LAS VEGAS (KLAS) — The federal government has released a 584-page document detailing possible solutions to an invasive species that poses “an unacceptable risk” to another fish that’s listed as threatened.
When it’s all said and done, officials want to give smallmouth bass a cold shower — or a cool bath, anyway — to discourage them from reproducing.
Make no mistake, the U.S. Bureau of Reclamation’s plan is a detailed “Cool Mix” strategy on how to reduce the threat to the humpback chub in the Colorado River below Glen Canyon Dam. Smallmouth bass are voracious predators, and they’ve started to establish populations below the dam where the chub is struggling to survive. Biologists say the bass will feed on the chub, their eggs, and pretty much anything else that will fit in its mouth.
It’s an urgent situation for water managers, and Reclamation intends to begin implementing solutions this summer and continue through 2027. A 30-day public comment period begins after the document released today is
A “Cool Mix” would reduce the river temperature, and has a wider environmental impact, churning up sand from the river bottom and carrying it downstream to build up beaches that are used by rafters and others in the Grand Canyon.
Smallmouth bass have been found around river mile 16, and Reclamation’s proposal would target river conditions from Lee’s Ferry to the confluence with the Little Colorado River (river mile 61).
This all started as Lake Powell dropped significantly in recent years. Smallmouth bass there are only near the surface, where they thrive in warm water. The penstocks that carry water through the dam to hydropower turbines were far below the surface in colder water, and bass never went through the dam. The drought that began in the year 2000 changed all that and the water pipes are now closer to the surface of the water where the bass live.
The warmer water near the surface of the lake is also going downstream, creating conditions that are better for smallmouth bass once they get through the dam.
And with lower lake levels, there was more sediment in the water.
Reclamation has already been working to disrupt bass reproduction, scheduling “High Flow Experiment” (HFE) releases that sent water jetting out of Glen Canyon Dam through an alternative set of pipes called the River Outlet Works — a set of four pipes, each with a capacity of 3,750 cubic feet per second (cfs). Those pipes draw water from lower — colder — levels of the lake.
But this year, Reclamation revealed that River Outlet Works is showing serious signs of wear and tear. Damage discovered after the HFE release in 2023 led Reclamation to forego the annual blast of water. Last year’s 3-day HFE carried enough water downriver to raise the level of Lake Mead by more than 2½ feet.
Reclamation is assessing proposals to coat the inside of the River Outlet Works with a protective layer, a project that could take quite a bit of time.
The HFE releases have been effective, but they could further damage the pipes. Engineers have set a release limit of 3,150 cfs through each of the pipes, and Reclamation continues to look at other methods of disrupting bass as spawning season nears.
These changing conditions have led to Reclamation recommending a “Cool Mix” option as the preferred method for disrupting the smallmouth bass below the dam. If needed, Cool Mix strategy would be activated based on water temperatures in the river below Glen Canyon Dam. Daily water temperatures measured at river mile 61 would have to go above 60˚F for three consecutive days.
“We still have time to prevent smallmouth bass from becoming fully established below the dam, but that’s only if we have a variety of tools available to us,” Reclamation Upper Colorado Basin Regional Director Wayne Pullan said. “Recognizing the ecological threat that smallmouth bass pose on the Colorado River downstream of Glen Canyon Dam, it is important that we have the ability, by adjusting the timing and other features of flows from the dam, to disrupt smallmouth bass spawning this season.”
The Cool Mix alternative is described here: “The Cool Mix Alternative would involve strategic water releases from both the penstocks and river outlet works to maintain a daily average water temperature below 15.5˚C (60˚F) from below the dam to either below Lees Ferry (river mile 15) or the Little Colorado River (river mile 61) (USGS 2022). The quantity of water released through the river outlet works would be determined by predicted temperatures at the river outlet works and penstocks during the flow, ensuring the minimum necessary release to meet the water temperature goal.”
“The goal is to interrupt smallmouth bass spawning by reducing water temperature below the level when smallmouth bass initiate spawning.”
These releases would vary throughout the year depending on temperature conditions.
The document released today also indicates Reclamation continues to analyze how the releases affect sediments flowing down the Grand Canyon.
Other alternatives detailed in the document include “cold shock” and “flow spike” alternatives. There’s also an alternative to release less water, which would affect deep-water pools more than the other strategies.
The LTEMP (Long-Term Experimental and Management Plan) modifications released Wednesday also cite the federal government’s intention to manage dams on the Colorado River to protect Lake Powell’s level to make sure it doesn’t go below 3,490 feet surface elevation. That decision will certainly affect how much water is stored in Lake Mead, America’s largest reservoir, as well as reservoirs above Lake Powell in Colorado, Utah and Wyoming.
As reservoir levels drop, wildlife and water supply issues will be entwined as never before along the Colorado River. Needs to protect species will conflict with contractual obligations to deliver water to states and tribes. And recreation will also be affected, as it has already when Lake Mead and other recreation areas make adjustments as water levels drop.
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