Glen Canyon Dam managers will flush cold water from deep in Lake Powell into the Colorado River to help prevent invasive smallmouth bass from colonizing the Grand Canyon, federal officials announced Wednesday.
The hope is that cooling the river below ideal bass-spawning temperatures could thwart a feared explosion in the predator’s numbers, buying time for bass-removal efforts to save federally protected humpback chubs throughout the canyon.
When biologists call for cold water, the U.S. Bureau of Reclamation releases it through the dam’s bypass tunnels, mixing it with the hydropower plant’s warmer surface water to create a “cool mix” in the river below.
Reclamation officials said they will use the cool mix to minimize bass spawning this summer while keeping other options available for future years. The plan is to spike the river with cold water when it reaches 60 degrees Fahrenheit, a plan favored by the U.S. Fish and Wildlife Service.
“The service endorses this action because the science indicates that the risk of smallmouth bass establishment is reduced through cold water discharges intended to disrupt their spawning,” the agency charged with recovering threatened native fish concluded in its biological opinion about the cool mix plan. “Under conditions where smallmouth bass or other warm-water nonnative predatory species become established in the Grand Canyon, the predation threats to humpback chub become greater.”
The plan does not increase the amount of water that the government will send downstream and into Lake Mead during the year, though it will come with the cost of some lost hydroelectricity production.
The stretch of river flowing from Glen Canyon Dam through the Grand Canyon and into Lake Mead is a native fish haven that allowed chubs to rebound to the point of being removed from the endangered species list. Then, in 2022, biologists discovered hundreds of young smallmouth bass in the stretch of the river between the dam and Lees Ferry, portending a potentially catastrophic invasion if those fish were able to reproduce and stock the entire canyon with bass.
The fish are thought to have originated from parents that once swam in Lake Powell and then survived a trip through the dam’s turbines before spawning in a warm backwater slough a few miles downstream.
Non-native species:As Lake Powell shrinks, voracious smallmouth bass are staging for a Grand Canyon invasion
The U.S. Interior Department is studying potential river and dam alterations to block further migration from Powell and to discourage spawning in the slough, possibly including an anchored net above the dam and a new channel to allow cooler river water to flow through the slough. Meantime, federal biologists have worked to remove the fish and occasionally dose the slough with a fish-killing agent.
The cool mix releases will draw water through the dam bypass tunnels, which are 100 feet below the hydropower intakes and, therefore, draw from the colder part of the lake.
Smallmouth bass tend to reach a mature length of 200 millimeters and start spawning around age 3, biologist Drew Eppehimer of the U.S. Geological Survey's Grand Canyon Monitoring and Research Center told members of the Glen Canyon Dam Adaptive Management Program’sadvisory working group this spring. That could mean that those young fish that emerged in the river during 2022 could spawn and trigger a much larger invasion within the next year.
Bass growth also depends on warmer temperatures, though, so it’s possible the fish won’t all reach maturity soon. Last year’s big snowmelt and infusion of water into Lake Powell may prove fortuitous. It naturally lowered the temperature of water flowing past the dam, Eppehimer said.
“But we do have the slough,” he said. “We do have various backwater habitats that can warm up faster.”
Humpback chubs evolved to survive both cold rushes of springtime meltwater and warm summers of lower flows. The current need to cool summer flows is a symptom the dam’s profound alteration of the ecosystem. Unnaturally cool summer releases at one time were considered a threat to the native fish, but they also seemed to prevent warm-water predators like bass from establishing populations.
After two decades of drought, Lake Powell’s decline brought warmer surface water closer to the intakes and contributed to the river’s warming. Managers now seek to reverse some of the warming by drawing from deeper in the reservoir.
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The hope is that cooling the river below ideal bass-spawning temperatures could thwart a feared explosion in the predator’s numbers, buying time for bass-removal efforts to save federally protected humpback chubs throughout the canyon.
When biologists call for cold water, the U.S. Bureau of Reclamation releases it through the dam’s bypass tunnels, mixing it with the hydropower plant’s warmer surface water to create a “cool mix” in the river below.
Reclamation officials said they will use the cool mix to minimize bass spawning this summer while keeping other options available for future years. The plan is to spike the river with cold water when it reaches 60 degrees Fahrenheit, a plan favored by the U.S. Fish and Wildlife Service.
“The service endorses this action because the science indicates that the risk of smallmouth bass establishment is reduced through cold water discharges intended to disrupt their spawning,” the agency charged with recovering threatened native fish concluded in its biological opinion about the cool mix plan. “Under conditions where smallmouth bass or other warm-water nonnative predatory species become established in the Grand Canyon, the predation threats to humpback chub become greater.”
The plan does not increase the amount of water that the government will send downstream and into Lake Mead during the year, though it will come with the cost of some lost hydroelectricity production.
The stretch of river flowing from Glen Canyon Dam through the Grand Canyon and into Lake Mead is a native fish haven that allowed chubs to rebound to the point of being removed from the endangered species list. Then, in 2022, biologists discovered hundreds of young smallmouth bass in the stretch of the river between the dam and Lees Ferry, portending a potentially catastrophic invasion if those fish were able to reproduce and stock the entire canyon with bass.
The fish are thought to have originated from parents that once swam in Lake Powell and then survived a trip through the dam’s turbines before spawning in a warm backwater slough a few miles downstream.
Non-native species:As Lake Powell shrinks, voracious smallmouth bass are staging for a Grand Canyon invasion
The U.S. Interior Department is studying potential river and dam alterations to block further migration from Powell and to discourage spawning in the slough, possibly including an anchored net above the dam and a new channel to allow cooler river water to flow through the slough. Meantime, federal biologists have worked to remove the fish and occasionally dose the slough with a fish-killing agent.
The cool mix releases will draw water through the dam bypass tunnels, which are 100 feet below the hydropower intakes and, therefore, draw from the colder part of the lake.
Smallmouth bass tend to reach a mature length of 200 millimeters and start spawning around age 3, biologist Drew Eppehimer of the U.S. Geological Survey's Grand Canyon Monitoring and Research Center told members of the Glen Canyon Dam Adaptive Management Program’sadvisory working group this spring. That could mean that those young fish that emerged in the river during 2022 could spawn and trigger a much larger invasion within the next year.
Bass growth also depends on warmer temperatures, though, so it’s possible the fish won’t all reach maturity soon. Last year’s big snowmelt and infusion of water into Lake Powell may prove fortuitous. It naturally lowered the temperature of water flowing past the dam, Eppehimer said.
“But we do have the slough,” he said. “We do have various backwater habitats that can warm up faster.”
Humpback chubs evolved to survive both cold rushes of springtime meltwater and warm summers of lower flows. The current need to cool summer flows is a symptom the dam’s profound alteration of the ecosystem. Unnaturally cool summer releases at one time were considered a threat to the native fish, but they also seemed to prevent warm-water predators like bass from establishing populations.
After two decades of drought, Lake Powell’s decline brought warmer surface water closer to the intakes and contributed to the river’s warming. Managers now seek to reverse some of the warming by drawing from deeper in the reservoir.
Link
