WW-07694     2002  

Tillage Best Management Practices for Water Quality Protection in Southeastern Minnesota

Southeastern Minnesota's Water Resources and Soils

The Lower Mississippi River Basin of southeastern Minnesota is a varied landscape, rich in both soil and water resources. It contains more than 11,000 miles of rivers and streams that drain to the Mississippi River. Of these, 736 miles are cold and clear enough to be designated as trout streams. Although the region is considered "lake poor," it does include 115,000 acres of lake area. The counties included in this area are Houston, Fillmore, Mower, Winona, Olmsted, Wabasha, Goodhue, Dodge, and parts of Dakota, Rice, LeSueur, Steele, Waseca and Freeborn. As the area's population grows, and tourism and recreation become higher priorities for economic development, it will become ever more important to protect the area's high-quality water resources from potential degradation. Impaired waters will have to be restored to allow high-quality recreational experiences such as swimming, boating and fishing.

Productive loess soil supports crop cultivation on sixty percent of the land surface, making cropping the area's predominant land use. Two distinct areas of loess (wind-deposited silt loam soils) dominate the regional landscape (see Figure 1, page 6). One area has relatively deep to shallow deposits of loess overlying limestone and sandstone bedrock (MLRA 105) and has exceptional water- and nutrient-holding capacity. The rolling topography and gradual-to-steep slopes give the soils good internal and surface drainage (see Figure 2, page 7). The other major area, on the western edge of the rolling landscape, is characterized by gently sloping to flatter fields. The loess soil in this area developed as a cap overlying glacial till (MLRA 104). This soil also has good water- and nutrient-holding capacity, but has much poorer internal drainage because of the fine-textured subsoil. Loess soils, particularly those on rolling landscapes with steep slopes, are susceptible to erosion. To add to the problem, the southeastern region receives the state's highest average annual rainfall (30–32 inches), which often occurs as intense storms. The abundant moisture greatly benefits crop production, but it also creates the challenge of trying to control erosion and the transport of sediments to surface waters.

Why Erosion and Water Quality Are Concerns

Soil erosion, caused mainly by snowmelt and rainfall runoff, threatens the long-term productivity of the soil and contributes to the degradation of surface water quality. Suspended sediment from upland soil erosion, stream bank erosion and other sources can injure aquatic organisms. Light cannot penetrate as far, which limits the growth of rooted aquatic plants that are needed for a healthy stream ecosystem. Sediment deposits reduce pool depth, destroy fish spawning areas and reduce the ability of pollution-sensitive species such as trout and smallmouth bass to survive and reproduce. Of 42 officially listed stream reach impairments—based on monitoring—in the Lower Mississippi River Basin, the cause of impairment in 19 cases is turbidity, or "cloudiness," usually the result of suspended sediment in the water. Stream monitoring shows that turbidity or suspended sediment is a widespread problem throughout the basin, including in the Mississippi River.

Agricultural land uses have changed over the past few decades. A much higher proportion of farmed land is in row-crop production today than in the recent past, making it increasingly important that row-crop fields be managed to control soil erosion. In 1999, corn and soybeans occupied 80 percent of the total crop acreage in a nine-county southeastern Minnesota area, compared to 64 percent in 1975, according to the Minnesota Department of Agriculture. A dramatic increase in soybean acreage has come at the expense of hay, pasture and small grains. This means the potential for soil erosion—and the associated decreases in soil productivity and water quality—has risen substantially over the past two decades.

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