Why aren't experimental floods helping native fish below Glen Canyon Dam?


Before Glen Canyon Dam tamed it in 1963, the Colorado River flowed red with mud, and the seasons ruled its temperature and flow. Today, the river is a vastly different ecosystem. Now, it's the color of a tropical ocean because the dam holds back sediment, withering the beaches that river travelers love for camping. And the river now hovers at an unnaturally cool 46 degrees Fahrenheit year round, thanks to cold water that pours out of the dam from deep in Lake Powell. Four of the river’s native fish species are gone, and one, the humpback chub, is endangered.

In 1996, the first of four experimental floods surged from Lake Powell, amid hopes that the burst of water would churn up sediment that would fatten sandbars and provide fish habitat. The floods did restore sandbars for a time, and the experimental surges became an official part of dam operations in 2012. But flushing out the canyon has yet to benefit native fish – particularly the endangered humpback chub – as researchers and managers hoped.

In a recent study, ecologists teased apart some of the reasons why. They looked at how the river’s food webs are impacted by floods, and found that a large artificial flood in 2008 changed the simple food webs of introduced rainbow trout near the dam, increasing the blackflies and midges that the trout feed on, which boosted the non-native population -- a potentially troublesome development for native fish since trout compete with them for food, and sometimes prey on their young. Further complicating things, the flood had little impact on the more complex food webs of the native humpback chub, flannelmouth sucker, or bluehead sucker, which live further downstream in the Grand Canyon.

“The conservation implications are that we’ve got to be careful about these huge management actions, because what benefits one resource (like rainbow trout) may not benefit another (like the chub),” says Wyatt Cross, an aquatic ecologist from Montana State University involved in the study.

Now, managers are trying to balance the need for beach-restoring floods, which increase non-native trout numbers upstream, with the need to maintain chub habitat in the Grand Canyon. That left some researchers asking the question: If big, artificial floods didn’t help humpback chub as expected, what are the root causes of their low numbers, and what will help them thrive?

The Colorado River below Glen Canyon Dam.

One thing likely to prevent them from thriving is non-native rainbow trout, which were introduced in the ‘60s, and are now part of a highly valued sport fishery near the outflow of Glen Canyon dam (check out Cally Carswell’s 2012 HCN story for more on that). The recent food web study found that native fish in the Grand Canyon are already eating all of the available food (mostly insects), meaning there may not be enough forage to support both the native fish and rainbow trout should trout swim downstream into the chub’s territory and compete for prey. Or, the trout could simply prey on the chub. (As a side note, no one is sure why invertebrate populations are so limited in the Grand Canyon, but it may be a combination of colder water temperatures and unnatural flow patterns resulting from the dam, both of which can impact insect life cycles.)

“If we could get chub expanding to downstream reaches in Grand Canyon, whatever competition or predation is exerted by trout near the Little Colorado River (where the chub live) might be more acceptable,” says Ted Kennedy, an aquatic biologist with the U.S. Geological Survey’s Grand Canyon Monitoring and Research Center, who worked on the study.

Endangered humpback chub live only in the Colorado River system. They have a lifespan up to 30 years.

Despite being limited by food availability, the warm water-loving chub has gradually been making a comeback. They've likely benefited from recent increases in water temperatures as drought has lowered Lake Powell, and from trout control measures. Managers have removed rainbows from the Grand Canyon main channel in the past. The Fish and Wildlife Service may do that again, if trout numbers increase, and they march downstream. The National Park Service is already controlling rainbow trout, along with brown trout, in the side streams Shinamu Creek and Bright Angel Creek.

But a majority of the Grand Canyon’s roughly 10,000 chub live in or around the Little Colorado River, and biologists worry that if that single population becomes diseased, or something toxic spills into the river, it could doom the entire species. So National Park Service biologists have also been reintroducing chub to additional side streams, and this May, they discovered chub from an introduced population spawning for the first time in Havasu Creek, in the Grand Canyon — an encouraging sign.

In spite of all of this human intervention, chub are still rare in many sections of the Colorado River's main channel in the Grand Canyon, leading Kennedy to question what's keeping them from flourishing. He isn’t entirely surprised that the experimental floods haven’t helped the chub population. In a dammed river with such drastically altered natural flow patterns and temperature regimes, a two or three-day simulated flood, “is not going to fix everything that ails the ecosystem,” he says. Kennedy is now looking at how other aspects of dam operations, like daily spikes in flow, or big flow changes that come from seasonal water delivery, affect insects and food webs.

Given the logistics and expense of running frequent research trips in the Grand Canyon, and the difficulty of collecting insects from the river, researchers struggle to collect enough data to understand how the dam’s daily and monthly operations, not just floods, affect insects that fish feed on. So Kennedy and his collaborators have enlisted the help of commercial river guides to put out insect traps each night at camp. Kennedy is now looking at the first round of data from 2012, and they are starting to see how shorter-term changes to river flow and water temperature may be affecting insect emergence, which is a good sign that they'll start to be able to draw conclusions about how normal dam operations impact insect populations.

“Glen Canyon Dam itself has clearly affected food webs throughout the whole river,” Kennedy says. “But in spite of all of the research, we don’t have a clear idea of (the impact of) daily and monthly operations of the dam are affecting food webs and invertebrate populations.” With the help of river guides, that could soon change.

Sarah Jane Keller is an editorial fellow at High Country News. Glen Canyon Dam by Christian Mehführer via Wikipedia Commons. Humpback chub image by Randall Babb courtesy of Arizona Game and Fish Department.