rior to construction of Glen Canyon Dam, the Colorado River would warm seasonally from near freezing to about 85°F. Since construction of the dam, cold water (about 50°F) has been drawn year round from the depths of Lake Powell through fixed level intakes and then released downstream. Near the dam, these cold releases are tolerated by the (non-native) trout fishery, but are below optimal. As the water moves further downstream, it warms to about 60°F, but this is not quite warm enough to allow endangered warm water fish (humpback chub) to reproduce in the mainstem of the Colorado River.
In their biological opinion on the operation of Glen Canyon Dam, the U.S. Fish and Wildlife Service recommended that temperature controls at the dam be investigated by Reclamation. Increasing the dam's discharge temperature is believed to be a key element in the recovery of native fish (humpback chub) near the Little Colorado River. Reclamation agreed to study the feasibility of temperature controls in its EIS on the operation of Glen Canyon Dam.
To appreciate the interest in temperature controls, one has only to look upstream at another major dam in the system. Much like Glen Canyon Dam, discharge temperatures from Flaming Gorge Dam were cold and steady before temperature controls were installed in 1978. After the dam was retrofitted with a selective level withdrawal structure, release temperatures in the summer were increased from 50°F to a peak of nearly 70°F.
The addition of temperature controls at Flaming Gorge Dam has had a remarkable impact on the river system below the dam. Trout growth rates immediately below the dam have increased significantly in response to the warmer water. At the same time, native fish are doing better downstream (in the reach near the Yampa River). In their 1981 report on macroinvertebrates and fish populations above the mouth of the Yampa River, Paul Holden and Larry Crist of BIO/WEST concluded that, “The outlet modification of Flaming Gorge was expected to increase downstream water temperatures during spring, summer, and fall periods, which it did. This caused the Green River above the mouth of the Yampa River to follow a more natural yearly temperature regime. This in turn created conditions acceptable to more benthic invertebrate taxa, because [temperature] cues for life history development were present. Increased diversity, due to a more equitable distribution of abundance among taxa, was a significant result.”

They went on to observe that diversity and reproductive success of warm water fish increased near the mouth of the Yampa River while cold water species declined in abundance and predominance. In many ways, the situation and problems at Glen Canyon Dam appear to be remarkably similar to those at Flaming Gorge Dam.
Preliminary work done by Reclamation on temperature controls for Glen Canyon Dam suggests that releases from the dam could be warmed in July, August, and September. During the warmest part of the season, release temperatures might be increased by up to about 18°F (release temperature would be up to about 68°F). Much as they did at Flaming Gorge, these warmer releases are expected to improve growth rates for the cold water sport fishery immediately below the dam. Then, as the water moves downstream to the vicinity of the Little Colorado River, it would warm enough to support recruitment of young native fish. Warmer summer flows may also increase the biodiversity of the invertebrate population by providing seasonal temperature cues, but may cause some potential adverse impacts that need to be investigated. For example, warm water released from the reservoir caused Flaming Gorge Reservoir to cool. Lake Powell is larger and may not have this problem, but if it does, it may impact the lake's forage fish which are sensitive to winter temperatures. Another concern is that warmer water in the lower reaches of the river may allow non-native (warm water) fish to compete with native fish. There may be ways to deal with these problems. These and other potential impacts will be studied in detail over the next several years.
In 1994, Reclamation developed its plans to study the feasibility of retrofitting Glen Canyon Dam's deep, cold water intakes with adjustable level intakes (a selective level withdrawal structure). The potential cost of the facility is estimated at between $60 million and $100 million. Funding to begin working on the evaluation was received from Congress in 1996. Two studies are currently underway, and more work will follow. The two studies include: (1) temperature modeling of the river/reservoir system and (2) a study to look at how warm water releases might impact the productivity of the river below the dam. These studies should be completed in 1997. In 1998, Reclamation plans to begin an environmental assessment of the potential impacts, costs, and benefits of the facility and its operation.

David Trueman, Program Manager
Bureau of Reclamation