A wetter than normal springtime has served to wipe away most of the last remnants of snowpack above west-central Idaho’s Dworshak Dam but has also served to fill the valued reservoir of water well in advance of the annual launch of flow augmentation for migrating salmon and steelhead.
As of Monday the reservoir had risen to within a half foot of full pool, 1,600 feet elevation. Dworshak is now being operated in a combined spill-power generation mode aimed at maintaining the reservoir while holding within the state prescribed cap – 110 percent – for the creation of total dissolved gas in the water immediately below the hydro project. Elevated TDG can be harmful to fish.
Dworshak backs up the North Fork of the Clearwater River, which flows into the Clearwater, the Snake and then the Columbia River. Those rivers hold both hatchery fish and wild stocks that are protected under the Endangered Species Act.
Recent rain events have resulted in observed conditions that exceed the current water supply forecast, according to a statement released by the U.S. Army Corps of Engineers, which operates the dam. Therefore, the schedule for Dworshak releases has been adjusted to account for this increased forecast.
Based on current forecasts and modeling results, the project was to be operated through the end of this week at full powerhouse (approximately 9,500 cubic feet per second outflow) with spill up to the TDG cap ( about5.0 kcfs) for flows in excess of powerhouse capacity.
On or about June 30, when inflows decrease sufficiently, the project will reduce discharges to approximately 7.3 kcfs outflow (using two of the dam powerhouse’s generating units) and hold that outflow through approximately July 9.
The reservoir will be maintained at a consistent elevation over the July 4th holiday to provide safe recreation conditions.
On or about July 10, outflows will increase for summer flow augmentation operations. The reservoir’s cool water stored at depth is tapped in late summer both to boost flows for migrating salmon and steelhead and to help bring down temperatures in the lower Snake.
According to the Corps the schedule will be adjusted as necessary based on changing forecasts and conditions, which can change from day-to-day during the freshet.
NOAA Fisheries’ Federal Columbia River Power System biological opinion calls for augmentation flows that draw the reservoir down by 65 feed by the end of August to 1,535 and then to 1,520 by the end of September. The BiOp judges whether the FCRPS, of which Dworshak is a part, jeopardizes the existence of listed fish and outlines actions designed to improve survival.