he water is 20 inches below average and a foot lower than just a year ago.

PHOTOS/Slide Presentation: Lake Superior drops to record lows: Go to http://www.usatoday.com/weather/gallery/2007/w070614_lakesuperior/flash.htm

The dropping levels have had serious environmental and economic consequences. Wetlands have dried up. Power plants run at half capacity. Cargo ships carry partial loads. Boaters struggle to find a place to dock.

The changes can be seen all along the 2,800-mile shore of Lake Superior, the coldest and deepest of the Great Lakes. The water has receded, sometimes 50 feet or more, from its normal shoreline.

Lake Huron and Lake Michigan are at low levels, as well, although not quite as extreme.

Researchers at the University of Minnesota and elsewhere study whether Lake Superior's low water levels are a result of global warming. The average water temperature of Lake Superior has risen 4.5 degrees Fahrenheit since 1979.

A drought and warm weather are the immediate cause of the drop in water levels. In the past year, precipitation was 6 inches less than the average of 31 inches. The lake's southern shore had a green Christmas in 2006. The ice and snow pack that usually cover the lake arrived late, allowing water to evaporate.

"It's been a long time since we've been this low, but it has happened," says Tim Calappi, a hydraulic engineer for the Army Corps of Engineers, which tracks water levels. "We still think this is within the range of what's normal, but we have to wait and see."

Superior isn't the only prominent North American lake or reservoir at a severely low level. Lake Mead near Las Vegas and Lake Powell on the Utah-Arizona border are about half full. Florida's Lake Okeechobee recently set a record low.

Many people living near Lake Superior don't buy drought or warm weather as the reasons for dropping water levels — a conspiracy theory is more popular. They say Lake Superior was drained through the St. Mary's River to raise the levels of Lake Huron and Lake Michigan.

"It's like the tide went out and didn't come back," says Dan Alexander, a commercial fisherman in Baraga. "We know what it is. They drained the lake." The water is so low he had to find a new place to dock his 38-foot boat.

Calappi says it's a myth that the Army Corps drains Lake Superior to help other lakes with presumably more powerful benefactors. He says the amount of water that flows out of Lake Superior is established by an international agreement with Canada. The water flow is regulated by how much water is permitted to pass through hydroelectric plants on the St. Mary's River, which connects Lake Superior and Lake Huron and, indirectly, Lake Michigan.

The Edison Sault Electric power plant in Sault Ste. Marie, Mich., will operate at less than 50% capacity this year because its water flows have been slashed as a result of the low lake levels, the company said. That pushed the company to buy high-cost power elsewhere and increase rates.

Other problems:

•Cargo ships run partly empty, especially those that carry heavy materials such as coal and iron ore.

On a recent trip, the 1,004-foot freighter James R. Barker had to leave 7,000 tons of coal behind, so the boat would draft 26 feet under water, instead of 29 feet.

"We need more rain, and we need more dredging," says Robert Dorn, senior vice president of Interlake Steamship Co., which owns the ship.

Adolph Ojard, executive director of the Duluth (Minn.) Seaway Port Authority, says cargo ships have lightened loads about 5%. For ships averaging $6 a cargo ton and making 40 trips a year, that amounts to about $1 million in lost revenue per ship, he says.

•Large beds of wild rice that grow in wetlands have gone dry. Wild rice beds in the Kakagon Slough of Bad River in Wisconsin have been hit particularly hard.

•Recreational boaters find fewer berths everywhere along Lake Superior. Smaller boats compete for fewer spaces. Owners of big boats not suitable for shallow water are sometimes forced to move on or spend the night in deeper waters.

In Marquette, Mich., the water is so low, the city had to build two-step stairs for people to walk down to their boats. The landings are supposed to be level with the boats.

"It's a mess. There's not much to tell people with deep-keeled sailboats other than, 'There's no place for you anywhere,' " says Hugh Leslie, parks and recreation director in Marquette (pop. 20,714), the largest Michigan town on the lake.

'We're not really beach people'

In Marquette, boulders line the shore to prevent waves from washing out Lakeshore Boulevard. Today, the lake is more than 50 feet from the road.

The receding water has created wide swaths of scenic beach, but even this has created problems. Changing currents at South Beach in Marquette carved a 4-foot crevice in the popular family beach. "It cut the beach in half and exposed drainage pipes," Leslie says.

Elsewhere along Lake Superior, the beaches are wider than usual but they aren't expected to attract larger crowds. Because of the cold, "here in Duluth, we're not really beach people," says Ann Norris of the city's Parks and Recreation Department.

Scott Brossart, engineer for the Army Corps in Duluth, says some dredging will be done to make the commercial channels in Lake Superior ports a little deeper. In Washington, Congress is considering more money for dredging. But the corps doesn't work in recreational harbors.

"We're getting requests to dredge from everywhere this year, but I have to tell them we don't do that," Brossart says.

Away from shore, Lake Superior is doing fine. A 19-inch drop doesn't make a big difference in a lake that is 1,330 feet at its deepest.

The fishing has never been better. Alexander says he's catching huge amounts of trout and whitefish. For now, he's waiting, like everyone else, for the water to rise.

"It seemed normal last October," Shalifor says. "Then it dropped and never came back."

Contributing: David Onze of the St. Cloud (Minn.) Times