MCWD issues “State of the Watershed” report

The Minnehaha Creek Watershed District (MCWD) presented its annual State of the Watershed report to the public April 8. The MCWD, an autonomous government agency, is responsible for regulating water quality along a watershed area that runs from north of Lake Minnetonka all the way south and east to Minnehaha falls. The report detailed projected lake levels, gave a water quality report card for area lakes, and presented practical spring cleanup tips for citizens.

"Every year at this time, citizens in Minneapolis and throughout our watershed wonder about the potential for spring flooding and how clean the water is in our lakes," says Eric Evenson, MCWD Administrator. "Upstream, Downstream, The Annual State of the Watershed program provides insights into the issues that contribute to flooding and effective how-to tips for citizens to put into practice that help reduce pollutants in our lakes, creeks and streams."

In South Minneapolis, Lake Nokomis received an overall grade of "C" for its water quality. According to the MCWD, that places the lake between the 30th and 70th percentiles in terms of its suitability for aquatic activities like swimming, fishing and boating. The grade is determined by three measurements: the level of total phosphorus in the water, the level of chlorophyll-a in the water, and the "Secchi disk transparency" of the lake.

Phosphorus is a chemical commonly found in fertilizers. It tends to run off from lawns and storm drains into lakes and streams. A high level of phosphorus is responsible for increases in algae levels and more frequent algae blooms. Algae blooms can, in turn, choke off other aquatic species by consuming much of the oxygen in the water and blocking sunlight from reaching below the water surface. Even more important, for recreational users, a high level of algae makes for a rather mucky day at the beach.

The chlorophyll-a measurement serves a similar purpose: chlorophyll-a is the green pigment in plants. It is essential to photosynthesis, and a measure of its presence in water estimates algae abundance.

The Secchi disk measurement tests the water clarity directly: scientists lower an 8-inch white disk into the water and measure how far it can be lowered before it disappears. It sounds simple, but "It's actually surprisingly well defined at which point you can't see it" says consulting environmental engineer John Thene. The shorter the distance you can see into the water, the more muck there is.

Lake grades can be improved, the MCWD notes, but it takes work on the part of the public—particularly those living closest to the lakes. It may take changes in drainage patterns, flirtation, or detention of storm water. Not all lakes have the same potential, they say, since shallow lakes can only have so much Secchi visibility.

One major step that the MCDW has undertaken to improve the condition of the lake is the construction of special phosphorus-trapping ponds in areas where runoff is particularly bad. A set of three such ponds was constructed for Lake Nokomis with the help of the Blue Water Commission, a citizen-led group assembled to address water quality concerns.

Called wetland ponds, settling ponds, or storm water detention ponds, these natural-like products of environmental engineering function by capturing contaminated runoff before it enters a larger body of water. This allows phosphorus to settle at the bottom, becoming food for wetland plants. "We design them such that they have enough detention time and the water stays there long enough that algae will grow, consume phosphorus from the storm water, and die and settle out, so that the water coming out of the ponds has a significantly lower phosphorus concentration than what flows into them," says Jon Thene. He notes that the ponds seem to be working. "It's probably too early to tell, but last year we had a very high precipitation year, and in spite of that saw a slight decline in lake phosphorus." (Rain causes the runoff of phosphorus from the soil into the lake.)

The new pond sites at Lake Nokomis have been named Amelia (the original name of Lake Nokomis), Gateway and Nokomis Knolls. The ponds will be integrated into the landscaping, with each of the three ponds surrounded by distinctive wildflowers, grasses and water plants. The ponds are active this year and landscaping should be in full bloom by this summer. Pond construction began in fall 2000 with completion in the summer of 2001. Within the 200-acre park, 5.88 acres of wetlands were expanded to 8.2 acres.