The dirt on river sediment
MONTEVIDEO -- The Minnesota River continues to carry more and more sediment to Lake Pepin, but it's not because we're losing the battle to reduce erosion in farm fields.
Increasingly, the evidence is indicating that the proliferation of artificial drainage has greatly accelerated the rate of erosion from the banks of ravines, streams and the river itself.
"The rate of non-field erosion is increasing, and it's not natural,'' said Dr. Shawn Schottler, an environmental engineer and senior scientist with the Science Museum of Minnesota.
Schottler and Dr. Carrie Jennings, a geologist with the University of Minnesota, outlined what is being discovered during a multi-year study focused on the sediment load building in Lake Pepin. They spoke during the annual meeting of Clean Up the River Environment on Saturday in Montevideo.
"The story of Lake Pepin is the story of the Minnesota River,'' said Schottler.
The Minnesota River contributes 38 percent of the water to Lake Pepin, but it provides 85-90 percent of the sediment, the study has shown. The Mississippi and St. Croix Rivers are the other major sources of water to the lake.
The Minnesota River's glacial legacy makes it naturally prone to erosion, but we've speeded up that process dramatically.
Prior to European settlement, sediment in the Minnesota, St. Croix and Mississippi Rivers was coming to Lake Pepin at a rate that would take 3,600 years to fill the lake.
Now, the sediment will fill the lake in only 300 years, said the two.
Schottler has been "finger printing'' the sediment in Lake Pepin to determine its source. He is able to differentiate between soils that originate in farm fields and those from river, stream and ravine banks.
From the start of farming to 1950, field soils accounted for the increasing volume of sediment reaching Lake Pepin. Sediment cores show that during this period the amount of sediment coming from farm fields continued to grow each year.
Scientists expected the sediment load to hold steady or begin decreasing after the 1950s. By then, 95 percent of the perennial prairie cover had been converted to farming and no new field sources of sediment were being created. Also, conservation practices and new technology were helping reduce field erosion.
And in fact, Schottler said the total mass of sediment coming from farm fields each year has not grown greatly from 1950 to today. It may be slightly declining.
But the overall load of sediment being carried by the river has continued to grow each year since 1950, with the exception of drought years.
Analyses taken of sediment cores in Lake Pepin and other water bodies in the basin show why: The amount of sediment coming from the banks of the river, and tributary streams and ravines is growing. These non-field sources of sediment now accounts for 67-70 percent of the sediment load reaching Lake Pepin, the reverse of what was the case in 1950, when farm fields were the major source.
"I think we've made our rivers more erosive over time, but I said I think,'' said Schottler. While we know where the added sediment is coming from, more research is needed to show why.
Both researchers said they suspect that artificial drainage is playing the major role. "We've changed the terms of it,'' said Jennings of the natural erosion in the glacier-carved landscape. "We've given it a lot more water to do its work.''
The great expansion of artificial drainage, along with a trend towards a wetter climate, means the natural process of erosion is happening much, much faster, they believe.
The Lake Pepin study could help guide future environmental policy in the region, the speakers noted. Funds aimed at reducing farm-field erosion might be more effectively spent to mitigate the erosion caused by drainage practices, noted the two.
They also noted that research is needed to look at how we can reduce erosion from drainage. It might be better to outlet tile lines directly into the river, instead of in ravine banks, for example.
Schottler also pointed out that the sediment coming from non-field sources does not carry the high amounts of phosphorus and nitrogen as sediment from fields. Its environmental impact is not the same.