Stormwater in Your Neighborhood

Why is there a pond in my backyard?

Ponds are built to clean stormwater that runs off roofs, lawns, driveways and streets. The water flows into the street, down the gutter, through a grate and into the storm sewer.


From the sewer, water usually ends up in a pond or another type of treatment system. Stormwater needs treatment because urban development – homes, streets, etc. – contributes pollutants such as phosphorous and sediment to stormwater that we don’t want to send downstream to natural lakes and rivers. 


Phosphorous comes from leaves, grass clippings, and dog poop; excess phosphorous is the main reason that algae grows in ponds and other water bodies.algaeExcess sediment (e.g. sand, soil) that gets into rivers or lakes can cloud water, which makes it hard for fish to find food and hard for sunlight to reach aquatic plants. Phosphorous is sometimes attached to sediment.

Ideally, phosphorous and sediment stay in stormwater ponds while clean water leaves the pond and moves downstream to lakes, wetlands, and the Mississippi River. Having algae grow in ponds is normal because so much phosphorous is stored there. It is better to have algae in ponds than in natural water bodies where it can negatively impact fish and plants as well as recreational activities. 

Every 20 years or so the accumulated sediment and other pollutants are excavated from the pond floor so there’s space in the pond for more sediment. 

Treatment of stormwater in manmade ponds keeps natural lakes and rivers sufficiently clean so people could use and enjoy them. Swimming and fishing, for example, were difficult or unwise in polluted water bodies. In the land of 10,000 lakes and a few major rivers, polluted water wasn’t good for business or pleasure.

Below is an aerial view of the pond and infiltration basin in the Cayden Glen neighborhood of Cottage Grove. 

Pond-infiltration Drawing

 Stormwater treatment was first made mandatory by the passage of the Clean Water Act by Congress in 1972. Since then, federal, state, and local laws and ordinances have been enacted with the goal of keeping natural water bodies clean. These laws and ordinances mean that ponds are constructed when a new neighborhood or parking lot greater than 1 acre is built.

While ponds are the most common stormwater treatment practice, Cottage Grove also has infiltration basins and underground treatment systems. 

Infiltration basins are often located next to ponds. When it rains, runoff flows into the pond first so sediment and other pollutants can be captured. Ponds are sometimes known as pre-treatment basins because they treat water before it goes into an infiltration basin. As the pond level rises, water flows through a pipe into an infiltration basin where water can seep into the ground. Allowing water to infiltrate into the ground is considered the best-case scenario. Any pollutants that were not removed as water flowed through the pond, will be removed by the soil as the water makes it way down into groundwater aquifers. And sometimes the sheer volume of water that flows off hard surfaces like roofs, driveways, and streets is a pollutant itself. It’s too much water flowing through the urban environment too fast. It’s best to let it infiltrate near where the rain fell. 

Cottage Grove has sandy soil that allows for quick infiltration. 

In commercial districts – like 80th Street South at East Point Douglas Road – where development is dense and land is valuable, ponds and infiltration basins are not cost-effective. Large retailers with big parking lots in Cottage Grove and throughout the Twin Cities often have underground storage pipes that can store up to a million gallons of runoff water. The pipes have segments where sediment and other pollutants are captured. The pipes allow stored water to infiltrate slowly into the ground over a couple of days.

Check out this YouTube video to see how underground storage pipes manage stormwater.

Pond inlets and outlets are surrounded by rock, called rip rap. The rip rap, and a layer of synthetic fabric which is placed under it, prevent erosion. Erosion would undermine the pipe and perhaps lead to cracks or other failures.  Outlet_riprap

Rip rap is also placed on top of berms that separate ponds from infiltration basins. The rip rap is placed about 6 inches to a foot below the rest of the berm to create an emergency overflow. In a large rain storm, the pond might be filled to capacity and beyond. The emergency overflow provides a wide area where a large volume of water can flow into the infiltration basin. The rip rap and synthetic fabric under it stabilize the overflow and prevent erosion when water flows over.IMG_3840

This photo shows the pond on the north side of 65th Street east of Hinton Avenue in the Calarosa development. The concrete outlet structure sticks up on the left. Water flows through this structure and out to storm sewer under the street. To the right is an inlet with rip rap. Barely visible in the center of the photo are two concrete pipes within a berm. Water flows through these two pipes into a filtration basin, which can be seen in the distance. 

A filtration basin is slightly different than an infiltration basin. A filtration basin has plastic pipes buried underneath to capture water after it is filtered through a layer of special soil. The plastic pipes then route water into the storm sewers. Filtration basins are used in situations where the existing soils are full of clay and not conducive to infiltration. Or when other regulations prohibit infiltration. 

The filtration basin in the picture is planted with native grasses and wildflowers. The pond is surrounded by tall native prairie grasses. The grasses have a long root structure that binds with soil and prevents erosion on the banks of the pond. The grasses also provide habitat and food for pollinators, birds, and other living things. Native flowers among the grass provide aesthetics as well. The City’s policy is to maintain native grasses around all stormwater basins. 


Silt fence and bio-logs can be seen around construction sites. They are installed to prevent soil from leaving the construction site during rain. Because there is no grass or other vegetation covering the soil, rainwater runoff could carry soil off the construction site. Silt fence and biologs prevent sediment from leaving a construction site. They should remain in place until the area is sodded or other vegetation is established. 

What Can You do to stop Stormwater Pollution?

  • Wash your car on your lawn or use a local carwash: chemicals from the soaps used to wash your car can get into storm drains and pollute local rivers and ponds.kayak
  • Reduce the amount of fertilizers you use or use organic fertilizers: runoff from your lawn can bring fertilizers to storm drains. By cutting down on the fertilizers used and/or using organic fertilizers you can reduce the impact of this fertilizer on stormwater pollution.
  • Keep track of what goes into storm drains: Do not pour anything into a storm drain you wouldn’t pour into the river or a local pond.
  • Maintain your car or truck: leaks from the vehicle can end up in storm drains and cause pollution. 
  • Consider removing impervious surfaces at your yard: These surfaces can aid in the transportation of pollutants to stormwater drains.
  • Dispose of Cigarettes correctly: Cigarettes that end up on sidewalks and other surfaces can pollute stormwater.
  • Collect Rainwater using a rain barrel: By collecting water with a rain barrel there is less water going to the storm drain and in turn less polluted water running into local waterways. Use the collected rain to water outdoor plants, flowers, and trees.