Friday afternoon the sun peaked through the clouds above Lake Oroville and a rainbow arched over the Feather River. It was a welcome sight for state engineers who were battling the lake’s worrisome rise with torrential releases down the reservoir’s broken concrete spillway. The break in storms and a drop in the volume of water pouring into the huge reservoir gave dam operators hope that they could keep lake levels from hitting an elevation of 901 feet — the point at which uncontrolled flows would start washing down an unpaved emergency spillway that has never been used in Oroville’s 48-year history.