In 2024, managers deliberately cooled the Colorado River to protect the Grand Canyon from invasive smallmouth bass. 2 years later, no bass recruitment had been detected, but the effort cost $19.1 million in replacement power

Managing a major river is often a balancing act between protecting ecosystems and maintaining essential infrastructure. At Glen Canyon Dam, officials took an unusual approach in 2024 by deliberately cooling part of the Colorado River to prevent invasive smallmouth bass from reproducing near the Grand Canyon.

The strategy appeared to work biologically. Monitoring detected no smallmouth bass recruitment in 2024, while preliminary results from 2025 also found none, although scientists conducted less field sampling. But the environmental measure came with a major energy cost: $19.1 million in replacement power in 2024 and another $6.5 million in 2025.

Why the river was cooled

Historically, water released below Glen Canyon Dam was generally colder than 57°F, keeping the river below the roughly 60°F temperature associated with smallmouth bass spawning.


Declining levels in Lake Powell changed that protection. Warmer water and fish living near the reservoir's surface moved closer to the dam's intakes, increasing the possibility that smallmouth bass could enter the Colorado River downstream.

The invasive predator poses a particular threat to native fish, including the humpback chub, which was reclassified from endangered to threatened in 2021 but remains federally protected.

How “Cool Mix” worked

The idea behind Cool Mix was to keep river temperatures at or below 15.5°C, or 60°F.

Operators combined water released through the dam's power-generating penstocks with colder water drawn through river outlet works located about 100 feet lower. The total amount of water released from Glen Canyon Dam did not change, but some water bypassed the turbines.

Cool Mix began on July 9, 2024, after river temperatures crossed the biological trigger. The National Park Service later reported that the operation prevented smallmouth bass spawning from the dam to the Little Colorado River confluence, about 76 miles downstream.

The cost of cooling the river

The conservation result came with a substantial electricity cost.

During the 132-day 2024 operation, managers bypassed 892,694 acre-feet of water and lost 372,341 MWh of generation. Replacement-power costs reached $19.1 million.

In 2025, managers shortened the operation and moved the temperature target upstream from river mile 61 to river mile 30. Bypass volume fell to 376,119 acre-feet, lost generation dropped to 145,227 MWh and replacement-power costs declined to $6.5 million.

Acting Reclamation Commissioner David Palumbo said the approach “strikes a good balance between achieving biological objectives and reducing hydropower impacts.”

No recruitment detected, but monitoring was reduced

Preliminary 2025 monitoring recorded 38 smallmouth bass, compared with 254 at a similar point in 2024. Scientists found no evidence of recruitment in either year.

However, researchers cautioned that the 2025 field effort was reduced, making continued monitoring important.

The result does not mean smallmouth bass have disappeared. Instead, it suggests that manipulating river temperature may prevent or disrupt reproduction and give managers more time to develop other control measures.

Why Cool Mix ends in 2026

The Bureau of Reclamation decided not to implement Cool Mix flows in 2026, citing significant hydropower-generation concerns under already strained conditions.

The agency plans to focus instead on rapid-response fish removal, modifying more backwaters, testing new control tools and pursuing longer-lasting structural solutions.

One example came in 2025, when Reclamation and the National Park Service reshaped a warm slough about 3.5 miles below the dam. Water movement through the site increased from roughly zero to about one foot per second, making the habitat less favorable for bass spawning.

A costly conservation lesson

Cool Mix showed that temperature can be used as a barrier against an invasive predator. It also revealed the financial consequences of using dam operations for ecological protection.

Replacement-power costs reached a combined $25.6 million across 2024 and 2025. Lower hydropower generation can also affect the Basin Fund, which supports operation, maintenance and infrastructure replacement across the Colorado River Storage Project.

The experiment therefore leaves managers with a difficult question: how can they protect native fish without repeatedly sacrificing large amounts of hydropower?

For now, the answer is shifting toward targeted fish removal, habitat changes and structural solutions. The latest decision shows that an approach can succeed biologically while still proving too expensive to repeat every year.

Source: ECONEWS

FAQs:

Q1. Why was the Colorado River cooled?

Managers cooled the river to make conditions less suitable for invasive smallmouth bass reproduction. The goal was to protect native fish in the Colorado River.

Q2. What is Cool Mix?

“Cool Mix” was a dam-management strategy that combined colder and warmer water releases. The mixture was adjusted to keep downstream river temperatures near or below 60°F.