Government forecasters are projecting that Lake Superior, the largest of the five, will fall to its lowest level for September since modern recordkeeping began nearly a century ago. The amount flowing out of the lake at its outlet, the St. Mary's River, has plunged too, and would have to rise by a staggering 50 per cent to reach the average of the past century.

Levels on Lakes Michigan and Huron are also sagging, Ontario is down, as is Erie – although the latter, the smallest by volume, has been the least affected.

What's going on? While there is no scientific certainty about what's ailing the Great Lakes – which together form the world's largest interconnected body of fresh water – some fear global warming is at work, causing them to shrink.

For Michigan and Huron, there is an added concern: Human meddling may have made them spring what amounts to a giant leak. Environmentalists contend that decades-ago dredging near Sarnia is causing them to lose an enormous amount of water – estimated at about an extra 10 billion litres a day, or enough to fill 4,000 Olympic-size pools.

The falling water levels aren't news to Gary Vent, whose home overlooks Georgian Bay, an arm of Lake Huron, near Waubaushene, Ont. He can see the effects: The shoreline that used to be 50 metres from his house is now more than 150 metres. Newly emerging land from the drying lakebed means that, where he docked his boat just six years ago, he now plays golf.

"You can hit a solid seven iron," he says. "I've got a golf course now instead of a place to park my boat."

Water levels on the Great Lakes go through seasonal fluctuations driven by the flow and ebb of the spring snow melt. They also experience lengthy – perhaps as long as 30 years – alternating cycles of high and low readings that occur for unknown reasons and can cause levels to vary by a metre or even more over the years.

The lakes contain about one-fifth of all the fresh water on the planet. Although a drop of a metre may not seem like much compared to what they contain, about 99 per cent of the lake water is considered a legacy of the last ice age and is basically non-renewable.

Only about 1 per cent is replenished each year through precipitation, and has to offset what flows out of the lakes through the St. Lawrence River into the Atlantic Ocean. And while the lakes are big, they're not limitless. Except for Superior, their average depth is less than 100 metres (compared to oceans that plunge more than 3,000 metres.)

The lakes are now in the midst of one of their periodic down cycles. But this one seems much more extreme than usual, lasting for nearly a decade – prompting questions about global warming.

Computer model projections generally show that the lakes will shrink as climate changes cause air temperatures around them to rise. Not only will the lakes themselves become warmer, leading them to lose more water to evaporation, but the land is likely to become drier, reducing the supply of groundwater to streams that feed the lakes.

At this point, global warming is speculation, but Mary Muter thinks she has another explanation for why things are worse this time, at least for Lakes Michigan and Huron, which are joined at their northern tips and consequently have the same water level.

Ms. Muter, head of the environment committee for the Georgian Bay Association, is advancing a controversial theory that the two lakes are inadvertently being drained because of the effects of dredging on the St. Clair River in the early 1960s.

She believes dredging has made the river bed more susceptible to erosion, which over time has deepened it, and caused more water to flow into Lake Erie. "The drain hole is getting bigger," she says.

In 2005, the group commissioned a study that estimated the erosion had caused an extra daily outflow of about 3.2 billion litres. An update issued last month, using more recent estimates on lake levels, upped the figure to 10 billion litres, leading to concerns that the ecological integrity of the lakes was being damaged. Since 1970, the extra drainage has lowered the levels of the lakes by an estimated 60 centimetres, the group says.

The International Joint Commission, the organization that oversees the lakes for the Canadian and U.S. governments, has announced that it will study water levels on Lakes Superior, Michigan and Huron, but it isn't expected to report its findings on the giant leak for another three years.

Ms. Muter says the problem could easily be fixed by piling large boulders into vulnerable parts of the riverbed to shore it up, something she says should be done right away, given that most of the water draining away is a non-renewable resource.

"I consider it bordering on immoral to not address this," she says. "How much longer are we going to wait to do the right thing?"

Environment Canada wants to see what the IJC concludes before considering mitigation measures. "We'll be looking to see what they find out in their study to see if [riverbed erosion] is a primary factor or whether it's a secondary factor or no factor in the change in levels that's occurring," says Ralph Moulton, a senior engineer at Environment Canada.

Lake Superior is also undergoing travails. It's experienced below-average water levels for nearly 10 years, the longest period of below-average readings on record. Mr. Moulton says the lake is "almost certainly" going to set a new record for the lowest September, eclipsing the previous low-water mark set in 1926, probably by a few centimetres.

Lake Superior levels are being driven down in part by a lack of rain in the area it drains. Last year, precipitation was at its lowest since the mid-1920s.

But another worrisome development is that Superior has been getting warmer, a reflection of higher air temperatures around the lake. According to a study by University of Minnesota researchers released this year, summer water temperatures rose about 2.5 degrees from 1979 to 2006.

There is no mistaking the warming – the lake has less ice cover in winter. And with Superior not freezing over as much, more of its water is being lost to evaporation.

Meanwhile, Lake Ontario is about a fifth of a metre below its average level, and Lake Erie is down by a little more than a tenth of a metre.

Mr. Moulton says Environment Canada modelling indicates that under all the scenarios used, global warming will cause water levels to drop, possibly by as much as 1.2 metres by 2050, although he also says these simulations show precipitation will rise, which hasn't occurred.