The St. Lawrence Seaway is a series of canals that make it possible for Ocean-going ships to make their way from the Atlantic Ocean to the Great Lakes as far as Lake Superior.
The canal was formally opened in 1959 by Queen Elizabeth II and President Dwight Eisenhower who cruised up the canal on the Royal Yacht Britannia.
There are four postage stamps associated with the opening of the St. Lawrence Seaway. The first of these was the stamp issued by Canada to honor the royal visit of Queen Elizabeth II for the opening of the canal. This stamp, one of Canada's most beautiful in my opinion, was based on the famous portrait of the queen by Pietro Annigoni. This is Scott #386:
The second and third were the first joint issue of the United States and Canada and were designed to commemorate the opening of the canal itself. These stamps, designed by Arnold Copeland, Ervine Metzl, William H. Buckley and Gerald Trottier, feature the Canadian Maple Leaf and the United States Eagle within interlocking rings superimposed over the Great Lakes. First day of issue was June 26, 1959 at Massena, NY, USA and Ottowa, ON, Canada. The US stamp is Scott #1131 and the Canadian stamp Scott #387:
A complete treatment of these issues with emphasis on the story surrounding this famous error can be found in "The 1959 St. Lawrence Seaway Joint Issue and its Invert" by Charles J.G. Verge published in 2009 on the 50th anniversary of the issue.
(A second USA / Canada joint issue was produced in 1984 for the 25th Anniversary of the opening of the canal (Scott # 2091 & 1015 respectively). However, this issue had similar rather than identical designs - unlike the 1959 issues.)