Falling water levels mean lighter loads, more trips for Great Lakes shippers.(Knight Ridder Newspapers)

DULUTH, Minn. _ The grimy docks of the "twin ports" of Duluth and Superior, Wis., are clogged. There are mountains of coal from Montana and Wyoming and mounds of taconite, iron ore ripped from the Iron Range in Minnesota and northern Wisconsin, awaiting shipment to the steel mills and power plants of the East and Midwest.

Times here should be good. Beyond the coal wharves, reaching like metallic fingers into an impossibly blue Lake Superior, the waters are ice-free. The shipping season opened March 16, the earliest ever, thanks to a mild winter. Now, the 1,000-foot freighters, the fabled "lakers" of the Great Lakes trade, nose to shore as if hungry for cargo.

But what the unpracticed eye cannot see is ships riding higher than usual in the water as they head 250 miles across the lake, through the locks at Sault Ste. Marie, and farther south into the other Great Lakes.

With water levels in the lakes approaching historic lows, the giant ships are being forced to leave millions of tons of cargo, and the dollars they represent, on the docks.

Superior, the deepest of the Great Lakes, is 1,200 feet deep in places, but the lighter loads are necessary so ships can negotiate falling water levels in the Great Lakes' 26 locks and in the slips, in places such as Buffalo, N.Y., and Port …