Artificial floods over 14 years helped rebuild sandbars in the Grand Canyon but may have pushed non-native rainbow trout into an area frequented by the endangered humpback chub, according a U.S. Geological Survey study.
The study found that 75 percent of sandbars monitored were larger after the floods, which were created by stepping up releases from Glen Canyon Dam.
The goal of the floods: stirring up sediment to rebuild sandbars thought to create habitat for native creatures such as the humpback chub. Far more sediment moved through the canyon before the dam was built.
Theodore Melis, who headed the study, recommended conducting the floods twice yearly based on the results.
“We only have 10 percent of the sediment entering the Grand Canyon from pre-dam time,” Melis said. “We don’t know actually how much sediment was required in the pre-dam era to keep rebuilding and maintaining sandbars.”
The study found that the population of rainbow trout spiked 800 percent in 2008 around the confluence of the Little Colorado River and Colorado River, where the humpback chub is common, and continued through 2010. However, Melis said that doesn’t necessarily mean that the floods washed trout, a predator of the chub, into the area.
“It could all be about timing,” he said.
The artificial floods were timed to destroy trout eggs, and Melis and other contributors said future floods can timed to mitigate trout populations. They also recommended manually removing trout near the Little Colorado River for a time to protect the humpback chub, something that was done in 2006.
The U.S. Department of the Interior will use the study as it decides whether to approve a draft environmental assessment recommending artificial floods every spring and fall for a decade. If approved, the floods could begin as early as this fall.
John Jordan, who represents the sports fishing industry on a board that advises the Interior Department on Grand Canyon operations, said there’s too much emphasis on controlling trout.
After hearing a presentation by Melis and others involved in the study last week at Arizona State University in Tempe, Jordan noted that the chub population has increased by 50 percent since 2000.
“The humpback chub population is doing well as of the present time in spite of an increasing rainbow trout population,” he said.
Beverly Heffernan, manager of environmental resources for the Bureau of Reclamation, said the Interior Department would take into account recommendations from the advisory board as it makes a decision. It also will consider costs, she said.
“We have to make sure if we are committing something that we can pay for,” Heffernan said. “It is a delicate balance we are trying to reach right now.”
Sandy Bahr, director of the Sierra Club’s Grand Canyon Chapter, said she’s concerned that the flooding could boost trout populations. Manually removing trout isn’t a sustainable solution, she said.
“It is time to make hard decisions,” she said. “Continuing to sustain these fish [trout] doesn’t make sense.”