How Cold is the Colorado River in the Grand Canyon?

 by Amanda Podmore, Grand Canyon Director

The first time I rafted through the Grand Canyon in July 2008, a friend bet me $20 that I wouldn’t jump into the water at Lees Ferry, the boat launch 15 miles downstream of Lake Powell and Glen Canyon Dam. 

The day of the bet, air temperatures at Lees Ferry hovered around 106 degrees. It was an easy but painful $20 that involved lots of shrieks as I plunged into the 53-degree Colorado River.

Fast forward to August 2022 when I launched on another Grand Canyon trip. That day, my friends and I lounged for hours in 67-degree water at Lees Ferry while we rigged our boats and made dinner. It was the perfect water temperature to complement a hot summer day.

But why was the water so much warmer? And how were these warmer waters impacting fish and other aquatic species living in the Colorado River below Glen Canyon Dam?

Low water levels at Lake Powell lead to warm water temperatures downstream of the dam

Ever since Lake Powell started filling in 1963, the Colorado River’s natural, muddy flows have been caught behind the behemoth Glen Canyon Dam. Huge loads of sediment settle out of the water upstream where the river enters Lake Powell. When the reservoir level is high, water is pulled from the depths of the reservoir through the dam’s hydropower penstocks, resulting in clear and very cold water in the Colorado River below.

The past two decades in the Colorado River Basin have been the driest in the past 1,200 years, and between the drought and climate change, water supplies are dwindling. Lake Powell’s water level in August 2022 was near record low due to reduced snowmelt, high aridification (drying of the soil), and high water demand.

Low levels at Lake Powell mean that water at the top of the reservoir, which is warmed by the sun, drops dangerously close to the hydropower penstocks, large tubes where water flows through the dam to generate electricity. Non-native fish, like smallmouth bass, live in this warm layer of water and can pass through the penstocks along with the warm water. If smallmouth bass survive the harrowing journey through the penstocks, which they often do, they find themselves below the dam in a new habitat with a new food source: young humpback chub.

Smallmouth bass threaten native fish species, humpback chub

Humpback chub. U.S. FISH & WILDLIFE SERVICE

Usually, warm water is beneficial to threatened and endangered native fish of the Grand Canyon, like the humpback chub. Humpback chub used to live throughout the Colorado River Basin millions of years ago. Today, 92% of humpback chub live in the Grand Canyon, with a large population taking refuge in the warm and muddy waters of the Little Colorado River. Millions of dollars have been spent on native fish recovery throughout the basin, and by 2021, humpback chub populations rebounded enough to move the species off the endangered species list. It is now considered threatened, but no longer endangered.

Unfortunately, smallmouth bass – predatory fish with big appetites – like warm temperatures too. They were introduced into Lake Powell for sport fishing in 1982, when the reservoir’s water levels were high and the fish stayed in the warm surface waters far above the dam’s penstocks.

Low Lake Powell levels and the sudden appearance of smallmouth bass below Glen Canyon Dam in 2022, along with evidence that they were reproducing, marked an existential threat to the humpback chub’s comeback.

Cool mix flows can disrupt smallmouth bass spawning

In response to this new threat of smallmouth bass gobbling up native fish in the Grand Canyon, scientists, tribes, and conservation groups put their heads together to come up with a creative solution.

Water managers can flush colder water into the Grand Canyon from outlets in the dam 100 feet lower than the penstocks. This cools the river below the temperature at which smallmouth bass will spawn (60.8 degrees Fahrenheit), thus slowing their spread into humpback chub habitat farther downstream.

On July 3, 2024, the U.S. Bureau of Reclamation finalized a plan to allow these experimental cool mix flows. Just six days later, water temperatures at the Little Colorado River in the Grand Canyon exceeded 61 degrees Fahrenheit for three days in a row, triggering the first experimental cool mix flow. Since then, the dam has released cool mix flows to keep water temperatures cooler.

Experimental cool mix flows show promising results

Humpback chub like warm, muddy waters. A large population lives at the confluence of the Little Colorado River (right) and Colorado River (left). SHANE MCDERMOTT

We are just a few months into the experimental cool mix flows, and scientists are reporting finding no new smallmouth bass under 1 year old. This indicates that the experimental plan to protect native fish may be working. Now is the time to prevent additional smallmouth bass from entering and establishing populations in the Grand Canyon. We believe the Bureau of Reclamation should see the plan through to avoid reaching the tipping point — already reached in the Upper Colorado River Basin — where the only way to protect native fish is lethal removal of non-native species with little benefit to native fish.

What’s more, the cool mix flows work toward a more culturally sensitive approach to fish management in the Grand Canyon. In 2022 and 2023, water managers applied a water-based poison to a smallmouth bass spawning area above Lees Ferry, an action that the Pueblo of Zuni opposed because of the tribe’s concerns regarding the taking of life in the canyon. Options like cool mix flows, which prevent smallmouth bass reproduction, provide alternatives to killing, poisoning, and causing the death of non-native species.

In August 2024, I was fortunate to launch on another Grand Canyon river trip. This time, the water at Lees Ferry was back to a chilling 53 degrees. We floated downriver for the first two days of our trip on cool mix flows, lending to exceptionally cold (but welcome) splashes during our first big rapids.