"That's a dramatic fall," Cynthia Sellinger, a hydrologist at the US Great Lakes Environmental Research Laboratory, told Reuters. "Lake Superior has been in and out of an extreme drought since 2003, and now the drought has got more extreme on the lake's western basin."

Lakes Huron and Michigan, into which Superior flows, are similarly low -- down 1 metre (3.3 feet) in the last ten years -- leaving dried out marshes and some inaccessible ports.

Meanwhile, some of the shallows and riverbeds used by fish species such as salmon and trout for spawning have dried up.

In the last 30 years, precipitation has decreased while evaporation has increased, leading to higher water temperatures in the three upper Great Lakes. Lakes Erie and Ontario are the lower of the five, which make up the world's second-largest body of unfrozen, fresh water behind Russia's deep Lake Baikal.

Average spring temperatures in northwestern Ontario, the Canadian shoreline of Lake Superior, were at least 2 degrees Celsius (3.6 degrees Fahrenheit) above normal this year. The warmer temperatures melt ice on the lake, resulting in more water lost through evaporation.

Sellinger said there is a 15 percent to 20 percent probability that Lake Superior, the northernmost lake, will reach record low levels for at least a couple of months this year.

It is only 6 cm (2.4 inches) above its lowest levels, which were recorded in the 1960s when the biggest and most controversial dredging project took place on the St. Clair River, a major shipping route near Detroit that connects the upper and lower lakes.

This week, a study showed that the amount of water flowing through the St. Clair River and, eventually, out to the Atlantic Ocean, is about 2.5 billion gallons (9.5 million cubic metres) a day -- triple what was previously thought.

"How much water are we going to let drain out before the erosion can be stopped?" Mary Muter, chair of the Georgian Bay Association's environment arm, said of the group's findings.

The group of Canadian homeowners on Lake Huron used data from the US National Oceanic and Atmospheric Administration for its waterflow measurements, and compared that with findings from a study it commissioned two years ago.

The association blames regular dredging for navigation on the St. Clair River and wants sills and a gate to be built so that water levels won't drop any further.

Iron ore and grain are among the biggest cargoes shipped on the lakes, which are connected to the Atlantic through the St. Lawrence Seaway's system of locks and canals, which opened in 1959, allowing ocean-going vessels into the industrial heartland of North America.

In 1962, the US Army Corps of Engineers deepened the St. Clair River channel by 0.6 metres (2 feet) to accommodate commercial shipping. US and Canadian governments planned to construct underwater sills to stem the flow, but never did because water levels in the upper lakes rose to record highs in the 1970s.