In the recent Minnesota Soybean Research and Promotion Council’s Sponsor Board article on MinnPost.com, University of Minnesota researcher Satish Gupta’s research on the sources of turbidity in the Minnesota River is cited in support of the conclusion that the observed turbidity in the river was from “natural causes.”
The following statement was made in the article: “… turbidity in the Minnesota River is caused primarily by erosion of streambanks that has been occurring for centuries.” This statement is true. What the Council’s article does not tell the reader, however, is that the rate of streambank and stream bed erosion in a watershed has increased significantly over time with changes in the runoff coefficient of the Minnesota River watershed and the increased efficiency of the watershed’s drainage system.
The runoff coefficient increased dramatically over the last century or so, during which time the original prairie sod was converted to cultivated fields and the agricultural drainage system expanded to deliver this runoff to the tributaries and main stem of the Minnesota River much faster than the unaltered drainage system did before the age of modern agriculture.
This increase in volume of water and alteration of peak flows is what causes rivers to begin to change their channels by eroding river beds and banks. It is by this erosion process that a river changes its channel adjusting its cross-sectional capacity to accommodate the “modified” hydrology of the watershed. This is basic fluvial geomorphology 101. Neither of these changes in the watershed is “natural,” and both are directly related to agricultural practices!
Conclusion is not supportable by science
While there are surely other non-agricultural hydrologic modifications that contribute to the problem as well, the conclusion stated in the Sponsor Board article that the elevated turbidity in the Minnesota is “natural” and not traceable to agricultural activity in the Minnesota River Basin is not supportable by science and is fundamentally incorrect. I suspect that even Gupta would have significant reservations regarding the Soybean Research and Promotion Councils’ interpretation of his work as presented in this article.
I hold several degrees in the environmental sciences and I’m retired (after 28 years of service) from the Minnesota Pollution Control Agency (MPCA), where I served as regional director out of Rochester and Detroit Lakes. During my employment with the MPCA I was required to study and identify various sources of water pollution impacting the streams and lakes of southern and northwestern Minnesota. Modified hydrology of watersheds in these regions from agricultural activities including increased runoff from cultivation practices and expansion of artificial drainage systems were identified as major factors in increasing stream bank and stream bed erosion.
These hydrologic modifications have been identified as major contributors to the suspended sediment and turbidity loads of the Red River of the North and of the Minnesota River.
I refer you to the Nonpoint Water Pollution Plan prepared by the Minnesota Pollution Control Agency submitted to and approved by the U.S. Environmental Protection Agency in conformance of requirements of the Federal Clean Water Act. (The plan can be seen here. Click on: 2008 Minnesota Nonpoint Source Management Program Plan and see Chapter 4.3 Rivers and Streams Strategy.)
Willis Mattison, of Osage, Minn., is a retired regional director for the Minnesota Pollution Control Agency.