3.1 Water Quality
- 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.
- 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
- 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
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.
- 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
- 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