Artificial Flooding May Help Grand Canyon

 

By David Frey, 2-09-11

When Glen Canyon Dam blocked the natural flow of the Colorado River to create Lake Powell, it unleashed a torrent of effects downstream, including in the Grand Canyon, where the once-muddy river became a blue waterway where native plants and animals struggled to survive.

After 15 years of experimenting with artificial releases meant to mimic historic floods, scientists say periodic high flows can help rebuild the sandbars that provide habitat for native fish and campsites for rafters. But they say it’s unclear how long the effects will last and if they can survive outside the drought conditions that have gripped the West.

And they say the artificial flooding will never bring the river back to the way it used to be.

“There’s really no way we can expect to have a pre-dam landscape by implementing high flows,” said Paul Grams, a hydrologist with the Grand Canyon Monitoring and Research Center and a co-author of a report released Tuesday that examined the findings of three experiments of high flows from the Glen Canyon Dam stretching back to 1996.

Since Glen Canyon Dam was completed in 1963, it not only trapped the water that used to flow through the canyon lands. It also trapped the sediment that made its way to the Grand Canyon. More than just dirt, this mix of sand, gravel and clays played a key role in forming ecosystems for native plants and animals.

The dam also took away the river’s historic ebbs and flows from floods to trickles. These changes in water volume, which helped shape the river, were replaced with a steady flow throughout the year. Changes in temperature, flow and sand contributed to losses of native fish, invasions of nonnative species, the erosion of sandbars and the narrowing of river rapids.

In an effort to re-create natural flooding, researchers experimented with high-flow releases from the dam in 1996, 2004 and 2008.

They found artificial floods increase the size and number of sandbars downstream, but it’s unclear how long the sandbars can survive under ordinary dam operations.

Scientists found the sandbars are most successful when the releases take place during times of normal flooding, when more sand is flowing from tributaries downstream from the lake.

The high flows seem to help native fish, researchers found, but they may also encourage nonnative rainbow trout. That’s good news upstream, where rainbow trout are encouraged for fishermen, but it’s bad news downstream, where the trout can muscle out native species, like the endangered humpback chub.

“Sandbars are important,” Grams said. “They’re a resource for river recreationists, a resource for riparian vegetation, riparian ecosystems. The underwater part of sandbars creates habitat for native fish.”

Prior to the dam, these sandbars formed naturally as sand flowed downstream and deposited on the shorelines. They created quiet backwaters used by young native fish. After the dam was built, it captured most of the sand that used to flow downstream.

The first experiment to mimic natural floods was something of a failure. The water was released, but the sandbars didn’t form they way researchers expected. Scientists had expected the sand that had built up on the river bottom to flow downstream, said Ted Melis, deputy chief of the Grand Canyon Monitoring and Research Center, but that didn’t happen.

So they went back to the drawing board, this time releasing the water when the sands were coming down the tributaries. In those experiments, sandbars formed, but they were fragile. While they could form in a matter of hours, they tended to disappear within days or months, although many showed growth over time. The research found sandbars also were vulnerable when more water was being released from the dam.

That creates another question researchers still don’t know the answer to, Melis said. These experiments were conducted while the West was in the midst of a prolonged drought and water releases from the dam were minimal. But in wetter years with higher annual flows from Glen Canyon, Melis said, “all bets are off.”

The Bureau of Reclamation is hoping to launch further studies to determine if these bursts of high water will help the ecosystems downstream or not. But even if they do help, they are still a far cry from the floods that rolled down the Colorado River before the dam was completed.

“The kind of sediment conditions and stages that are associated with annual peaks pre-dam, neither of those can be replicated now,” Melis said.

Essays in the Range blog are not written by High Country News. The authors are solely responsible for the content.

Originally posted at NewWest.net

Follow David Frey at www.davidmfrey.com and on Twitter.

Image of high flow release from Glen Canyon dam in 2008 courtesy USGS.
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