Stormwater pond

Stormwater ponds keep pollutants out of lakes and rivers.

In residential developments, you will often see small ponds throughout the neighborhoods. These are stormwater ponds, created to collect stormwater runoff which is rainwater or snowmelt that runs off streets, parking lots, rooftops and compacted lawns.

The ponds are designed to look like natural lakes or wetlands, and as such are often mistaken for landscaping features. But they are there to prevent flooding and collect stormwater and pollutants like heavy metals, sediment, bacteria, oils and nutrients.

The ponds collect and filter the water, keeping pollutants out of lakes and rivers.

The filters: Each pond has a “buffer” around it. A buffer is an area of plants surrounding the pond, usually about 10 feet wide but sometimes up to 30 feet wide.

This buffer strip slows down and filters stormwater running off yards before the water reaches the pond. Removing the native plants in the buffer strip or mowing straight down to the pond gives polluted water direct access to the pond. Protect the buffer and you protect the pond.

But why does it turn green? Two factors can cause green ponds. The first is related to nutrients, specifically phosphorus, which causes algae blooms. These ponds are designed to capture pollutants, such as phosphorus, so they will rarely be pristine, but homeowners around the ponds can unknowingly increase phosphorus in ponds.

Another green factor is duckweed, which is often mistaken for algae. Duckweed is a tiny, free floating green plant that can form thick blankets across a pond.

Duckweed an important food source for birds and wildlife and it has such good water filtering capabilities it is used by water treatment plants. So, if its duckweed greening your pond, relax and enjoy the wildlife and water treatment.

Homeowner tips:

  • Do not dump grass clippings or leaves in the pond or near storm drains. This will cause algae blooms. Sweep or rake up any leaves from curbs and storm drains to keep them from washing down.
  • Do not pour chemicals, oils, etc. in or near the pond, or allow any household or automotive chemicals to drain into the storm drain. Use biodegradable soaps for outdoor cleaning.
  • Sweep up grit and debris on driveways and sidewalks, do not hose it into the street.
  • Capture stormwater runoff from roofs and driveways in rain barrels or rain gardens.
  • Pick up animal waste and dispose of it in the trash.

Mosquitos: As mosquito larvae develops in standing water, increased mosquitoes near ponds is a valid concern of residents.

If ponds are functioning correctly, it limits breeding sites for mosquitoes.

Additionally, treatment for mosquitoes is provided by the Metropolitan Mosquito Control District. The Metropolitan Mosquito Control District is responsible for protecting the public from disease and annoyance caused by mosquitoes, black flies, and ticks.

The district controls mosquito populations by targeting the larval stage using various control materials including Bti, a bacteria that inhibits larval growth.

If mosquitoes are a concern, the district has an interactive map where residents can log in and see how and when stormwater ponds have been treated for mosquitos. Visit and click on “Services” and then “District maps.”

Madeline Seveland is an education coordinator with Carver County Water Management. She can be reachedat