(North Plains, OR)  --  Authorities are offering a cash reward for information on the disappearance of a 20-year-old Washington County woman.  Crime Stoppers of Oregon is offering a 25-hundred-dollar reward for information on Allyson Watterson, who's been missing for more than three weeks.  Watterson was last seen December 22nd with her boyfriend, Benjamin Garland.  Police say Garland claims he and Watterson were hiking near North Plains when they got separated, but investigators say there's no evidence that that's the case.