WILLMAR -- Naturally occurring river bank erosion appears to be a much bigger contributor of sediment to the Minnesota River than is being factored into our strategy for cleaning it up, according to Dr. Satish Gupta, professor of soil science at the University of Minnesota. He spoke at the Hawk Creek Watershed Project annual meeting on Wednesday in Willmar.
Reducing the sediment load in the Minnesota River is considered one of the most important objectives towards improving the river. Monitoring data at Fort Snelling has shown that the river carries 625,000 tons of sediment to the Mississippi River each year. Its sediment load is 3.6 times that of the Mississippi River, and 22 times that of the St. Croix.
Gupta and graduate students conducted research in the Blue Earth and Le Sueur Rivers, which are considered two of the largest contributors of sediment to the Minnesota River.
They found that many miles of riverbank on both rivers show evidence of major sloughing and erosion.
Much of the sloughing and obvious erosion they discovered was occurring on the upper portion of the banks, sometimes as much as 40-100 feet above the water line.
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Their research showed that seepage, capillary action, freezing and thaw cycles, and other natural mechanisms -- as well as the nature of the soils in the basin -- were responsible for the erosion. The glacial till soils in the landscape hold together firmly when dry, but add water, and those same soils have the strength of a wet doughnut, according to Gupta.
Water moves freely through the topsoil to the river. The steady seepage and capillary action is evident at the riverbank, where the flow is manifested by the erosion of river bank soil. From the vantage point of the river, we see the result as sloughing.
Gupta said the research also showed that lens of gravel and sand that naturally occur in the landscape also readily move water and contribute to sediment problems. As river banks slough and expose these areas to the flow of the river, they are readily undercut, aggravating the erosion problems.
That led to the question: How much did it all add up to?
The researchers employed a laser-based system to measure actual changes in the river banks. With the aid of computers, they produced a three-dimensional image that could help measure the amount of sediment actually being lost from river banks and entering the waters.
In the Blue Earth River, Gupta said a "conservative estimate'' determined that 56 percent of the sediments carried in the water come from this natural river bank erosion. Most of the strategies to reduce erosion in the river assume that most of the sediment is caused by human activities in the landscape, and natural processes, he said.
Gupta suggested that we should not necessarily be surprised by the natural erosion. He noted that the Dakota name for the river -- which was translated as tinted or colored water -- may speak to the natural tendency of the soils in this landscape to erode into the river.
Of course, sediment layers in Lake Pepin show that erosion in the Minnesota River has grown tremendously since white settlement and the plowing of the prairie began.
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Gupta said there is no doubt that farming practices -- and especially the transport of sediment via tile lines -- are major sources of sediment.
Nonetheless, Gupta pointed out that there will always be a certain level of sediment in the river that has nothing to do with human practices on the landscape. "I think it is a natural process,'' said Gupta of the river bank sediment in the Minnesota River. "What are you going to do about it?''
One possible answer may be perennial vegetation. Participants at the meeting pointed out that the nature of the topsoil in the landscape has been greatly altered by the elimination of the deep, thick rooted prairie vegetation that once filled the basin.
Gupta said it is possible that the return of perennial plants or other strategies may reduce the natural erosion, but the research hasn't been done yet to tell us. In fact, his research was ended by budget cuts. He said there remains a great deal about the natural erosion process in the basin that is still unknown.
