Glen Canyon Dam holds back Lake Powell, one of the largest man-made reservoirs in the United States. It and Lake Mead, which straddles the Arizona-Nevada line, are key to ensuring Colorado River water gets to the 40 million people and the more than 7,000 square miles of farmland that depend on it.

Drought and climate change already have reduced the river's flow, forcing seven Western states to adopt a plan earlier this year to ensure the lake elevations don't dip too low.

The environmental groups — Save the Colorado, the Center for Biological Diversity and Living Rivers — have said climate change will lead to such scarcity of river water that Glen Canyon could become inoperable.

They sued the Bureau of Reclamation in U.S. District Court on Tuesday. Aside from wanting the dam's 2016 operating plan to be revised, they want to force a discussion on draining Lake Powell and removing the dam.