Water levels declined in 1998 and have remained low, forcing ships to take on lighter loads and sparking concern about shorelines and wetlands in the Great Lakes, the world's largest supply of fresh water and a major commercial shipping route for Canada and the United States. Iron ore and grain are among the biggest cargoes shipped on the lakes.

"It's a pretty different mindset to come off 30 years of above-average water levels and to suddenly, since the late 1990s, have below-average levels," said Scott Thieme, chief of the US Army Corps of Engineers' Great Lakes Hydraulics and Hydrology Office in Detroit.

Lakes Huron and Michigan, where water levels have declined the most, are down about 3 feet (one metre) from 1997 and about 20 inches (50 cm) from their 140-year average, according to the US National Oceanic and Atmospheric Administration's Great Lakes Environmental Research Laboratory.

When home owners on Lake Huron's Georgian Bay noticed wetlands were drying up, the Georgian Bay Association funded a US$223,000 report that last year concluded shoreline alterations such as dredging and erosion in the St. Clair River, at the bottom of the lake, were responsible.

In partial response, US and Canadian governments approved funding for a US$14.6 million study of the upper Great Lakes by the International Joint Commission, which resolves border disputes and was denied funds for a similar study in 2002.

Depending on what it finds, the commission could recommend changes to the amount of water that flows out of Lake Superior, the first and largest in the chain of lakes.

NATURAL CAUSES?

Water levels in the Great Lakes have always fluctuated, but experts point to climate change, dredging, private shoreline alterations and even lingering effects of glaciers to explain the latest changes -- the decline of Lake Huron and slightly higher water levels in Lake Erie, into which Huron flows.

The most controversial of several dredging projects was in 1962, when the US Army Corps of Engineers deepened the St. Clair River channel by 2 feet (60 cm) to accommodate commercial shipping.

"When they dredge a river, it's like taking a straw and widening it," said hydrologist Cynthia Sellinger, who helped plan the upper Great Lakes study, which begins this summer.

US and Canadian governments approved the 1960s dredging on condition that submerged sills be built to compensate for water lost from Lake Huron, and they started a series of studies.

But by the time the studies were completed in the 1970s, water levels in Lakes Huron and Michigan were at record highs, and no one wanted sills that would raise levels even more.

Experts are unsure why water levels in the upper lakes rose soon after the St. Clair River dredging. But they say that major climatic events usually coincide with changes in water levels.

The 1930s Dust Bowl drought coincided with then-record low levels in the Great Lakes. And the most recent decline was in 1997, when a strong El Nino brought warm, dry temperatures to North America, Sellinger said.

In addition, above-average temperatures since 1998 mean less ice forms on the Great Lakes and the rivers that flow into it, and more water evaporates away, Sellinger said.

And then there is something called post-glacial rebound, or the slow rise of the earth's crust, that could partly explain declining water levels in Lakes Huron and Michigan.

"The area around Georgian Bay (Lake Huron) is rising faster than the area around Lake Erie so it may be that the land has just tilted and more water is flowing out," Sellinger said.

DECLINING REVENUE

For every inch water levels go down, ships bound for destinations outside North America forfeit about US$8,400 in freight revenue, said Dennis Mahoney, president of the United States Great Lakes Shipping Association.

Saginaw and other ports have done emergency dredging to accommodate ships and barges that can be hundreds of yards long.

But Lake Superior's largest American ships carried 3,000 fewer short tons of cargo last year than in 1997, when water levels were 12 inches (30 cm) higher, according to the Lake Carriers' Association.

"Obviously water levels are crucially important to this industry, and we have been in a period of decline," said Glen Nekvasil, vice president of corporate communications for the association.