In Wake of Experimental Floods, Scientists Track Grand Canyon’s Vanishing Sand

By Melissa Sevigny
Published: Wednesday, December 21, 2016 - 2:35pm
Updated: Thursday, December 29, 2016 - 1:33pm
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(Photo by Melissa Sevigny - KNAU)
Two bypass tubes spill water during the November 2016 high flow experiment. To reach peak flow, all four will open.

The Grand Canyon is running short on sand. Sediment that once washed down the Colorado River to form beaches is now trapped behind Glen Canyon Dam. Scientists want to find out if artificial floods are rebuilding the beaches.

Enormous floods once thundered through the Grand Canyon, but that’s a rare sight these days.

So there’s a lot of excitement as a crowd gathers at the base of Glen Canyon Dam. A massive tube cranks open, and water gushes out. The calm river begins to churn.

“There’s a mist over the river, it’s really truly awesome to see,” said Katrina Grantz of the U.S. Bureau of Reclamation. She helped plan this four-day flood. It’s like watching an Olympic-size swimming pool empty into the river every two and a half seconds.  

“The more water that we can get out, the more sediment that’s stirred up, and the more we can rebuild those beaches,” Grantz said.

Beaches in the Grand Canyon are vital for plants and wildlife, and for people who need places to camp.

When Glen Canyon Dam was completed in the 1960s, those beaches began to vanish. Scientists thought controlled floods, or “high-flow experiments,” might fix the problem.

Rob Billerbeck, Colorado River coordinator for the National Park Service, said the first attempt in 1996 was only a partial success. “At that time, they thought just running the high flow would pick up sediment off the channel bed and redistribute it,” Billerbeck explained.

Turns out, there wasn’t enough sand.

It’s all stuck behind the dam.

Billerbeck said they learned it’s best to flood the river just after tributaries dump a bunch of fresh mud in the Colorado. “That really maximizes it,” he said. “That way the water that’s moving downstream, that wave of water, will pick up the highest concentration of sediment to redistribute.”

For decades scientists have rafted the river before and after these events, measuring sandbars and the silt suspended in the water. This is the seventh high-flow experiment.

Paul Grams, a hydrologist at the U.S. Geological Survey, pulls up a photograph taken before the flood. It shows a thin layer of sand along the shoreline.  “And then the photo taken immediately after the high flow shows a large bare sandbar sticking out into the river channel,” he said.

That’s just what he wants to see. But Grams said there’s a problem: “At least 80 percent or more of the sand is below the water.”

It’s tough to measure sand in this unseen world. That’s the only way for scientists to know if controlled floods will continue to work, years or decades from now. It’s like checking your bank account, said Matt Kaplinski, a geologist at Northern Arizona University.  

“You’ve got to have sand in the bank to build the sandbars,” Kaplinski said.

Kaplinski and his colleagues use a sonar instrument to bounce sound waves off the bottom of the river. They use the data to make color-coded elevation maps that reveal deep pockets of sand and stretches of bedrock.  

“You map it once and several years later you map it again and you compare those two surveys and you look at the change in bed elevation,” Kaplinski said. “And you can determine what’s been eroded and what’s been deposited. Figuring out how the river works is a pretty lofty goal.”

But it’s a goal that’s changing the way the river is managed. The U.S. Bureau of Reclamation just signed a plan to continue the experimental floods for another 20 years. 

Science