Lake Calhoun once again aced Minnehaha Creek Watershed District’s annual lake grades report, earning its seventh consecutive “A” grade for water quality.
The report assigns letter grades to lakes in the 181-square-mile watershed district based on measurements of water clarity, algae growth and nutrient levels. This year’s report is based on samples collected between May and September of 2014.
The grades typically do not change much from year to year and provide a limited picture of the overall health of a body of water, said Kailey Kreatz, a watershed district water quality technician.
“That’s not factoring in invasive species and excessive plant growth or zebra mussels covering all the rocks for swimming, so just because it is an ‘A’ doesn’t mean it’s a perfect ecological system,” Kreatz said.
That’s why the district is planning a shift to a new, more comprehensive reporting system by 2017. The new Ecosystem Evaluation Program, shortened to “E-Grade,” will rate bodies of water on a wider range of factors, including flood control, biodiversity, habitat diversity, recreation, drinking water supply and nutrient cycling.
E-Grade will also distinguish between deep lakes, shallow lakes, streams and wetlands. The more comprehensive reports will be released on a 10-year cycle, but the watershed district still plans to release annual lake grades as the new system gets up and running.
On this year’s report, very lake in the Chain of Lakes scored above a “C,” which is considered average water quality for the Twin Cities metropolitan area. Lake Harriet’s “B+” was the second-highest grade in the chain, followed by Cedar Lake’s “B.” Brownie Lake’s “B-” was a full letter grade higher than its last reported grade in 2012, and Lake of the Isles scored a “C+.”
All of those lakes, part of the Minnehaha Creek sub-watershed, are monitored by the Minneapolis Park and Recreation Board, which is responsible for providing the water quality data to the watershed district. Other major lakes in the sub-watershed include Lake Nokomis (“B-”), Lake Hiawatha (“B-”) and Powderhorn Lake (“D”).
The 2014 open-water season was marked by heavy rains and flooding in the late spring and early summer. Testing didn’t register any “noticeable, significant changes” in water quality due to flooding, Kreatz said, but all the rain likely carried pollutants from lawns and streets into Minneapolis lakes.
“Phosphorous peaked a little higher in some of the lakes in the Chain of Lakes, which could definitely be due to the increased runoff from the rain that we had,” she said.