Lake Mead water levels have a 50 percent chance of falling below a threshold that will trigger an official shortage, federal water managers have said, which would result in a significant reduction of water shared by three states. The lake stores water for Arizona, California and Nevada. If it falls below 1,075 feet above sea level, or about 35 percent capacity for the reservoir, federal regulations would kick in mandating radical cuts to the share in water received by the states.