Management of Lower Deschutes water temperatures has begunJuly 22nd, 2010
July 1 marked the beginning of a new era of managing temperatures on the lower Deschutes. Hydro operators adjusted some controls on the new Round Butte Selective Water Withdrawal Tower (SWW) and the lower Deschutes River grew cooler. They had begun mixing the cold, deep water in Lake Billy Chinook with warmer water on the surface.
Under the new regulatory rules for the Pelton Round Butte Hydroelectric Project, PGE and our partner, the Confederated Tribes of Warm Springs, must adjust the temperature of the discharge water to match what it would be without the dams.
The large water volume within Lake Billy Chinook not only stores water, but also stores the water’s heat or cold. Since the reservoir first filled in late 1964, the hydro project has had one deep outlet from which it discharged cool winter water in the spring time and warmer summer water during the fall. Fall discharges have been warmer than state and tribal water standards. With the new SWW, operators can now withdraw warmer surface water or mix it with cold water taken 240 feet deep.
Warming the water slightly during the spring and early summer will speed up biological activity from what it has been since 1964 during those seasons. Steelhead fry and rainbow trout fry will emerge from their redds (gravel nests with fertilized eggs) about two weeks sooner. Insect hatches will also happen earlier. This will increase the food available to young steelhead, trout and Chinook salmon. Zooplankton in the water from the reservoirs will also increase the food supply for young salmonids rearing in the lower Deschutes.
Upstream from Maupin, juvenile fall Chinook salmon rearing in the lower Deschutes should greatly benefit from the warmer early season temperatures, more abundant food selection and faster growth. Fall Chinook salmon migrate to the ocean the same spring/summer they emerge. Until now, their growth was unnaturally delayed until late summer by the abnormally cool spring water temperatures. Warmer, more natural temperatures will now allow faster growth during spring and earlier emigration when conditions in the lower Columbia river are more suitable for survival.
The SWW was built, in part, to facilitate water temperature management in the river downstream. Very cold, dense water from the Metolius River will accumulate during winter and early spring, while warmer surface water exits Lake Billy Chinook to faciliate fish passage.
Mixing of the accumulated cold water from the bottom intake with warmer surface water starts July 1. Following what is known as Blend 17, about 15 percent of the water discharged into the lower Deschutes near Warm Springs throughout July will be cold bottom water. The amount of cold water in the mix is schedule to increase monthly, reaching 50 percent during October. This shift in temperature back to the natural thermal cycle will mean the temperature of the water discharged at River Mile 100 (just below the Reregulating Dam) in May, June and July will be warmer than in recent years. But in August, September and October, it will be cooler than it has been since 1964.
Temperature targets not based on historic temperatures
It is important to note that the temperature modeling we use to arrive at the temperature targets is not based on some sort of averaging of historic temperatures from before the dams were built. Instead, it uses a calcuation based on the flow and temperate of the three tributaries of Lake Billy Chinook and the average air temperature. This calculation, or regression equation, is designed to discharge temperatures as if the Pelton Round Butte Project were not on the Deschutes. If inflo temperatures change, due to hot or cold weather or other factors, the natural themal potential downsteam of the hydro project will also change.
The associated graph shows the historical temperature problem caused by the delay in the temperature cycle in the lower Deschutes River, and how mixing surface and deep water at the SWW from July through October will shift the temperatures back to, or below, the natural thermal potential (e.g., the temperature that would occur at River Mile 100 if the dams were not present).
Click to view larger water temperature graph and explanation.