River Operators Approveby Mike O'Bryant
The Technical Management Team this week approved summer operations at Dworshak Dam on the North Fork of the Clearwater River that will save 200,000 acre feet of Dworshak's stored water in order to augment and cool flows at Lower Granite Dam on the Snake River through mid-September.
The NOAA Fisheries 2000 Biological Opinion for the Columbia River hydroelectric system calls for using Dworshak Reservoir's water through Aug. 31 as a way to keep water temperatures below 68 degrees Fahrenheit at Lower Granite Dam's tailrace and to arrive at a reservoir elevation of 1,520 feet by that date.
The plan approved by TMT this week reserves Dworshak's cooling waters for two additional weeks. It calls for a reservoir elevation of 1,535 feet by Aug. 31 and to arrive at the 1,520-foot elevation in mid-September.
The approved summer operation is very similar to the plan proposed in past years by the Nez Perce Tribe and the Idaho Department of Fish and Game, which is designed to reserve some Dworshak water for late migrating chinook salmon. However, TMT salmon managers also acknowledged that summer conditions may change and that if tailwater temperatures at Lower Granite Dam begin to approach 68 degrees that could prompt a change to the operation.
"1,535 feet is the target (on Aug. 31), but as we go through the season we'll have to be cognizant of the temperature (in the tailrace) and how much water is still available," said Dave Statler of the Nez Perce Tribe.
According to the systems operational request submitted this week by salmon managers, saving some Dworshak water for September supports a "recently documented life history pattern of yearling fall chinook. Particularly, scale analysis indicates that 40 percent - 50 percent of adult fall chinook returning to Lower Granite Dam were yearling migrants. This suggests that value exists in assuring environmental conditions be maintained into September for the benefit of this life history pattern."
While 90 percent of Snake River wild subyearling chinook and 97 percent of hatchery sub-yearling chinook have passed Lower Granite Dam by Aug. 31, limited information about the Clearwater River population of chinook indicates they pass the dam later, the SOR said.
Tailwater temperatures at Lower Granite Dam were rising towards 68 degrees last week, prompting the U.S. Army Corps of Engineers, which operates the lower Snake River dams and Dworshak Dam, to increase flows from Dworshak to 9,800 cubic feet per second, with release water temperatures of 45 degrees. The tailrace temperature at Lower Granite Dam on July 2 was 67.8 degrees, but an increase in Dworshak flows along with lower air temperatures, resulted in tailwater temperature dropping to 66.9 degrees by July 5.
Outflows at Dworshak Dam will rise to about 12 kcfs Monday, July 12, according to the SOR. Full powerhouse is 9.8 kcfs, so Corps will spill about 2 kcfs with this new operation. Unless weather and temperature conditions change, that flow will be maintained through Aug. 8 and flows will decline to full powerhouse on Aug. 9 and continue through Aug. 31 when the reservoir level is expected to be 1,535 feet. The additional water will be used to keep Snake River flows up and cooler through mid-September, arriving at a reservoir elevation of 1,520 feet in mid-September.
Technical Management Team: www.nwd-wc.usace.army.mil/TMT/index.html
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