Discharge at Dworshak pinched

Flows below Dworshak have been reduced due to a malfunctioning turbine.
Flows below Dworshak have been reduced due to a malfunctioning turbine.

by Columbia Basin Bulletin

OROFINO — A primary source of cool water used to improve Snake River salmon summertime migration conditions was pinched Aug. 15, leaving fish and hydro system management to debate how to make the best out of a bad situation.

Unit 3 at Dworshak Dam – the largest of the three generating units at the dam in terms of water passing capability ‒ went out of service at 3:30 p.m. last Friday resulting in a reduction of outflow from 9,800 cubic feet per second down to 6,500 cfs.

From a fish migration perspective, running as much cool water as possible through the turbines is the preferred discharge method because water going through the turbines stirs up little total dissolved gas (TDG) in the river below while helping improve migration conditions.

Another water release option is spill over the top but that creates more TDG that can threaten fish health.

Federal, state and tribal fish managers asked the U.S. Army Corps of Engineers, which operates the dam, to push up spill to restore outflows to previous levels of about 10,000 cfs in order to increase the probability that water temperatures downstream will be at desired levels.

Such a boost in spill would create more TDG than is allowed under water quality guidelines. But the systems operational request (SOR) signees suggest that the corps seek temporary waivers from the state of Idaho and the Nez Perce tribe that would allow a discharge of up to 10,000 cfs, which is estimated by the corps to produce TDG levels of up to 120 percent. The state standard is 110 percent.

Getting the desired water out and holding down TDG levels to water quality limits might be impossible.

“Something has to give,” said the state of Idaho’s Russ Kiefer.

Dworshak is located in west central Idaho on the North Fork Clearwater River about three miles upstream from its confluence with the Clearwater River, which feeds into the Snake.

Spill was started at the dam following the unit breakdown in order to push as much water through the dam as possible to augment flows for salmon spawners headed up upriver, and for whatever juvenile migrants that might be in the river on their way toward the Pacific Ocean.

Cool waters from Dworshak’s depths are called on late each summer to bring down water temperatures in the Snake River. A goal is to hold water temperatures in Lower Granite’s tailrace and in the fish ladder at or below 68 degrees.

The fall chinook are headed farther up the Snake, the Clearwater and other tributaries. Because of their endangered status, many of the Snake River fish, both fall chinook and steelhead, are the subject of numerous research studies that collect data aimed at tracking their migration status and other information.

The 2014 adult Snake River fall chinook return is expected to be a record.

Current DWR operational data may be found on the following website: http://www.nwd-wc.usace.army.mil/report/projdata.htm#D

The Lower Granite adult trap was shut down earlier this month when temperatures reached 70 degrees because at that temperature migrating salmon and steelhead, as well as other fish, are already being stressed.