Scientists are studying the long-term implications because big storms are more common.
The rain that flooded Duluth last week also flushed an unprecedented quantity of dirt, pollution and bacteria into Lake Superior -- enough to make experts worry about the long-term environmental consequences on the largest and clearest of the Great Lakes.
One day after the storm, sediment runoff made the lake opaque for miles along the shore, local researchers say. Satellite photos show a wide swath of mud streaming into the lake from the Duluth harbor almost all the way to the Apostle Islands. "We don't know what's going to happen because we've never seen this," said Elizabeth Austin-Minor, an associate professor of chemistry at the University of Minnesota Duluth's Large Lakes Observatory.
Lake Superior, notable for its clear, rocky depths, is different from smaller inland lakes -- and even the other Great Lakes -- because it has relatively little "food." As a result, it doesn't produce the quantity of algae and plants, and the diversity of aquatic life typical of other lakes.
But researchers said the historically intense rain -- as much as 10 inches -- that fell on the Duluth area last Wednesday might change that, in at least the western arm of Lake Superior.
The water swept nitrogen, phosphorus, carbon and bacteria into the lake -- a huge influx of food for the ecosystem.
"We don't know how quickly the lake will reset, or if it's pushed itself into a different state," Austin-Minor said. "You may have changed the lake from being antiseptic to one that is not so antiseptic."
From the deck of the Blue Heron, the UMD research vessel where she was collecting samples, the lake's appearance was bizarre, she said.
"It looked orange. It was visually disturbing to be on a lake that looked orange."
And her measurements showed that the water was so dirty that no light could penetrate to a depth of at least 30 meters.
Analyzing what's in the water is just getting started, researchers said. But they expect that, in addition to sediment, they will find such pollutants as lead and mercury that have been sequestered in local soils and in sediment on the bottom of wetlands and the St. Louis River estuary.
"There is potentially a whole host of contaminants flushed into the lake'' with the sediment, said Erik Brown, a geology professor at UMD. Not to mention the umentionables from overloaded sewer systems, pet waste and even wild critters in the woods. "Everything that is on the landscape," he said.
One of the crucial scientific questions, he said, is figuring out where all the sediment settles, because that will determine the long-term effect on the lake's ecosystem.
Big storms, more often
Understanding the lake's new dynamics is important because major rain events are occurring more often.
Between 1958 and 2007, the Upper Midwest saw a 31 percent increase in "intense" rainfalls -- those 1-in-100 storms -- compared to previous decades, according to the National Climatic Data Center.
The rainfall in Duluth last Tuesday and Wednesday, measuring from 7 to more than 10 inches across the city, was in some places nearly twice what is regarded as Duluth's 1 percent-chance rainfall.
Climate scientists say big rainfalls, particularly from intense thunderstorms, are a symptom of ongoing climate warming. (Warm air holds more water vapor than cooler air.)
Brown said his quick review of data from the Minnesota Climatology Working Group shows that rainfalls of more than 2 inches have come and gone in a decades-long cyclical pattern for Duluth. The number was high in the late 1800s, then dropped in the 1930s, but has been rising steadily again since the 1960s.
But, compared to the last century, the landscape around Lake Superior now has far fewer forests and far more farm fields and cities, making it much more vulnerable to erosion, he said.
Only time will tell how fast the sediment settles, and where, but data gathered in the next year will be critical in predicting the lake's future, said Austin-Minor.
"What it's going to do is anyone's guess," she said.
Josephine Marcotty • 612-673-7394