DULUTH, Minn. (AP) - Sand, mud and gravel from last week's torrential rains and flood in northeastern Minnesota are spreading into western Lake Superior, giving it a brown tint visible in satellite photos from space.
Hundreds of tons of material fouled the lake miles out from river mouths. Boaters have reported entire trees floating in the lake, and satellite photos show a ring of brown around the Twin Ports, the near North Shore and up the South Shore to Chequamegon Bay.
The erosion spill may be unprecedented, said Erik Brown, acting director of the Large Lakes Observatory at the University of Minnesota Duluth. And the muddy mix isn't just at the top of Lake Superior, Brown said.
Clay-laden waters from the Duluth, Minn.-Superior, Wis., Harbor Basin pour out past the Aerial Lift Bridge through the ship canal and into Lake Superior recently. (AP photo)
"We had people out on the water taking samples, and it was the same right down to the bottom at 100 feet deep," Brown told the Duluth News Tribune. "They were recording essentially zero light penetration from the top to the bottom."
The water was so cloudy that tiny creatures called copepods, which usually rise out of the depths only at night, were swimming about at midday.
"They thought it was night," Brown said.
Crews netting fish to assess lake trout populations found visibility of just six feet in the water this week compared with being able to see down 35 feet last week, said Josh Blankenheim, Minnesota Department of Natural Resources fisheries specialist at French River.
"I've never seen quite this much sediment coming out of the rivers," Blankenheim said.
The mud, muck and clay already are settling to the bottom. But the exact impact of all that sediment is uncertain. There are concerns that the sediment will cover lake trout spawning reefs and make them unsuitable for eggs to properly incubate and hatch.
"That's a possibility, if that clay silt fills in between the rocks on the reefs," Blankenheim said. "But we don't really have any research that tells us what's going on down there."
Brown said that each tiny grain of clay also carries nutrients and pollution, such as pesticides. And if there's too much turbidity, even the Duluth water plant sometimes shuts down its pumps, supplying city water off its reserves and waiting for the lake water to clear.
As many as 10 inches of rain pounded the Duluth area over two days last week, breaking a more than century-old record.