Warning signs have been posted along the Deschutes River on the south end of Bend because of cougar sightings this past week.   Bend Parks officials say there have been three cougar sightings in the south river canyon area, upriver from Farewell Bend Park, and as far south as the River Rim subdivision. They are trying to determine if the cat is just moving through or spending time in the area. The signs warn trail users to use caution, and officials say if you confront a cougar, stop, make noise, act tall and large, and slowly back away.