Salt Lake City, Utah
Released On: October 04, 2005
The combination of the record five-year drought (1999-2004) and above average runoff into Lake Powell during the spring and summer of 2005 have put large amounts of sediment and organic matter from the reservoir's delta into the lake. These substances consume oxygen as they decay which, in turn, results in declining dissolved oxygen concentrations in the Colorado River immediately below the dam. While this phenomenon occurs to some degree every summer and fall, oxygen levels this year have reached a level that is causing concern for the health of the trout fishery in the river below the dam. The trout fishery below the dam is administered by Arizona Game and Fish.
As the river moves downstream, the water cascades through rapids, quickly raising the dissolved oxygen levels, so there is less impact or concern for endangered native fish in the reaches of the Grand Canyon. However, there may be some impact to the non-native trout fishery as fish remaining near the dam become lethargic or temporarily move downstream to more oxygen-rich waters.
Reclamation has been conducting short-term experiments to evaluate the relationship between dam operations and dissolved oxygen concentrations. Researchers from the Grand Canyon Monitoring and Research Center are trying to determine if low releases spread among varying numbers of generating units will inject more oxygen into the water. The turbines draw some air during operation, especially when operated at very low output levels. Various operational combinations of three to five units have been tested.
While some changes in dissolved oxygen have been noted as a result of the experiments, a concern also has emerged. Operating the units at very low levels is both inefficient and damaging. Because the generating units were designed to operate efficiently only at higher releases, normal operations for low flows would be met by using fewer units. If the units are operated at too low a level, the risk of damage to the turbines increases significantly. While units have been operated that low in the past for various system and emergency reasons, every effort possible has been made to minimize the length of such operations to protect the generating units. Additionally, such "rough operation" also is inefficient in both the generation of power and the use of water.
The results of studies conducted over a previous weekend indicate the turbines can contribute to improved dissolved oxygen concentrations in the water being released from the reservoir. The studies remain inconclusive concerning the best balance of turbine operations, or whether other alternatives should be developed.
The issue of declining dissolved oxygen will self-correct itself later in October or early November when the water in Lake Powell "turns over." This happens when the cooling air temperatures and seasonal winds mix reservoir waters, essentially causing the upper, better oxygenated water to mix with the lower level water that contains low oxygen.
Reclamation's operations are conducted in accordance with the 1996 Record of Decision following the Operation of Glen Canyon Dam Environmental Impact Statement. They comply with provisions of the Grand Canyon Protection Act and applicable elements of the overall Law of the Colorado River that deals with the entire Colorado River system. Additionally, Reclamation's operations are in compliance with the Endangered Species Act. Trout, as an introduced non-native fishery, are managed in the 16-mile stretch from the Dam to Lee's Ferry by Arizona Game and Fish.
Reclamation will continue to work with the Grand Canyon Monitoring and Research Center, Arizona Game and Fish, and Western Area Power Administration to seek solutions.
DOI | Recreation.gov | USA.gov
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