MANKATO — Backyards all along the bluffs of this picturesque country subdivision southeast of Mankato are slowly falling into the Le Sueur River.

A combination of geological forces and modern farm drainage has been creating a more powerful river that’s stripping the dirt from the riverbank and sending much of it downstream into the Minnesota River.

This has long been the case, though both natural and artificial forces appear to be accelerating erosion in recent decades.

Remedies typically involve massive amounts of rock with wire caging, an expensive proposition. 

But thanks to a $247,000 grant funded by the new sales tax-funded Clean Water, Land and Legacy Amendment, this area of the Le Sueur will get a new effort to stop the erosion.

Using natural materials, mostly timber and mud, engineers fill in part of the river and protect the bare bluff.

It’s cheaper than the rock and creates habitat for fish and other animals. It’s been used in northwest Minnesota and elsewhere in the country, but it remains to be seen whether the method will work in rivers in southern Minnesota.

It focuses on four sites along River Hills Lane, a rural subdivision to the southeast of Mankato that is essentially a peninsula surrounded by the winding Le Sueur.

We need help

Homeowners have appealed to the Department of Natural Resources and the Blue Earth County Soil and Water Conservation District for help.

Aerial photographs leave little question as to their plight.

One resident is facing a drop as steep as 60 feet. If the erosion doesn’t stop, their homes will have to be moved or fall into the river.

The problem is too expensive for the homeowners to solve. An engineer estimated in 2005 that one site would cost $433,000 to shore up with traditional stone methods.

Local governments don’t have that much cash sitting around, either. The county’s conservation district has a budget of $16,000 for 2010 for projects of this type, district manager Jerad Bach said.

But the Legacy fund, approved by voters in 2008, offered a much larger pot of money. With grant-writing help from Julie Conrad at Blue Earth County, they wrote a request that scored higher than similar proposals in Faribault and Waseca counties.

It’s called a “Toe Wood-Sod Mat with Woody Debris.”

The “toe” is the area that’s shored up next to the vulnerable bluffs, protecting them from the river. The “toe” will mainly be tree trunks imbedded into the riverbank and filled in with mud and sod taken from nearby.

Even if the new method is successful, it will only fix four out of the 800 eroding bluffs in Blue Earth County. The assumed lifespan for projects like this is 10 years, but officials hope this effort lasts longer.

Recipe for erosion

All rivers erode, but the geology, geography and land use of the Minnesota River watershed primes it to wash away more dirt than most. And the Le Sueur River is the best example of how you make a dirty river.

It drains only 7 percent of the watershed of this part of the Minnesota River, said Todd Kolander, a clean water specialist with the Department of Natural Resources who’s based in Mankato. But the Le Sueur contributes between 30 percent and 40 percent of the sediment found in Lake Pepin, a reservoir on the Mississippi River.

“The Le Sueur is considered a hot-button issue for looking at the amount of sediment contributed downstream,” Kolander said.

The story begins about 11,500 years ago, when the immense glacial Lake Agassiz drained, creating the River Warren, progenitor of the Minnesota River. The massive river bit deep, carving a valley 70 meters into the earth near Mankato.

This left the river’s tributaries hundreds of feet higher than their destination. Those smaller rivers, sped up by the descent to the Minnesota, are still carving through the watershed and seeking a stable path.

Another natural explanation is the increase in rainfall seen during the past century, said Conrad, with Blue Earth County. In Waseca, annual rainfall averages have increased 30 percent since the first half of the 20th century.

Then there’s us, people.

Kolander of the DNR said: “I think a bigger emphasis needs to be on altered hydrology in our watersheds. It’s one that a lot of people don’t want to look at.

“Look at watersheds like the Le Sueur, where 90 percent of the land is in agriculture or urban development.”

Humans don’t need to make it rain more to make bigger rivers. They only need to drain rainfall faster so the rivers swell and knock against their banks.

A dirtier river hurts habitat for fish and other animals along with changing the relationship between the river and its flood plain. Instead of a natural river system, you end up with a big drainage system.

Common interest

Clean water is good for everyone, but the project was funded partially because it benefits many groups.

For the landowners, it’s about saving backyards. The township would like to keep its road safe. The Department of Natural Resources and other agencies aim to reduce downstream sedimentation. Anglers and canoeists want cleaner and more productive streams.

The new approach also is attracting attention.

“We’re trying to show something innovative,” Bach said.