A massive reservoir known as a boating mecca dipped below a critical threshold on Tuesday raising new concerns about a source of power that millions of people in the U.S. West rely on for electricity.

Lake Powell’s fall to below 3,525 feet (1,075 meters) puts it at its lowest level since the lake filled after the federal government dammed the Colorado River at Glen Canyon more than a half century ago — a record marking yet another sobering realization of the impacts of climate change and megadrought.