For every inch that Lake Michigan water levels go down, a commercial freighter has to leave 270 tons of cargo on the dock.
With lake levels hovering around record lows, that's a lot of cargo that isn't moving.
The United States has 57 flagged ships on the Great Lakes, and Canada has 62 (the Canadian ships on average are smaller).
"Over the course of a season, we can leave as much as 400,000 tons of cargo unmoved," says James Weakley, president of the Cleveland-based Lake Carriers Association.
That's a drag on the economy, Weakley says, because ships can move some cargo more cheaply than rail. He estimates it would cost America $3.6 billion more a year to move by rail what now moves on ships.
Lower lake levels can directly impact America's steel industry, the heart of which is around the Great Lakes, because ships move the raw materials for steel, he says.
"At some point, we are making less steel in the United States and North America and making foreign steel more competitive," he says.
Shipping has been hit by a double whammy, he said. First, when water levels were high in the 1980s, the federal government cut back on dredging in harbors and shipping lanes. Now, water levels are low, but dredging hasn't resumed.
Read a Jan. 7 Sun-Times editorial about lower lake levels here.
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