The U.S. and Canadian Seaway Corporations marked the opening of the Seaway's 58th navigation season today with the transit of Canada Steamship Lines' Thunder Bay through Lock 3 on the Welland Canal.  The ship, carrying a load of road salt, will be replenishing stocks depleted by ice storms which repeatedly struck Eastern Canada over the winter.  "The Great Lakes St. Lawrence Seaway System continues to be an environmentally sustainable, vital route for commerce in the global supply chain," said Betty Sutton, Administrator of the U.S. Saint Lawrence Seaway Development Corporation.