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Muddying the waters
by Mark Steil, Minnesota Public Radio
August 3, 2004

Riverbanks along the LaSueur show clear signs of erosion. Researchers are trying to understand the erosion process (MPR photo/Mark Steil)

The effort to clean up the Minnesota River is running into some unpleasant realities. Some of the river's problems may be unfixable. Take the high bluffs that flank parts of the Minnesota and its tributaries. They routinely send tons of soil sliding into the water. Engineers could stabilize the banks, but there's not enough money available. Muddy water affects a river's health. It harms both plant and fish life.

	Warren Wagner

Worthington, Minn. — Warren Wagner and Mark Bosacker have been watching the problem over the years. They recently took a canoe trip on the Le Sueur River to see the problem first-hand.

The trip is barely under way when Wagner spots something. An old tree has fallen. It's top is in the river, the roots on shore.

"Still got semi-green leaves on it, so that just fell in about a week ago," says Wagner.

One week ago this river was flooding. Runoff from heavy rain pushed the Le Sueur out of its banks. Mark Bosacker is paddling in the front of the canoe, he says the downed tree illustrates the power of moving water.

	Roots

"The main current just bounces, like a bullet ricocheting off a wall and undercuts the trees," says Bosacker.

But that's not all that's happening. As the water bores into the bank its carrying off truck loads of soil. Much of that sediment finds its way to the Minnesota River. The Le Sueur joins the Blue Earth River near Mankato. Just a few miles on the Blue Earth empties into the Minnesota.

At some places along the Le Sueur, the water flows directly against vertical sandy banks. The scarred soil bleeds directly into the river. At other spots banks rise 200 feet high. Here, both the top and bottom are problems. At the water line soil washes out.

Higher up untold tons of sediment cascade down whenever the top lip breaks off. Sometimes long tree roots dangle in the air where a chunk has fallen. Warren Wagner says when heavy rains fall the river rises quickly, aggravating river bank erosion. He says the Le Sueur can rise four or five feet in a few hours.

	Mark Bosacker

"The farmers and you cannot blame them, like to drain the water off the field as quick as they can," says Wagner. "So the water gets drained through tile into the county ditch. And the name of the game is to get rid of the water as quick as you can. So the rivers come up very quick and once they're that high of course they drop down very quick."

The water coming off farm fields carries soil with it. Many people believe farmland runoff is the main cause of river sediment. It not only carries soil directly but also washes out stream banks.

But a study done several years ago shows there may be more to river sediment than just farmland runoff.

Satish Gupta is a professor at the University of Minnesota. He studied bank erosion on the Blue Earth River, the stream the Le Sueur eventually joins. The study was funded by several groups, including some farm organizations.

	Steep banks

Using laser beams and computers he was able to precisely map the river bank. The next year he went back and did it again. By comparing the two snapshots he was able to calculate soil erosion. His conclusion: much of the soil in the Blue Earth came from river bank erosion.

"As much as 56 percent of the load in the year 2001 to 2002 may be from the banks sloughing over there," says Gupta.

That finding surprised many people but even more startling was Gupta's second major conclusion. Most of the erosion came from the tops of banks, not at water level.

"If you look at the bank you'll see the top of the bank is 30-40 feet away, vertically, from the bottom of the bank. So that tells us the top of the bank is sloughing maybe faster than the bottom of the bank," says Gupta.

	Pointing to a problem

Gupta says what's happening is much like a west coast landslide. Most people have seen dramatic pictures of California houses sliding into the ocean when the top lip of a bluff collapses.

Gupta says river bank erosion is similar. Rain saturates the top layer of the bluff. The water logged mass breaks off, picking up more soil as it rumbles down to the river.

If Gupta's findings are accurate, they have big implications for cleaning up the Minnesota River and its tributaries.

Simply put, controlling farm land erosion is not enough. Something must be done to reduce river bank sediment.

Gupta says its very expensive to stabilize steep banks and wonders if the public is ready to spend what it takes. In the meantime the erosion continues. Gupta spoke to farmers along the river. One old timer told him he's monitored a steep bluff on his farm for about 60 years.

	River bank

"The bank has moved about 30 feet," Gupta says. "So that's about six inches every year. And it's a quarter of a mile long bank so if you calculate all the volume, it's 150 feet tall, that's quite a bit of sediment."

Gupta says Minnesota residents saw a dramatic example this summer of how the erosion works. The same heavy rains that pushed the Le Sueur River out of banks collapsed a portion of a bluff along highway 169 near Mankato in the Minnesota River Valley. The landslide closed the road for a time.

The effects of bank erosion can clearly be seen on the Le Sueur River. Sandy-looking bluffs rise nearly straight up from water's edge. But they have virtually no vegetation on them. Up close water trails are etched in the soil. Warren Wagner calls it natural erosion, he says there's not much anyone can do to stop it. Mark Bosacker though says modern farming practices have made the situation worse.

"I don't want to be picking on the farmers, we need farmers, we need food," says Bosacker. "But it's just a fact that much of the land that's being farmed up on the top of the river valley used to be potholes and sloughs and the water would be retained there for a while. And slowly get into the river."

He says rapid farmland runoff makes the river an erosion powerhouse. The fast flowing water carves soil from stream banks and the river bottom. It carries it effectively downstream.

He says in a slower moving river much of the sediment would quickly settle to the bottom.

University of Minnesota professor Satish Gupta says there's no doubt modern farming has aggravated river problems, but no one knows to what extent. Finding an answer to river sediment is a problem much of the mid-section of the country is interested in.

The Minnesota River and its tributaries are a major source of pollution for the Mississippi River. Some of it reaches the Gulf of Mexico. That gives the bluffs of the Le Sueur River an important role in a major ecological event.

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