The idea was first proposed in the late 1800s -- to build a series of locks to permit oceangoing ships to travel from the coast into the upper Great Lakes.
The idea came to fruition in the spring of 1959, after the U.S. and Canada worked together to develop and build what is known as the St. Lawrence Seaway.
Duluth Port Director Adolph Ojard said the Seaway system has huge economic value to the region.
"Over the past 50 years we've moved in excess of 2.5 billion metric tons of commodities through the seaway. The value of that was in excess of $375 billion," Ojard said. "So it is a vital waterway. It has a tremendous economic impact on the region. That will remain and that will continue."
There's a strategy to the system, which is designed to benefit both manufacturing and mid-continental grain producers. Ships bring steel from Europe and iron ore from northern Canada to manufacturing centers like Michigan and Ohio.
"Ships then discharging cargo in these lower-lake manufacturing areas would then come up to the western Great Lakes, Duluth and Thunder Bay, loading grains for international cargos around the world," Ojard said.
But unpleasant surprises turned up in the ballast water of ocean ships -- more than 100 potentially destructive non-native species now can be found in the Great Lakes, and at least 30 of them are in Lake Superior.
They include Eurasian ruffe, now the most prevalent fish in the Duluth-Superior harbor, and zebra mussels -- infamous for clogging water intake pipes and smothering underwater surfaces with their tiny shells.
"Since the seaway opened, the No. 1 source of new invasive species entering the Great Lakes has been from the discharge of ballast water from ocean vessels," said Jennifer Nalbone of the group Great Lakes United.
The Great Lakes invasives are spreading into other fresh waters, Nalbone said.
"The zebra and quagga mussel have been here just over 20 years, and they have now reached the West Coast," Nalbone said. "That's tragic. That's just tragedy."
Nalbone said the saltwater ships account for only 7 percent of Great Lakes shipping, and that their value to the region is exceeded by the costs of dealing with non-native creatures.
Nalbone's group, Great Lakes United, wants the seaway closed to saltwater ships until the U.S. and Canada "solve this invasive species crisis. Until they can assure that ocean vessels can come in and discharge clean ballast water, the risk associated with dumping unclean ballast water in the Great Lakes is too high."
Duluth Port Director Ojard said closing the seaway would not only be costly, it's unnecessary.
That's because saltwater ships are now required to flush their ballast for new water before entering the Great Lakes. Ojard said the rules appear to be working, since no new invasive species have been found since 2006.
Closing the seaway, he said, would shift freight to trains and trucks, leading to the use of significantly more fuel and producing significantly more pollution.
"The unintended consequences of the closure of the seaway would be very profound," said Ojard. "In this day, when we're trying to reduce our environmental footprint, we would increase the environmental footprint."
What's next for the seaway is also up for debate. Global warming is the wild card. Dr. Frank Millerd from Ontario, Canada's Wilfrid Laurier University has studied potential outcomes.
He said warmer water could worsen the spread of non-native fish, now held in check by cold temperatures. And Millard said lake levels could be heading down for good.
"Mainly because of increased evaporation. Also a shorter period of ice cover is expected, which will, of course, lead to more evaporation as well."
Millerd said ship owners could cope by loading less cargo. But he said other experts have an even dryer and more dire vision, in which some rivers connecting the lakes could dry up altogether.