Reclamation discontinues cold water releases from Glen Canyon Dam

PAGE, Ariz. - The Bureau of Reclamation, as a result of cooler weather, is stopping releases of colder water from the depths of Lake Powell into the Colorado River downstream of Glen Canyon Dam initially started to disrupt reproduction of smallmouth bass. Reclamation selected the 'cool mix' flow alternative in the Record of Decision for the Glen Canyon Dam Long-Term Experimental and Management Plan Final Supplemental Environmental Impact Statement.

The intermittent release of cool water began July 9, in an effort to maintain a daily average water temperature below 60 in the river down to river mile 61 (near the confluence of the Little Colorado River) where a core population of the threatened humpback chub is located. Cooler water temperatures deter smallmouth bass from spawning and maturing, making them less of a threat to native fish protected under the Endangered Species Act.

Preliminary results indicate the experimental cooler water flows were successful, with zero young-of-year smallmouth bass counted during increased monitoring of the Colorado River below the dam.

"Much of the prior research on smallmouth bass production suggested that these flows would work," said Reclamation Upper Colorado Basin Regional Director Wayne Pullan. "The fact that we did not capture a single young-of-year smallmouth bass during increased monitoring over the course of the summer and early fall is proof that science-based decision making and steadfast collaboration with our partners can yield positive results."

The cool mix flows consist of releasing water from the hydropower penstocks and the river outlet works located deeper within Lake Powell to release colder water downstream. These releases did not change the amount of water released from the dam, only the location from where the water was released. When water is released through the outlets, hydropower is not generated. Reclamation worked closely with the Western Area Power Administration throughout the experiment to mitigate impacts to power production.

"During these cool mix flows, Reclamation released water solely through the power plant when demand for power was at its peak, then released water through both the power plant and the river outlet works when there was a decreased demand for hydropower in the morning hours," said Reclamation Upper Colorado Basin Regional Power Manager Nicholas Williams.

Reclamation will continue to monitor the river in the coming weeks to ensure temperatures remain below the target threshold of 60 to river mile 61. In addition, the National Park Service will continue fish monitoring and counting smallmouth bass throughout the year.

Glen Canyon Dam impounds Lake Powell, the second-largest manmade reservoir in the United States and the largest water storage unit for the Upper Colorado Basin. As Lake Powell's elevation has declined and water released from Glen Canyon Dam has warmed in recent years, warmwater invasive fish such as smallmouth bass are closer to the dam's water intakes and are more likely to pass through the dam and successfully reproduce downstream in the Grand Canyon.

Reclamation continues to work with its partners to develop and implement additional preventative actions to combat nonnative species in the Colorado River. These actions include supporting the National Park Service in chemical and mechanical treatments and possible modifications to a shallow slough area at river mile 12 that is a prime spawning location for smallmouth bass and other invasive predatory fish. Reclamation is considering other methods for preventing or limiting future passage of fish through the dam.