Researchers get dirty in the Le Sueur River

By Tim Krohn
Free Press Staff Writer

July 20, 2008 11:31 pm

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The Le Sueur River is the bad boy of the Minnesota River Valley.
It’s not known whether it’s just his nature or if it’s how he was raised.
A team of geologists and researchers from across the country are here this summer to try to determine just that.
One thing they do know is that the Le Sueur, along with the Blue Earth River it empties into, carries more dirt particles into the Minnesota than any of the other tributaries in the basin.
Carrie Jennings, with the Minnesota Geological Survey, has long done significant research on the Minnesota River Valley and is one of those participating in the study of the Le Sueur.
The project involves the Minnesota Pollution Control Agency, Minnesota State University, the University of Minnesota’s St. Anthony Falls Lab and the National Center for Earth Dynamics, part of the National Science Foundation.
With people from universities such as Johns Hopkins, the group is being referred to as a researching “dream team.”
The team has rented a house in Mankato for use as a base.
Sediment hurting aquatic life
Lee Ganske, supervisor of the Minnesota River Basin Watershed unit, based in Mankato, said there are actually two separate but related projects taking place.
One is a biological study, conducted by MPCA staff, of all living creatures in the Le Sueur from tiny invertebrates clinging to rocks, to large fish and turtles.
The university research teams are trying to pinpoint where sediment in the river is coming from.
“In many cases, biologists will find that sites have poor biological conditions because of the sediment. This will bring to a new level the ability to pinpoint some of those sediment sources,” Ganske said.
The impetus for the massive study lies far from the Le Sueur, which wends its way from southeast of Mankato to where it empties in to the Blue Earth River, which flows into the Minnesota at Sibley Park in Mankato. It’s Lake Pepin, hundred miles to the northeast, that is in part pushing officials to solve the mystery of the Le Sueur River.
Pepin, which is fed by the Mississippi, is loaded with sediment — almost all of it coming from the Minnesota River. The Minnesota delivers just 25 percent of the water flow to Lake Pepin but carries nearly 90 percent of the sediment filling in the lake.
“There’s been a tenfold increase in sediment in Pepin and the major source is the Minnesota River. That’s well documented,” Jennings said.
Jennings said it’s not known exactly why the Le Sueur gives up so much soil, or if the sediment load coming from the river has increased significantly over time.
The team of scientists will conduct a variety of tests and ongoing research to decide exactly how much dirt is flowing from the river and where it’s coming from.
Long a dirty river
It is known the Le Sueur and Blue Earth have long poured dirt into the Minnesota River. Early explorers wrote of noticeable muddy water flowing out of the Blue Earth long before white settlement and farming in the region.
Modern agriculture and tiling of fields have put more sediment into the rivers. Jennings said some farmers in the Blue Earth River basin have taken part in using best-management practices, such as putting grass buffers around tile intakes and along drainage ditches. “It’s helped, but there may be better ways to do it.”
One thing researchers want to know is the changes in cut — width — of the Le Sueur as well as changes in its elevation.
When the Minnesota River valley was carved out about 9,000 years ago, the tributaries were higher than the Minnesota.
“All the tributaries, like the Blue Earth, would have been waterfalls, but those have been nicked now. We’re trying to estimate how rapidly ravines have deepened,” Jennings said.
They also want to measure another natural phenomena — the widening of the river at the mouth. Rivers — unless they hit hard rock — naturally widen at the mouth, with the expansion moving upstream.
“The Minnesota basin is relatively young,” Jennings said. “What it wants to do now is to continue moving up the tributaries to drain the whole watershed. The river will work up the landscape eventually and completely drain it.”
Discovering trends
Ganske said the detailed study of the Le Sueur watershed is possible because of funding through the state’s Water Legacy Act, passed in 2006.
“It’s part of a statewide effort where every major watershed in the state, about 80 of them, will have this biological assessment done,” Ganske said.
He said that while there are two efforts going on, researchers will share the information.
The biological study being done by the MPCA involves a variety of ways to capture living things in the river, including electrical shocking of fish, netting and collecting insects and other invertebrates and then analyzing them.
“We will be assessing about 70 different sites,” Ganske said. “They will be analyzed and we’ll try to put that into context with the habitat and other factors to provide a good picture of where things are good and not so good and where the potential is for improving things.”
Ganske said the collection and analysis of scientific data can be a slow effort. Because isolated events such as heavy rains, droughts or floods can dramatically change the condition of the river at any given time, long-range comparable data are needed.
“It takes a long time for trends to emerge from the fog of that year-to-year variability.”

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