Two conflicting Interior Department policies that guide how Glen Canyon Dam is operated -- majorly affecting Grand Canyon resources -- could be brought into closer alignment under a long-term dam management plan now in negotiations.

In a meeting held yesterday on the plan, Bureau of Reclamation and National Park Service officials talked about ways to better balance the need to restore the Grand Canyon's ecosystem with the necessity to move water from Lake Powell, which lies above the canyon, to Lake Mead, which lies below it.

Both reservoirs store water for millions of users in the Colorado River Basin. Under a suite of laws and agreements collectively known as the Law of the River, the four upper basin states -- Colorado, New Mexico, Utah and Wyoming -- must deliver a certain amount of water to the lower basin states of Arizona, California and Nevada, leaving some for Mexico as well.

But under the Grand Canyon Protection Act, Reclamation must also "protect, mitigate adverse impacts to, and improve" the Grand Canyon's natural, recreational and cultural resources.

"We're saying, OK, if you have a water volume that needs to be moved in a given year, what's the best way to move it, and what's that going to do to other resources?" Beverley Heffernan, Reclamation's environmental resources division chief for the Upper Colorado Region, said during a break in the meeting.

Under a 2007 agreement struck between the Interior Department and the Colorado River Basin states, Reclamation is to "equalize" water levels in the two reservoirs so that neither drops too low. That can only occur when there is an influx of water from the upper basin (LandLetter, Dec. 13, 2007).

Equalization involves releasing larger-than-usual volumes of water from Glen Canyon Dam, which forms Lake Powell just upstream from Grand Canyon, over a short period of time.

But sending a large pulse of water through the canyon can strip away sediment that's a crucial part of the Grand Canyon ecosystem. That sand is needed to build beaches and sandbars, which provide camping spots for rafters and hikers and form backwater habitat for the endangered humpback chub. It also helps protect archaeological sites in the canyon.

Equalization is at odds with a separate Interior policy that calls for conducting high flows when there's ample sediment in the Paria River, a major tributary of the Colorado, to try to move sand from behind the dam into the canyon to build beaches and sandbars. It's a major part of the effort to improve conditions in the park and lessen the resource damage caused by the dam (Greenwire, Nov. 28).

So at times, like during the artificial flood that Reclamation officials orchestrated two weeks ago and similar ones conducted in 2008, 2004 and 1996, water managers are sending more sand into the canyon. At other times, such as when they're releasing a large volume of water from behind the dam to balance out levels at Lake Powell and Lake Mead, they're scouring sand out of the canyon.

"A year or a few months of equalization can easily erode any deposition that occurred in the canyon in Grand Canyon National Park," said David Topping, a research hydrologist with the U.S. Geological Survey who studies sediment transport in the Colorado River system.

In the summer of 2011, for example, Reclamation released a fairly large, steady flow of about 26,000 cubic feet per second to equalize Lakes Powell and Mead. But those flows also carried a lot of sediment out of the canyon.

According to USGS, from mid-May through August of 2011, the Grand Canyon and adjacent Marble Canyon, which is also within the park, lost 2 million to 2.4 million metric tons of sediment to erosion. That's more than 50 percent of the sand eroded from the canyons since the end of 2008's controlled flood and "exceeds the total amount of sand likely eroded from Marble and Grand canyons since 2002," according to a presentation Topping made to officials at a meeting in January.

"It would certainly stand to reason that when you move 26,000 acre-feet of water for several months, that's going to have some resource effects," Heffernan said. "We know that's not a great thing for beach conservation."

The Park Service has encouraged Reclamation to lessen the effects from equalization between Lakes Powell and Mead. One way to do that would be to decrease flows and release the water over a longer period of time, said Martha Hahn, director of the science center at Grand Canyon National Park.

"One thing we've asked [is] if they can look at flexibility," she said. "Because if they can hold out to the next water year, that could make a huge difference. If they could do it over a few months, that would be good."

But she added: "It's really tricky. It's a real balancing act."

Heffernan said officials discussed that option during yesterday's meeting. But part of the problem, she said, is that water managers often don't know whether they'll need to release an equalization flow until the spring, when the winter snowmelt arrives, and that only leaves a few months during which to move the water from Lake Powell to Lake Mead.

"That's exactly the kind of thing we're working on," she said.

While theoretically a high flow release to move sediment -- such releases will be conducted every year under a new protocol adopted last year -- could also move water for equalization, the timing doesn't work very well, Heffernan added. High flows are typically done in the fall, when there's enough sediment in the Colorado River's tributaries to help build beaches and sandbars, while equalization flows are usually conducted in the summer, before the water year ends at the end of September.

But given the drought that's gripped the lower Colorado River Basin in recent years, and the predictions that climate change will likely decrease the river's flows in the future, there may not be too many years during which equalization will be an option anyway, she added.

"We haven't had that many equalization years, so you wouldn't think you'd have a lot of years like that," she said.

Reclamation hopes to present alternatives for the long-term management plan to the public in a few months, Heffernan added. A draft plan is scheduled to be released next fall.

Reese writes from Santa Fe, N.M.