An Abrupt Warming of the Colorado River

The downstream face of Glen Canyon Dam, which forms Lake Powell, America’s second-largest water reservoir. Water is released from the reservoir through a hydropower generation system at the base of the dam. Photo by Brian Richter

I’ve just returned from a glorious 17-day river float in dory boats through the Grand Canyon. Sublime beauty punctuated by adrenalin-pumping whitewater.

Most river runs through the Canyon begin at Lee’s Ferry, located fifteen miles downstream of Glen Canyon Dam. When we arrived at Lee’s Ferry to begin our trip on September 23rd, I headed straight for the river’s edge, anxious to dip my toes in the water.

Bracing for icy water, I was stunned when I realized how warm the river had become!

The river has been — since the construction of Glen Canyon Dam in 1963 — a frigid current of water released from the deep hypolimnion bowels of Lake Powell. For 40 years since the dam was built, the river’s temperature had fluctuated between 47 and 51 degrees Fahrenheit (8-11 degrees C), even in summertime when air temperatures in the bottom of the Grand Canyon regularly soar above 100 degrees F.

This icy river water has long posed a serious risk to river runners who become unintentional swimmers when dumped from their river boats in one of the Canyon’s 80+ big water rapids. When someone gets spilled from their river boat it becomes urgent to get them out quickly, before hypothermia sets in.

But our trip would be a very different one. When I dipped a toe at Lee’s Ferry in late September the river was a comfortable 69 degrees! (21 C).

Our knowledgeable river guides were quick to explain that due to the tremendous shrinkage of Lake Powell (the result of decades of water overuse and climate change), the reservoir has heated up and water released from the dam is now much warmer.

You can see this effect in the graph below. Lake Powell began losing its water volume very quickly during a series of severely dry years from 2001-2004. By the fall of 2004, the reservoir had lost nearly two-thirds of its capacity (orange line). The greatly diminished mass of water in Lake Powell began to warm to the highest temperatures recorded since its construction, and the releases of water from the dam into the Grand Canyon reached 58 degrees in October 2004, then 60 degrees in 2005 (blue line).

Lake Powell regained some volume from 2006-2018, but a precipitous drop since 2018 has sent the river’s temperature soaring again. This year’s 69-degree high point is a new record in the post-dam years. Lake Powell’s volume has shrunk to three-quarters empty.

I was of course very happy to know that hypothermia was going to be much less likely on our trip, and I was also pleased to remember that this warmer river is a good bit closer in temperature to its more natural, pre-dam condition. The warmer river helped us imagine what the wild Colorado would have been like, a feeling that was accentuated when we reached the confluences of the tributary Paria and Little Colorado rivers, both swollen with runoff from recent thunderstorms that turned the river a more natural reddish brown (Viva el Rio Colorado!) instead of its usual blue-green hue.

However, as I would confirm upon our return from the river, this warmer Colorado isn’t a good thing for the introduced trout fishery in the river. Trout don’t like warm water because warm water doesn’t hold enough oxygen to sustain them. The tailwater fishery below Glen Canyon Dam is famous for its abundance of big fat trout that attract thousands of fly-fishing enthusiasts each year.

One would think that the warmer river water would be better for native fish like the imperiled humpback chub, but the dropping reservoir levels are setting up a potentially disastrous situation for this endangered fish.

In recent decades, the humpbacks have been doing quite well in the Grand Canyon, particularly around the confluence with the Little Colorado River where warmer water joins the main river. The humpbacks have been recovering sufficiently that they were recently down-listed from ‘endangered’ to ‘threatened’ status under the US Endangered Species Act. But they’re now facing a bizarre twist of fate.

Lake Powell harbors a slew of invasive, introduced fish such as smallmouth bass that love to eat baby humpbacks. But until now, those predatory invaders in the lake haven’t been a threat to the humpbacks in the Grand Canyon because they hang out in the warmer upper layers of the lake, and the dam has been an effective barrier to their escape.

But as Lake Powell has dropped, the warm epilimnion water layer in Lake Powell has been lowering as well, and it is now at or near the same level as the hydropower outlets in Glen Canyon Dam. This means that the invasive predators may soon be able to pass through the dam’s penstocks in numbers that could put humpbacks in serious danger.

As many readers of this blog know, water solutions are oftentimes elusive and complicated. But there are many bright individuals working on these problems every day, and I remain ever hopeful for their success.

My sincere thanks to the extraordinary crew of river guides from OARS that piloted our safe yet adventurous journey. It has redoubled our hopes and efforts to keep the river flowing.

Dory floating through the Grand Canyon of the Colorado River. Photo by Brian Richter