Associated Press: No one had to twist Robert Redford’s arm to get him to narrate an IMAX film about a Grand Canyon river trip. Redford, a lifelong environmentalist, previously navigated the Colorado River and has a boat on Lake Powell, a manmade reservoir on the Arizona-Utah border created by a dam. He worries about water conservation as a drought threatens the region – and, visibly, the Colorado. "(The river is) endangered now on a number of fronts," Redford said by phone Monday from …