CARSON CITY, Nev.—The man-made lakes that store water supplying millions of people in the U.S. West and Mexico are projected to shrink to historic lows in the coming months, dropping to levels that could trigger the federal government’s first-ever official shortage declaration and prompt cuts in Arizona and Nevada. The U.S. Bureau of Reclamation released 24-month projections last week forecasting that less Colorado River water will cascade down from the Rocky Mountains through Lake Powell and Lake Mead and into the arid deserts of the U.S. Southwest and the Gulf of California. Water levels in the two lakes are expected to plummet low enough for the agency to declare an official shortage for the first time, threatening the supply of Colorado River water that growing cities and farms rely on. The agency’s models project Lake Mead will fall below 1,075 feet for the first time in June 2021. That’s the …