U.S. officials on Monday declared the first-ever water shortage from a river that serves 40 million people in the West, triggering cuts to some Arizona farmers next year amid a gripping drought.

Water levels at the largest reservoir on the Colorado River — Lake Mead — have fallen to record lows. Along its perimeter, a white “bathtub ring” of minerals outlines where the high water line once stood, underscoring the acute water challenges for a region facing a growing population and a drought that is being worsened by hotter, drier weather brought on by climate change.