U.S. officials on Monday declared the first 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.

States, cities, farmers and others have diversified their water sources over the years, helping soften the blow of the upcoming cuts. Federal officials said Monday’s declaration makes clear that conditions have intensified faster than scientists predicted in 2019, when some states in the Colorado River basin agreed to give up shares of water to maintain levels at Lake Mead.

“The announcement today is a recognition that the hydrology that was planned for years ago — but we hoped we would never see — is here,” said Bureau of Reclamation Commissioner Camille Touton.