Glen Canyon Dam 'flood' aids iconic park, but only in the short term

Posted: Nov 29, 2012

Written by

APRIL REESE, Greenwire
Glen Canyon Dam

The torrent of water released from Glen Canyon Dam last week succeeded in moving sediment downstream to build sandbars and beaches in the Grand Canyon, but many of those gains could be lost within a few months, scientists say.

The five-day surge, which sent an unusually large volume of water from Lake Powell into the canyon, is part of a decades-long effort to mimic pre-dam floods on the Colorado River to help restore resources in the national park (Greenwire, Nov. 21).

Last week's controlled flood began at noon Nov. 19 and concluded at 7 p.m. Friday after peaking midway through at 42,300 cubic feet per second. So far, it seems to have succeeded in moving at least some sand downstream: Scientists monitoring the effects of the release on the canyon have found newly created sandbars as far as 100 miles from the dam, and park officials have found new sand at a beach near Phantom Ranch.

On Monday, a team of scientists embarked on a trip down the canyon to measure the effects of the release at various sites and to retrieve memory chips from 35 time-lapse cameras along the canyon.

"I can tell you there are new sandbars down there, but that's not really the point," said Jack Schmidt, who heads the Grand Canyon Monitoring and Research Center. "The point is: How large are the sandbars, and in how much of Grand Canyon are there large sandbars?"

But over the next few months, some of the sand deposited by the flood will be eroded away as flows drop and the Glen Canyon Dam once again holds back much of the sediment, he added.

"It just will; it's inevitable," Schmidt said.

The dam will soon resume normal operations, which involve varied releases to maximize power generation during periods of peak demand. But the resulting fluctuating flows are bad news for conserving sediment, said Nikolai Lash of the Grand Canyon Trust. He believes the Bureau of Reclamation, which operates the dam, should follow high-flow releases with steadier flows to minimize erosion.

"For a short period of time, it will be relatively stable, but not very long, partly because there's pressure to get back to normal as soon as possible," he said. "Too quickly, we'll go back to fluctuating flows. The problem is that's the most erosional flow regime. It just pummels the sandbars."

But as winter arrives, electricity use will increase, and fluctuating flows are needed to meet that demand.

"December and January are high electricity demand months," Schmidt said. "When you have an enormous supplier of electricity and an iconic park, you have a constant balancing act."

Even with steady flows, the sandbars would still begin to erode, although probably less so, he added. One silver lining in the current drought, Schmidt added, is that low flows on the Colorado River will help limit erosion.

"The fluctuations will be between 8,000 cubic feet per second and 16,000 cfs through next summer, because we're in a drought," he said. "If the fluctuations were even larger, we'd have more erosion -- the amount of erosion is proportional to the amount of clear water you have to move after the flood goes down."

Long-term challenges

The dam, built in the 1960s, helped fuel growth in the Southwest by providing a reliable source of water and power. But those human benefits came at a tremendous ecological cost: The Colorado, whose Spanish name refers to the reddish hue it once had when it was thick with sediment, now runs low and clear most of the year. About 95 percent of the sand that once flowed down the river is now trapped behind the dam.

Those changes helped push the humpback chub, which historically took cover from predators in the murky, sediment-laden waters, onto the endangered species list. The lack of sediment also means fewer sandbars, which create nursery habitat for the chub, and smaller beaches, on which rafters and backpackers depend for camping sites and resting areas. Regular deposits of sand are also needed to keep archaeological sites buried and protected.

Last week's high flow was the fourth such release, following others in 1996, 2004 and 2008, but it was the first to be conducted under a new science-based protocol adopted by the Interior Department last year. It calls for more frequent high releases through 2020 when conditions are right -- typically, when the Paria River, a tributary of the Colorado downstream from the dam, has swelled with water and sediment after storms. That means high flows could be unleashed once or twice a year, depending on conditions.

Before last week's release, the Paria River had delivered 593,000 tons of sand to the Colorado between late July and the end of October -- enough to fill a building the size of a 100-yard NFL football field about 24 stories high. That's about 26 percent less than the Paria delivered in an average year, but it was still enough to merit a small controlled flood to give the downstream ecosystem a boost, Schmidt said.

The release didn't affect power generation much during the flood, because it was for such a short amount of time, said Lisa Meiman, a spokeswoman for the Western Area Power Administration, which is one of the parties that participate in the Glen Canyon Dam Adaptive Management Work Group. The panel was created in 1997 to advise the Interior secretary on ways to mitigate the effects of the dam on Grand Canyon resources, which is required under the Grand Canyon Protection Act.

"WAPA and Interior have been working really hard on the technical team to find ways to keep the river healthy but also satisfy the needs of power customers, since both of those objectives are really important," she said. "This was really a success of how you can meet differing needs through open communication and cooperation."

Lash of the Grand Canyon Trust said he's glad Interior will conduct more releases like last week's high flow over the next two decades, but he'd like to see officials pay closer attention to how the dam is managed after the bypass tubes are closed again.

"Now we'll want to see these high flows done on an annual basis, and learn from each one, and how best to sustain the benefits the high flows provide. And that could mean several months of steady flows," he said. "If we run three or four months of steady flows and the bars don't stabilize any better, we'll learn that and move on. But our thinking is the high flows followed by several months of steady flows would be the best way to keep sandbars intact."

The possibility of experimenting with various types of steady flows before and after high-flow releases has come up during discussions within the Glen Canyon Dam Adaptive Management Work Group about what to include in a forthcoming long-term management plan for the dam, expected next year. But it's unclear whether steady flows will be included.

"I do think what we've achieved with the high-flow protocol is commendable," Lash said. "But adding steady flows would be taking it one step further."

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