Environmental

NPS taking steps to protect native fish below Glen Canyon Dam

NPS taking steps to protect native fish below Glen Canyon Dam

The National Park Service (NPS) will begin efforts to remove invasive smallmouth bass and green sunfish from the Colorado River Slough below Glen Canyon Dam in the Glen Canyon National Recreation Area.

These non-native predatory fish were discovered breeding in areas where they have not previously been found in large numbers, threatening the recovery of humpback chub, which is listed as threatened under the Endangered Species Act. The humpback chub was downlisted from endangered in October 2021, in part based upon the success of the species in the Colorado River through the Grand Canyon. Because smallmouth bass are aggressive predators, failure to quickly control their population in the Colorado River above the Grand Canyon could likely lead to the demise of the humpback chub.

Water elevations in Lake Powell, which is impounded by Glen Canyon Dam, dropped to historically low levels in April 2023 and have had indirect adverse impacts on the federally listed and native populations of fish below the dam. The dam provides water to a 1,320 MW hydro powerhouse. Smallmouth bass and green sunfish live in the warmer levels of the lake’s waters, closer to the surface. As warmer water reaches the dam’s water intakes, the nonnative predatory fish have a greater chance of passing through the dam alive. This increases the threats to the native fish in the Grand Canyon and Glen Canyon’s downstream rainbow trout fishery.

Threats to native fish are increasing due to the warmer temperatures of water passing through the dam and related increased river temperatures below the dam, which increase non-native fish spawning and reproduction and allow for predation on native fish populations downriver. Juvenile smallmouth bass were found in the Colorado River below Glen Canyon Dam on June 30, 2022, and during July and August in 2023, underscoring the urgency of this emergent issue.

Beginning Aug. 26, 2023, NPS staff, will deploy the EPA-approved fish piscicide rotenone to kill these invasive, predatory fish. The treatment will be planned and conducted to minimize exposure to rotenone and protect the health and safety of humans, the environment, desirable fish species and livestock. Before the treatment, tests will be conducted to determine the minimum effective concentration of rotenone for use. Rotenone is a natural substance derived from plant roots. It has been effective at eliminating localized populations of smallmouth bass, including in the Colorado River Basin. An impermeable fabric barrier will be installed at the mouth of the slough for the duration of the treatments to minimize the exchange of water with the river.

Potassium permanganate, a chemical used to purify drinking water, will be added to the slough just above the fabric barrier to neutralize the rotenone. Should any rotenone enter the main channel, it will be diluted to concentrations that are insignificant to wildlife or humans, due to the volume of flow in the Colorado River. To facilitate the treatment, the Bureau of Reclamation will hold Glen Canyon Dam releases at about 10,500 cubic feet per second for 72 hours, noon Aug. 25 through noon Aug. 28.