Scott Watershed Management Organization

Lake O’Dowd and McMahon (Carl’s) Lake are both getting cleaner!

Good news!  Scott WMO can now definitively say that both Lake O’Dowd and McMahon Lake are getting cleaner.  Lake O’Dowd is located in the Louisville Township and in southwest portions of the City of Shakopee.  McMahon Lake is located in the southeast corner of Spring Lake Township. 

As you can see in the below graphs, Lake O’Dowd has been improving since about 2007. Lake McMahon has seen mostly improvements since around 2008. The graphs both show a reduction in the nutrient, Phosphorus.  Phosphorus is one of three parameters used to measure water clarity and cleanliness. Phosphorus impacts both of the other two parameters: chlorophyll-a levels (chl-a in the graph below) and transparency of the water (secchi in the below graph). Cholorphyll-a is a plant pigment and is used to measure the amount of algae.  Excess phosphorus causes algae and other plants to overgrow resulting in green and scummy water (high levels of chl-a).  In order for plant and animal communities in lakes to thrive, they need clean water.

The lakes are likely improving because:
  • Landowners surrounding the lakes have participated in both water quality monitoring and lake improvements, like native shoreline stabilizations.  Please see the McMahon (Carl’s) Lake Improvement factsheet for more information.
  • Much of the agricultural land draining to the lakes has been converted to large-lot residential (to view the changes between 1964 and 2015, please use this Land Use Map Viewer).  Large residential lots are better for water quality than both farmland and large urban areas because they maintain longer-living vegetation throughout the year.  Farmlands typically lack vegetation for much of the year.  Bare soil inevitably runs off into waterbodies, adding both sediment and nutrients.  Large urban areas often contain more paved surfaces, like roads and sidewalks, which increase stormwater run-off.  In many cases, stormwater run-off is not filtered prior to draining into lakes and rivers.  So, stormwater drains can directly carry pollutants to our waterbodies.
  • Both lakes are being treated for Curlyleaf Pondweed, which is an invasive plant that adds phosphorus into the lakes when it dies.  Both lakes have reduced levels of the invasive plant compared to ten years ago.  Please see our annual reports for more information on Curly-leaf Pondweed treatment and reduction.
  • Lastly, over ten years ago fertilizers containing phosphorus were banned from use within the metropolitan area.  Fertilizer and lawn clippings can easily run-off into storm drains and collect in lakes and rivers, which can greatly increase phosphorus levels.  The downward trend in phosphorus levels shown in the two graphs above suggest that when residents stopped using fertilizers containing phosphorus, water quality improved in both lakes. 
What happens now?
  • Both Lake O’Dowd and McMahon Lake now meet water quality standards for all three parameters.  Therefore, the Scott WMO requested and was granted removal of both lakes from the Minnesota Pollution Control Agency’s Impaired Waters listing.  
  • We can all enjoy the benefits of clean water: a clear look, healthier plant and animal communities, and probably fewer recreational disruptions (such as restrictions on swimming or fishing due to bacteria outbreaks). 
  • This is clearly a win, and we can all thank those residents living on both lakes for practicing good environmental stewardship.  However, maintaining clean lakes will require continued water quality monitoring and dedication to managing best practices (like native shoreline stabilizations).