When the American Century cargo ship glides beneath the Aerial Lift Bridge into Duluth, dock workers will spring to action, ready to load it with coal.
But when the 1,000-foot "laker" departs, its holds won't be full. Before workers can fully load the ship, it must first clear a shallow spot in the St. Mary's River after passing through the Soo Locks. The river connects Lake Superior to Lake Huron, which is at record low levels.
"They have to make sure they have enough under-keel clearance to pass over this rock section without doing damage to their ship," said Adolph Ojard, executive director of the Duluth Seaway Port Authority. "So they can't load any deeper."
Lake Superior also is low -- about a foot below its long-term average. But Lake Michigan and Lake Huron are even lower, nearly two feet below their averages. Their water levels have fallen for 15 years, the result of declining rainfall on the lakes, and increasing evaporation.
That's expected to significantly affect the economy of the Great Lakes region because even one inch of water makes a big difference for huge ships like the American Century.
"On your thousand-foot ships, an inch of water is over 260 tons," Ojard said. "So if you multiply that times the freight rate, then the fact that ship makes 45 trips a year, all of a sudden we see almost an entire cargo in that ship, one entire trip being consumed in the reduced draft."
Over the course of a year that can mean millions of dollars in lost revenue for a single ship, he said.
The condition is difficult to correct, as the low lake levels are compounded by a dredging backlog in the nation's ports.
Ojard said only half of the $1.6 billion collected from shippers every year through the federal Harbor Maintenance Tax is spent on what it was designed to do -- to maintain harbors. The rest, he said, is diverted to other federal spending.
"The problem is Mother Nature is working against us, and the money problems in Washington are compounding the issue," he said.
Water levels in the Great Lakes have been below normal since the late 1990s. On Lake Superior, they hit their all time low in 2007. Lakes Huron and Michigan hit their historic lows in December and January.
Drew Gronewold, a hydrologist with the National Oceanic and Atmospheric Administration, said the drop in water levels is largely the result of reduced rainfall and increased evaporation.
"Clearly what we're seeing here is a tendency for increased evaporative loss off of the lake surfaces, particularly off of Lakes Michigan, Huron and Lake Superior," he said. "That's a large reason why we see a significant drop in water levels, particularly in the late 1990s."
Jay Austin, a researcher at the Large Lakes Observatory at the University of Minnesota Duluth, has documented a strong connection between declining ice cover and warmer lake temperatures. A recent study in the Journal of Climate reports a nearly 80-percent loss of ice on the big lake in the last 40 years. Austin said that could lead to more evaporation.
"When you have more open water you have more surface to evaporate off of," Austin said. "You don't evaporate an enormous amount of water when you have good ice cover. Whereas when you take that ice cover away, you have a much greater potential for evaporation."
Scientists at the Oceanic and Atmospheric Administration is studying the interplay between low water levels, shrinking ice cover and warm water temperatures, Gronewold said. They have already concluded that climate change is playing a role in determining Great Lakes water levels.
"More recently, evaporation over lakes has steadily been increasing, largely due to increases in water surface temperature," Gronewold said. "That's a climate response, there's no other way to say it." How that will play out over the long term, he said, is hard to predict. That's because while a warmer climate could mean more evaporation, it could lead to a repeat of the heavy snowfalls seen over the Great Lakes this winter. It also could mean more rain falling over the lakes, the kind of deluge seen last summer, when a massive storm dumped 10 inches near Duluth and caused historic flooding.
In only 24 hours, Lake Superior rose between three and four inches, said Austin, of UMD.
"That's the entire lake, from here all the way to the Soo, going up by four inches," he said. "If you sat over in Munesing, or Sault Ste. Marie, you would have seen the lake level go up, even though rain was over here on the North Shore.
"That's a quarter of the supply that you normally get over the year and we got it in one day."
This winter's record snowfall in northeast Minnesota will likely bump up Lake Superior's water levels by several inches. But the Army Corps of Engineers predicts the lake will remain below its long-term average.