A late-season surge of rain and snow melt made a bad year better for the Colorado River, but it wasn’t enough to lift Lake Mead out of record-low territory. The reservoir that supplies 90 percent of the Las Vegas Valley’s drinking water bottomed out at 1,071.61 feet above sea level on July 1, its lowest level since May 1937, when the lake was filling for the first time behind a newly completed Hoover Dam.