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3.1 Water Quality

  1. The water quality in Minnehaha Creek as measured by total phosphorus and TSS concentrations is comparable to the MPCA's ecoregion guidelines.  Phosphorus and sediment loads in Minnehaha Creek increase upstream to downstream, although the impoundments at the major grade controls act as settling basins and trap some of the pollutants and sediment.  Average chloride concentrations are generally lower than state standards, and dissolved oxygen concentrations are generally sustained at levels sufficient to maintain aquatic life.  Monitoring for e. coli bacteria shows increasing concentrations from upstream to downstream at levels sometimes exceeding state standards. 
  2. Six lakes in the subwatershed have been designated Impaired Waters on the state's 303(d) list due to an excess of nutrients.  The District has petitioned to remove two of those lakes.  The District is preparing Total Maximum Daily Load (TMDL) studies, including plans to reduce phosphorus loads into the lakes, for Nokomis, Diamond, Hiawatha, and Powderhorn Lakes.
  3. The HHPLS established and the Lake Hiawatha TMDL assumes an in-creek total phosphorus concentration goal of 80 µg/L.   The HHPLS estimated that achieving this goal would require a 15 percent reduction in phosphorus loading to the creek from the entire subwatershed, excluding the Chain of Lakes and Lake Nokomis lakesheds.  However, not enough is known about in-stream processes and sources to partition that load reduction between loading from runoff and loading from in-stream sources such as streambank erosion, internal loading, riparian wetlands, etc.
  4. Development in the subwatershed has resulted in increased stormwater volumes conveying nutrients and sediment to the lakes and to Minnehaha Creek.  Significant efforts and investments by the District and the local governments have been made to improve water quality in the lakes through improvement projects, restorations, and nonstructural BMPs such as education and street sweeping.  As a result Brownie Lake, Cedar Lake, Lake of the Isles, and Lake Calhoun meet or exceed their water quality goals.
  5. Development, redevelopment, and reconstruction in the subwatershed may provide opportunities to obtain a net decrease in volume of stormwater runoff, nutrient and TSS loads conveyed to those water resources.