WILLMAR -- Like a stuck accelerator pedal, we've speeded up the delivery of sediment to Lake Pepin so that it will fill up in a mere 300 or so years, instead of the 3,600 years that natural processes would have required.
We've pointed the finger of blame largely at farm fields in the Minnesota River basin. The river is responsible for 25 percent of the flow and 85-90 percent of the sediment reaching the scenic lake.
We're discovering that erosion from farm fields is indeed part of the problem, but the bigger and growing source of erosion is from non-field sources, primarily stream banks, ravines and gullies, according to Dr. Shawn Schottler, senior scientist at University of Minnesota, and Dr. Carrie Jennings, a geologist with the University of Minnesota.
They described their work to "fingerprint'' the sources of sediment to the lake, and their surprising results at the annual meeting of the Hawk Creek Watershed Project in Willmar on Wednesday.
They found that the amount of sediment carried by the river has grown nine-fold since the arrival of agriculture to the river basin in the late 1800s. And without a doubt, sediment coming from fields started growing as soon as the first plows turned the prairie. By 1940 the amount of sediment coming from farm fields represented 52 percent of the sediment carried by the river, according to their research.
ADVERTISEMENT
Yet like cruise control already set at highway speed, the total load of sediment reaching the river from farm fields has remained pretty much the same ever since.
But the total tonnage of sediment from non-field sources has continued to grow, and steadily so.
Non-field sediment now represents 65 percent of the sediment, and this much is certain. "It's not natural,'' said Schottler.
Schottler said his hypothesis is that artificial drainage is the cause. "We have altered the hydrology and made our rivers more erosive,'' he said.
Work is needed to determine exactly what the mechanism causing the greater erosion is, and where the greatest erosion is occurring on stream banks, ravines and gullies. Are they sloughing away grain-by-grain, or are they calving large chunks of sediment like glaciers falling into the sea?
We don't really know, said Jennings.
We know the obvious: The greatest erosion is occurring in those basins where ravines and gullies are steepest, are comprised of erodible soils, and are located in close proximity to waterways.
By these standards, those most vulnerable to erosion are located primarily in the lower reaches of the Minnesota River basin.
ADVERTISEMENT
We also know that the runoff ratio - or amount of water that reaches the Minnesota River after a rain event rather than percolating into groundwater or evaporating in wetlands -- is increasing in the basin.
Again, the most dramatic increases are found in the lower basin, especially in the Blue Earth and Le Sueur River basins.
The increase in the runoff ratio is taking place upstream as well, apparently along with the proliferation of artificial drainage. The runoff ratio in the Chippewa River basin has increased by nearly 50 percent in recent years, according to the study.
In the Redwood River basin, where more has been done to channelize and connect drainage, the increase is closer to 200 percent.
In contrast, there has been no increase in the runoff ratio in watersheds where little artificial drainage has been installed, and that includes watersheds with growing urban populations, according to Schottler.
