Flood muddies Lake Superior like never beforeIf you are wondering where all that sand, gravel, mud, debris, dirt and clay went after last week’s torrential rain and flood, just look out at Lake Superior.
By: John Myers, Duluth News Tribune
If you are wondering where all that sand, gravel, mud, debris, dirt and clay went after last week’s torrential rain and flood, just look out at Lake Superior.
In what may be an unprecedented erosion event, the western reaches of the world’s largest freshwater lake looked more like a mud puddle over the past week as hundreds of tons of material fouled the lake miles out from river mouths. Boaters report entire trees floating in the lake.
The chocolate milk tint to the lake went farther out and farther down than previously noted, and was visible from space in satellite photos showing a ring of brown around the Twin Ports, the near North Shore and up the South Shore to Chequamegon Bay — the result of up to 10 inches of rain in two days last week that caused the worst flooding in modern times,
“It’s unprecedented, really. No one I’ve talked to has ever seen the lake like this," said Erik Brown, acting director of the Large Lakes Observatory at the University of Minnesota Duluth.
“And it wasn’t just on the top. We had people out on the water taking samples and it was the same right down to the bottom at 100 feet deep,” Brown said. “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, something no one had ever seen.
“They thought it was night,” Brown said.
Josh Blankenheim, Minnesota Department of Natural Resources fisheries specialist at French River, said crews netting fish to assess lake trout populations found visibility of just six feet in the water this week compared to being able to see down 35 feet last week.
“I’ve never seen quite this much sediment coming out of the rivers,” he said.
Already the mud, muck and clay — tons of clay — are settling to the bottom. But exactly what impact the sediment will have there 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 … but we don’t really have any research that tells us what’s going on down there,” Blankenheim said.
Brown notes 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.
Of all the lake’s major tributaries, by far the worst offender is the Nemadji River. Soil and water conservation experts have been studying Nemadji erosion at least since the 1970s and have been working to reduce upstream erosion to curb the clay washing down into Lake Superior.
According to a U.S. Geological Survey study in 1993, the Nemadji has the highest sediment load of any Lake Superior tributary in Minnesota or Wisconsin — about 131,000 tons of clay silt every year. That’s the equivalent of 23 big dump truck loads of clay unloaded into Lake Superior every day. And it’s possible that last week’s flood may have equaled that total in one event.
It’s not just an environmental concern. Nearly every year the U.S. Army Corps of Engineers must dredge about 33,000 tons of clay out of the Superior harbor, where the Nemadji flows into the lake, just to keep access open to the Superior waterfront, including the Burlington Northern ore docks.
The clay and sand, left by glacial Lake Duluth centuries ago, are a fragile base for an ecosystem much more prone to erosion than the nearby St. Louis River, which runs through more rocky and boggy country. While sediment core samples at the river’s mouth show scientists that the Nemadji has always been sending some clay to Lake Superior, it’s clear that the amount of clay has increased in the past 130 years. That’s when large-scale logging and farming started to affect the river’s 433-square-mile watershed in Carlton, Pine and Douglas Counties along the Minnesota-Wisconsin border.
Farming, development and roads all help speed the flow of water across that exposed clay and into the Nemadji system. And it appears that early logging of pines along the river’s often steep banks exposed the red clay to more and more severe erosion in the form of large slumps of clay that fall into the river during rainstorms and spring snowmelt.
Efforts to curb the erosion started in the 1970s with the Red Clay Project. They expanded in the 1990s and continue today, with much of the effort focused on planting trees to slow water runoff 60 miles and more upstream from Lake Superior.
Two years ago the U.S. Department of Agriculture announced a $249,000 project aimed at reducing the sediment that fouls the Midway and Nemadji rivers and pours into Lake Superior during snowmelt and after nearly every rain. The Natural Resource Conservation Service used the money to plant trees, mostly on private land, at key locations along the Nemadji and its tributaries. The government paid up to 90 percent of the effort to try to slow the flow of water.
The problem isn’t unique to the Nemadji, however. Up the South Shore of Lake Superior clay banks are the predominant feature, often slumping right into the water.
In Chequamegon Bay, sedimentation is considered one of the most pressing water quality issues by the Chequamegon Bay Area Partnership, a coalition of agencies, nonprofit groups and educational institutions. The group is focusing on Fish Creek for watershed restoration efforts. The creek near Ashland sends tons of clay into the bay of Lake Superior after big storms.
The problem “results in the loss of aquatic habitat," Tom Fratt, conservationist for the Ashland County Land and Water Conservation Department, said in a statement Wednesday. The clay sediment can even reduce boat access to the lake. And Ashland, like Duluth and Superior, gets its water from the lake. The clay sediment can “increase the cost of water treatment through additional filtering and disinfection," Fratt noted.