Retention and Detention Ponds
Retention and detention ponds are man-made bodies of water that collect runoff, including storm water runoff, from higher ground. These ponds are typically installed to control the runoff and erosion caused by new developments like a neighborhood, shopping center, road or parking lot.  

What's the difference?
A retention pond has a fluctuating water level, which chances depending on the amount of rainfall, but will always retain some water. Detention ponds are designed to prevent flash floods by holding storm water during periods of heavy rain to storm water runoff and releasing it slowly.

How does it fight pollution?
Retention ponds are typically built in highly developed areas where concrete surfaces prevent water from seeping back into the ground. Surface pollution is swept into the pond by storm water that carries oil that leaks from cars, pesticides, fertilizers and other harmful chemicals. This keeps them out of rivers and lakes, and eventually biological processes will break them down.

More information
Most cities require retention ponds for certain new construction projects. Find out if your city does before you build.