The Interior Department announced Friday that it will consider revising a set of guidelines for operating two major dams on the Colorado River in the first sign of what could lead to federal action to protect the once-massive but shrinking reservoirs behind them. The public has until Dec. 20 to weigh in on three options that seek to keep Lake Mead and Lake Powell from dropping so low they couldn’t produce power or provide the water that seven Western states, Mexico and tribes have relied on for decades.