In a significant move to safeguard the delicate ecosystem surrounding the Glen Canyon Dam, the US Bureau of Reclamation (USBR) has announced its formal engagement in developing alternative operational strategies. These strategies aim to disrupt the spawning of invasive fish downstream while also considering modifications to the High-Flow Experiment process. These proposed operational changes may extend through the year 2027.
This announcement comes following an environmental assessment conducted by USBR in August 2022, titled "Glen Canyon Dam/Smallmouth Bass Flow Options," which was made available for public comment on February 24. After a thorough analysis of the environmental assessment and the receipt of nearly 7,000 public comments, USBR determined the need for additional scrutiny.
On June 6, the Glen Canyon Dam Adaptive Management Work Group issued a directive for USBR to prepare a supplemental environmental impact statement, in alignment with the December 2016 Glen Canyon Dam Long Term Experimental and Management Plan Record of Decision.
The motivation behind these operational changes arises from declining water levels at Lake Powell, which have led to the epilimnion, the upper layer of the lake where most fish are located, drawing nearer to the water intakes of the Glen Canyon Dam. This shift has increased the likelihood of non-native fish passing through the dam and into the Colorado River. Moreover, the epilimnion, when discharged downstream, raises the river's temperature, creating ideal spawning conditions for the smallmouth bass, a predatory invasive fish species that poses a threat to the federally protected humpback chub and other native fish.
Camille Calimlim Touton, Reclamation Commissioner, stated, "We will continue our work to protect the river and its native species to the best of our ability as we plan our operations of the river. If smallmouth bass continue to spawn and establish below Glen Canyon Dam, there will likely be negative impacts to the humpback chub and other native fish species."
Wayne Pullan, Reclamation Upper Colorado Basin Regional Director, emphasized USBR's obligations under the 2016 Long-Term Experimental and Management Plan Biological Opinion to protect humpback chub. He noted that while humpback chub were recently downlisted from endangered to threatened, the number of sub-adults in the Grand Canyon region has been insufficient, warranting additional conservation efforts to bolster their population by enhancing survival and growth.
Approximately 60,000 humpback chubs are estimated to inhabit the area below Glen Canyon Dam. Some of them are found around the confluence of the Little Colorado River with the Colorado River, approximately 75 miles downstream from the dam, while a larger population resides in the Western Grand Canyon, starting about 175 miles downstream from the dam.
For the scoping process, USBR will assess various reservoir releases, considering temperature and flow velocity combinations. These analyses will encompass a flow option that does not utilize the dam's river outlet works to reduce water temperatures. The goal is to determine the effectiveness of these strategies in disrupting smallmouth bass spawning behaviour and preventing their establishment below the dam. Furthermore, USBR will update the high flow protocol sediment accounting process, incorporating the latest scientific findings.
