(Julie Jacobson | AP file photo) This Tuesday, April 16, 2013 file photo shows a bathtub ring marking the high water line as a recreational boat approaches Hoover Dam along Black Canyon on Lake Mead near Boulder City, Nev. A two-decade-long dry spell that has parched much of the western United States is turning into one of the deepest megadroughts in the region in more than 1,200 years, and about half of this historic drought can be blamed on man-made global warming, according to a study released Thursday, April 16, 2020 in the journal Science.