The seaway has always been the tale of two waters - salt and fresh, divided by nature but united by humanity. Even before construction began on the Montreal section, however, it was clear that mixing the ocean with the lakes came with risks. The building of the Welland Canal years ago allowed ships to circumvent Niagara Falls, but it also provided passage to the sea lamprey, a vicious "aquatic assassin," as Mr. Alexander describes it, that broke into the world's largest freshwater fish market with no natural predator to stand against it.
So perhaps it shouldn't have come as such a surprise when, in 1988, two biology students found an unusual shellfish on the bottom of Lake St. Clair, which lies between Lake Erie and Lake Huron. It turned out to be a foreign intruder that had hitched a ride on an ocean freighter and, of course, in the two decades since then, the zebra mussel has become legendary for the many millions of dollars in damage it has caused to its new habitat.
But it didn't come alone: Since the seaway opened, scientists estimate that as many as 57 foreign species (about one-third of the 185 now on record and almost all of those that have been found in the past 50 years) have arrived in the ballast water shed by saltwater ships. They have displaced native plants and animals, decimated fish stocks, even disrupted power plants.
The seaway is hardly the only cause of the Great Lakes' decline - aquaculture and recreational boating have done much damage, along with pollution from industry and agriculture - but many scientists believe that it is responsible for the most harm, and certainly let in the most destructive intruders.
Even worse, environmentalists point out, government agencies that regulate the seaway and shipping have been painfully slow to react. Only in the past two years have seaway authorities on both sides of the border made it mandatory that all ships - including those with just small amounts of ballast from ports overseas - flush their tanks in the ocean before entering the seaway. Even that isn't necessarily foolproof. Flushing may kill 95 per cent of what is in the tanks, but a troublemaker could survive.
So, 20 years after the zebra mussel arrived, "the threat still remains," says Jennifer Nalbone, an analyst with Great Lakes United, a cross-border environmental coalition. "It's a very sober anniversary."
Assessing the economic value of the seaway - and whether the environmental toll and human costs have been justified - is complicated. There is no doubt that having lots of cheap hydro as well as a watery highway has been important to manufacturing cities on the Great Lakes.
Statistics released this week show that more than 2.5 billion tonnes of cargo worth more than $375-billion have passed through the seaway, most of it between Canadian and U.S. ports.
Even so, annual tallies for "salties" have never reached the predictions made on opening day, and the early glow of having ready access to European markets - the romantic focus of those "glossy, flossy" celebrations - soon faded. Demand for grain moved to the west, other markets shifted as well, and long-distance container vessels grew too big to fit in the seaway's locks.
"It was a noble idea - it's been very valuable for domestic bulk cargo," says John Taylor, a transport specialist at Grand Valley State University in Grand Rapids, Mich. "But the seaway has been 'locked' in time. The world has evolved and the seaway has not been able to evolve with it."
Today, as Mr. Alexander points out in Pandora's Locks , only about 5 per cent of the world's container fleet can even squeeze into the Great Lakes. By 2007, the volume of cargo carried by ocean-going vessels had dropped to nine million tonnes from a high of 23 million in 1978, and even that figure was well off early expectations.