Erosion and Water Quality Concerns
Total suspended solids (sediment) and phosphorus are major water quality concerns in the Minnesota River Basin. Soils in the Basin have a high silt and clay content. Eighty-six percent of the suspended sediment in the Basin is characterized by fine particles of silt and clay that are easily transported by water. The Minnesota River carries more suspended sediment than most of the stateís rivers (MPCA, 1997). Excess sediment degrades the river system by filling reservoirs, destroying aquatic habitat, and reducing the riverís aesthetic qualities. Phosphorus originates from many sources in the Minnesota River Basin. Elevated phosphorus concentrations during low flow often indicate point sources (municipal and industrial sources), whereas elevated concentrations that occur mainly during higher flow periods can indicate nonpoint sources, i.e., sediment from eroding fields and construction sites and runoff water from fertilizer, manure, and plant debris.
Soil erosion that delivers sediment into surface waters of the Minnesota River Basin has been identified as a key source of nutrient (phosphorus) enrichment and turbidity (cloudiness) of the rivers. Phosphorus enrichment and cloudiness promotes algal blooms, reduces oxygen levels, and interferes with the biological and aesthetic well-being of the rivers. Improved crop residue management on agricultural soils is one practice that can reduce erosion and subsequent sediment and nutrient losses from these fields. Crop residue on the soil surface protects the soil from the impact of raindrops and minimizes the dislodging of soil particles. Crop residue on the soil surface may also improve infiltration of precipitation into the soil, thereby reducing surface runoff and increasing stored water in the soil profile for crop use.
A comprehensive monitoring program was conducted at sites near the entrance of the major tributaries to the Minnesota River by the Minnesota Dept. of Agriculture and Metropolitan Council Environmental Services during 2000-2002. Sediment loading to the Minnesota River came primarily from the eastern side of the Minnesota River Basin, notably the Blue Earth and Le Sueur watersheds, which receive greater amounts of precipitation and have greater runoff compared to watersheds upstream from New Ulm (MPCA, 2002). Sediment concentrations and yield (pounds of sediment per acre) were greatest for the Le Sueur, High Island, and Sand Creek watersheds in all three years. Sediment concentrations averaged 918 mg/L in 2000 for the Le Sueur River, 508 mg/L in 2001 for High Island Creek, and 404 mg/L in 2002 from Sand Creek. These flow-weighted mean concentrations substantially exceeded the turbidity-based threshold of 58 mg/L and the sediment range of 10 to 61 mg/L considered typical for streams in the Western Corn Belt Plains (MPCA, 1993).
Similar to sediment, total phosphorus loading was greatest for the Le Sueur and Blue Earth Rivers. Total phosphorus yields (pounds per acre) and concentrations were generally greatest for the Le Sueur, High Island Creek, Sand Creek, and Blue Earth Rivers. Flow-weighted mean concentrations of total phosphorus in these rivers ranged from 0.24 to 0.93 mg/L, substantially exceeding the EPA guideline of 0.1 mg/L. Because 70 to 80% of the total P in the rivers was in a particulate form (soil particles), conservation practices that reduce sediment loading can be expected to reduce most of the total phosphorus loading.