water downstream now, Dworshak clears space in its reservoir for later when the meltdown of an enormous snowpack begins in earnest.
Lighter flows from Dworshak during the height of the runoff would help hold down TDG levels at that point in time. High water causes dams to spill larger volumes. The more spill, the more TDG is created.
The Corps Steve Hall said given the current water supply forecast, there is from 3.1 to 3.5 million acre feet of water available to fill the 2 MAF space in the reservoir. That could change, depending largely on the weather.
An average of 6-7 kcfs per day in inflows would be needed to fill the reservoir by July 1, given the water supply forecasts made at the beginning of April, Hall said.
The Corps agreed to push flows back up to about 12 kcfs with up to 2 kcfs of that discharge coming in the form of spill. The operation began shortly after the TMT meeting on Wednesday. If flows downstream at Lower Granite surpass 120 kcfs, operations at Dworshak would be scaled back.