The Great Lakes: That giant sucking sound

How low can the water in the Great Lakes go?

Since the turn of the millennium, Canada’s shipping industry has played a strange game of inverted underwater limbo. As water levels in the Great Lakes have trended below norms, lakers have often been forced to carry less cargo to avoid running aground. Last year proved particularly difficult. Lake Superior fell to levels not seen since the 1920s, and reached record lows in August and September. “It was rather astonishing in the spring, when you went around and looked at the facilities and saw where the high-water mark was and where the new levels were,” says Guy Jarvis, director of engineering and harbourmaster for the Thunder Bay Port Authority. Lakes Huron and Michigan ended the year barely above their lowest levels on record (1964). Lakes Erie and Ontario have been only slightly below historical averages, but the problem extends to the St. Lawrence River: last fall, Montreal Harbour saw its lowest levels in 40 years.

Much of Central Canada’s commercial fleet is designed to navigate the St. Lawrence Seaway, which allows vessels to draw eight metres of water. But as the lakes fall, ports, terminals and connecting waterways may become too shallow to navigate at full capacity. “It has a large impact on the cargo we can carry per trip,” says Bruce Bowie, vice-president of operations for the Canadian Shipowners Association. “It increases our unit costs and the costs for customers. And you need more trips to carry the same cargo, so there’s more activity on the lakes and less availability of capacity.”

Not surprisingly, the environmental bogeyman of our times — global warming — is a serious concern to the shipping industry. Lake levels fluctuate constantly and are influenced by a host of factors. The lakes receive water from direct rainfall and runoff; upstream lakes transfer some of their water downstream through connecting channels. Simultaneously, they diminish through evaporation, the rate of which is influenced by water and air temperature and by ice cover. There’s no consensus on how much recent low levels have been influenced by man-made climate change. Ralph Moulton, who manages Environment Canada’s water level information office for the Great Lakes and St. Lawrence River, observes that heavy snow and rainfall during the early 1970s and mid-1980s lifted the lakes far above average. High temperatures during the 1930s and lack of precipitation in the mid-1960s resulted in lows comparable to those seen recently. Last year’s lower levels were due to unusually dry, hot weather, says Moulton, who adds he wouldn’t be surprised if nature eventually corrects the problem.

Predicting long-term water levels is a mug’s game. Environment Canada won’t hazard a guess beyond six months. But computer models suggest mean water levels in Michigan and Huron could drop by a metre — and in Ontario and Superior by about 40 centimetres — over the next two to four decades due to climate change. They also forecast the flow of water to the St. Lawrence River will likely fall between 4% and 24% by 2050. That wouldn’t turn the Great Lakes into the Aral Sea, but the implications for shippers would be profound. “Depending on the scenario used, significant water level declines may be experienced in all of the Great Lakes,” noted a recent joint Canada/U.S. government study. The report warned not only of reduced draft available for shipping, but also possible shifts in harbour sedimentation patterns.

If nature doesn’t sort it out over time, expect intensified calls for human intervention. Many areas of the Great Lakes have been dredged for well over a century to accommodate shipping. But the environmental impact of dredging is highly controversial. And a dredged channel not only allows ships greater draft; it also permits greater water flow, which some argue is akin to widening the drain. It’s estimated water levels on Huron and Michigan dropped by nearly 40 cm thanks to dredging along the St. Clair and Detroit rivers. So future dredging may need to be accompanied by weirs or other works to reduce water flow — and prevent a spiralling race to the bottom.