Lake Powell’s long decline may be on hiatus after this year’s snowy winter, but activists still are raising concerns that climate change could render Glen Canyon Dam inoperable.

This time, they are taking their concerns to court, asking a federal judge to invalidate the federal Bureau of Reclamation’s 20-year operating plan for the towering dam that impounds the lake because it fails to account for shrinking flows on the Colorado River and “conceals” the risks that trend poses for the 40 million people who rely on the river for water.

“Precipitation in North America is now coming more in the form of rain than it is snowpack, but our entire water-delivery system, especially for almost every community in the western United States is premised on having a snowpack, and the snowpack will melt, and there’ll be water available for the summer months,” said Dan Beard, former commissioner of the Bureau of Reclamation. “Now that’s changing. As a result, they really have an obligation to take climate change into consideration.”

Beard, author of “Deadbeat Dams: Why We Should Abolish the U.S. Bureau of Reclamation and Tear Down Glen Canyon Dam,” is a board member of Save the Colorado, one of the groups that sued the Interior Department on Tuesday in U.S. District Court in Arizona.

Beard’s group is joined by the Center for Biological Diversity and Moab-based Living Rivers in the suit asking the court to invalidate the bureau’s 2016 environmental study and order a new analysis, this time with dam removal as an alternative. Called the Long-Term Experimental and Management Plan, or LTEMP, the document updates the dam’s 1996 operating plan, with an eye toward maximizing electricity generation, meeting needs of downstream water users and protecting the environment inside the Grand Canyon.

“The Department chose not to fully consider several alternatives, such as Run-of-the-River, Decommissioning the Dam, and Fill Lake Mead First, which would better serve the Colorado River and its millions of users in [the] face of climate change impacts,” contends the suit, which is being handled by pro bono lawyers from the Earthrise Law Center in Portland. “For more than a decade, concerns regarding climate change impacts on declining surface water flows have occupied water management discussions and have been a major subject of scientific inquiry within the Colorado River Basin.”

Last year, lake levels approached historic lows at both Lake Powell and its downstream partner, Lake Mead, before rebounding, thanks to heavy runoff last spring from a damp winter. Long-term projections, however, indicate flows will continue to contract as the climate warms, Beard and others warn.

A Bureau of Reclamation spokesman declined to comment, citing an agency prohibition on discussing pending litigation, but the agency has addressed Save the Colorado’s arguments in publicly available documents.

According to Reclamation’s environmental impact statement, the agency relied on flow projections developed in 2012, which it concluded will remain relevant over the 20-year life of the dam’s operating plan. The plan spells out hourly, daily and monthly release patterns. The environmental review concluded flow uncertainties associated with climate change would not have influenced the agency’s final decision.

"That analysis did not evaluate the complete loss of power generation at Glen Canyon Dam, because decommissioning the dam would not meet the purpose, need, and objectives of the LTEMP," the agency wrote in response to Save the Colorado's comments submitted during the environmental review.

In past interviews, Reclamation officials said Lake Powell plays a vital role in storing water from the Colorado River. The reservoir marks the boundary between the river’s Upper and Lower basins; without it, officials said, the Upper Basin would not have been able to consistently meet its obligations to the Lower Basin.

“We are claiming that there won’t be enough water to release anything, and they blew off the climate-change predictions,” replied Gary Wockner, Save the Colorado’s executive director.

This year, the dam is expected to release 9 million acre-feet of water. The elevation of Lake Powell at the end of August was 3,619 feet above seal level, or 81 feet below “full pool,” holding 13.6 million acre-feet, which is 56% full. That’s up 48 feet from late last year, when it was at 38% capacity.

The environmental groups’ chief complaint is the agency relied exclusively on “historic” inflow data, without considering the lower flows climate-change models forecast. Instead of studying structural changes the groups argue are needed to preserve the Colorado River, the bureau is relying on incremental fixes, such as the recently adopted Drought Contingency Plan, as it hobbles from one crisis to the next.

Some models indicate Powell could fall below the level needed for producing power, and it would reach “dead pool,” at 3,370 feet, under the driest scenario. The suit contends this failure skewed the operating plan toward a “business as usual” approach that may not be sustainable much longer.

“We must throw ‘incrementalism’ out of the toolbox, take climate science seriously, and plan for so-called ‘Black Swan’ drought events on the Colorado River,” said Wockner, referring to the theory developed by scholar Nassim Nicholas Taleb. A black swan is an event that comes as a surprise and carries astonishing consequences but in hindsight seemed predictable.