In 2012, the Minnesota Pollution Control Agency (MPCA) prepared draft TMDL reports for turbidity in the Minnesota River and Greater Blue Earth River basins. During their public notice periods, both reports generated significant comments and requests for contested case hearings. In 2014, the state adopted new water quality standards for TSS that replaced the turbidity standard. As a result, the allocations for the turbidity impairments needed to be recalculated, and the 2012 draft TMDL reports were withdrawn from U.S. Environmental Protection Agency (EPA) consideration under Section 303(d) of the CWA. In the current report, TMDLs are developed using the TSS standard; these TMDLs replace the 2012 draft turbidity TMDLs. Impairments from both the Minnesota River and Greater Blue Earth River basins were combined into one report for efficiency. Land use in the watershed is dominated by agriculture, consisting of primarily corn and soybean rotations. There are also small amounts of urban area, wetland, and forest. Urban development is only significant in the Twin Cities Metropolitan Area. The primary sources of sediment in the project area include near-channel processes (e.g., bluff, ravine, and streambank erosion) and watershed runoff. Within each major watershed, near-channel sources account for between 63% and 83% of the TSS load. Much of the annual sediment load in the basin occurs in the spring as a result of snowmelt and spring storms in March through June. Drain tiling likely exacerbates sediment erosion in near-channel areas as a result of snowmelt and large storm events by increasing the rate and volume of water discharging to the river systems. The highest sediment concentrations occur during high and very high flow conditions.
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Minnesota Pollution Control Agency (St. Paul, Minnesota)
Minnesota Pollution Control Agency