The Columbia River defines the Pacific Northwest, from gigantic floods to thousands of shipwrecks, to the bucolic lake in the Canadian Rockies where the journey of the river begins. On the new Mossback Northwest special, we're revisiting last season's four-episode arc exploring the history and geology of the mighty Columbia River - putting a whole new perspective on "Roll On, Columbia." Mossback's Northwest: The Mighty Columbia River premieres Wednesday, April 2 at 7 PM and on the Cascade PBS ap