For the past three years, Montana State University professor Wyatt Cross took quarterly trips to the Colorado River in the Grand Canyon.
The views and rafting in the famed gorge were phenomenal, but Cross and 10 other researchers were spending most of their time involved in an intensive study into the river’s food web.
“We took three years to get the pulse of the ecosystem: who eats who and how much resources are available to feed all of the fish,” he said.
That meant days spent seine netting to capture fish to see what they were eating, and also counts of the many, many aquatic invertebrates that the fish eat.
“The food webs go from super simplified right below the dam to much more complex the farther away you get,” he said.
The study, published in the journal Ecological Monographs, also involved researchers from Idaho State University, the U.S. Geological Survey, Loyola University of Chicago, the Cary Institute of Ecosystem Studies and professor Bob Hall from the University of Wyoming.
Hall said the study helps to give scientists a broader picture of the ecosystem below the dam, one that also provides insights to similar dams elsewhere – like Flaming Gorge in Wyoming, as well as Yellowtail and Fort Peck dams in Montana.
“Dams form this huge disturbance in the rivers that considerably changes the ecosystem,” Hall said.
That’s evident in the fact that in the middle of the high desert of the Colorado Plateau, rainbow trout thrive in the waters of the Colorado River below Glen Canyon Dam – a place they aren’t naturally found.
Altered food webs
The rainbow trout are not native to that section of the river. They were planted there in the 1960s, after the dam was built to generate electricity, control water flows and created the popular recreational boating area of Lake Powell.
Because of the dam, the aquatic food web in the Colorado River below the dam has been substantially altered. Cooler water is now pulled from the bottom of Lake Powell and is dumped into the river below. The colder water favors different species than the natives that had evolved over thousands of years in the unaltered ecosystem.
“The same could be said for Fort Peck Dam,” Cross said. “Dams are these big incongruities and change the environment in several ways.
“But at the same time, these dams are really important,” he added. “We need to figure out how these ecosystems work because they are going to be with us for awhile.”
In part the study showed that near the dam, where water temperatures only fluctuated 8 to 12 degrees, there was a surplus of certain bugs, mainly midges and blackflies. There were so many bugs that the fish couldn’t eat them all. Invasive New Zealand mudsnails also thrived close to the dam.
Farther downstream the numbers of fish and fish food were more closely matched. Hall said that shows that the food web is more stable the farther the river flows away from the dam.
“We’ve known what dams have done for a long time in terms of fish assemblages,” he said. “What this study says is that the attributes of the ecosystem will recover as you go downstream.”
Study sites were distributed along a 240-mile stretch of the Colorado River below Glen Canyon Dam. During the three-year study, samples of more than 3,600 animal diets and 4,200 invertebrate populations were collected and processed.
The Bureau of Reclamation experimented in the spring of 2008 with increasing releases from Glen Canyon Dam to simulate pre-dam runoff conditions. It was thought such flushing flows might rehabilitate sandbars downstream, as well as create side-channel habitat for native fish rearing. What the researchers found instead was that such spikes in water flow seemed to benefit the nonnatives more than the natives.
“These food webs downstream don’t respond much to floods,” Cross said. “If you push other food webs with disturbances, they bounce around.”
There will be more papers published based on the research findings of the three-year study, Cross said. In the meantime, work on the river will transition to monitoring key attributes of the ecosystem to see how it changes over time, Hall said.
In the immediate future, the study gives fisheries managers more insight into an aquatic ecosystem that sustains a hugely popular rainbow trout fishery and will help agencies and the public make decisions on fish management.
“For years they’ve been managing for fish without knowing whether there was any food for those fish,” Hall said.