(Salem, OR)  --  A Salem man is being treated for serious injuries after he was thrown off his motorcycle and into the Willamette River.  Police say 73-year-old William Chamberland, Jr. lost control of his motorcycle early yesterday morning and collided with a van, hit a barrier on the south side of the Marion Street Bridge, then went airborne and plunged into the river.  A couple who were in a boat on the river provided aid to Chamberland, who's being treated at Salem Hospital for potentially life-threatening injuries.  The driver of the van stayed at the scene and cooperated with police.