The St. Lawrence Seaway is a series of canals, locks, and lakes giving ocean-going ships of the Atlantic access to the Great Lakes and dozens of inland ports, including Toronto, Cleveland, Chicago, and Duluth. A herculean engineering project built jointly by the Canadian and United States governments, the St. Lawrence Seaway has become an essential trade outlet for the Midwest. It has also become an inlet for exotic and often harmful aquatic plants and animals, from which the landlocked Great Lakes were historically protected. The seaway's canals and locks, completed in 1959 after decades of planning and five years of round-the-clock construction, bypass such drops as Niagara Falls and allow over 6,000 ships to sail in and out of the Great Lakes every year.

The initial impetus for canal construction came from the steel industries of Ohio and Pennsylvania. During the first half of this century...