Since 1998, Lake Superior has slipped to the shallower side of average. This year the lake could sink to a new low. Experts point to two main reasons for this: Less precipitation and more evaporation.

Each year, about 2.5 feet of water hits Lake Superior directly as rain or snow. Two more feet flow in through streams and ground water, which reflect precipitation that falls on land within the watershed. This year, precipitation has been short by about six inches across the Lake Superior Basin.

Less water in, more water out. Last winter’s sparse ice cover allowed more water to evaporate. Contrary to what one might guess, Lake Superior evaporates fastest from October to February when dry cold air from Canada moves over the warmer, wetter surface of Lake Superior, soaking up water like a sponge. When ice forms, it puts a lid on evaporation.

For at least the last 150 years, Lake Superior has lost and gained massive amounts of water (on the order of 20 cubic miles) while maintaining a remarkably even water level, varying only about four feet. Water levels on the other Great Lakes fluctuate much more (see table).

U.S. Army Corps of Engineers data for July 2007, based on an average height above sea level.

Consequences

From an environmental perspective, wetlands are traditionally sensitive to water fluctuations. An 11-inch water level drop changes a cliff face far less than it transfigures a gently sloping shoreline. This year, the lake line has receded more than 50 feet in some shallow wetland areas, forcing changes in the vegetation and wildlife habitat rimming the lake. Such dramatic shifts concern the tribes and other people who harvest wild rice from Lake Superior.

From an economic standpoint, cargo ships have had to lighten their loads and sometimes vessels have been excluded from harbors. Additionally, power plants at the Soo Locks are running at a diminished capacity.

For the shipping industry, a one-inch water level drop can mean over 250 tons of coal will be left on the dock when a thousand-footer weighs anchor. A two-foot drop, means that upwards of 6,000 tons, approximately 10 percent of a thousand-footer’s capacity, will be left behind. And it’s not like companies can just send more ships to pick up the slack.

"The shipping industry has a finite fleet in the Great Lakes," said Dale Bergeron, Minnesota Sea Grant’s maritime transportation educator. "There simply are no extra ships to send."

For Sea Grant’s View from the Lake boat tour trips this year, and for other research vessels, passenger ferries, and recreational boats, lower lake levels and silt have made docking challenging, if not impossible. The University of Wisconsin-Superior’s research vessel, the L.L. Smith, Jr., couldn’t dock at marinas in Ashland or Washburn, Wisconsin.

Just in case you are wondering, the U.S. and Canadian governments are NOT lowering Lake Superior to fill the other Great Lakes. It’s true that outflow is managed by the International Lake Superior Board of Control to keep the lake near its long-term average, but humans have truly limited control over such a mighty system. The outflow, which can reach 134,000 cubic feet per second, was approximately 40 percent of that maximum in July 2007.