Gov. Gavin Newsom is stepping into a labor dispute that could threaten the timeline of one of his flagship water projects: the planned Sites Reservoir north of Sacramento. Newsom (D) wrote to the Sites Project Authority Board of Directors on Friday expressing concern that the board’s choice to finalize a contract with Barnard Construction Co. to build the roughly $6.8 billion reservoir was alienating unions. “The Construction Manager you select must ensure that the project’s ambitious timetable is not disrupted by the potential for labor unrest,” Newsom wrote.  … Sites Reservoir would be the first major new reservoir built in California in decades. The project would divert water from the Sacramento River into an offstream reservoir capable of holding up to 1.5 million acre-feet of water.