What is the difference between a storm water Retention pond and a storm water Detention pond?
Retention Ponds or Basins: are designed to take and hold water indefinitely. Usually only releasing any captured water through evaporation into the air or infiltration into the soil. Retention Ponds can hold water for long periods of times, whole months or seasons. Usually considered as a VOLUME based measure.
Detention Ponds or Basins: are designed to temporarily hold water, with some kind of controlled outflow to let the water escape slowly. Usually with either a engineered outlet structure or a pump, the water stored within a detention basin will typically be evacuated within a few days or week. Usually considered as a FLOW based measure.
A common misconception is that the difference lies in whether or not there is always water in the pond. That is not true. You can find a Retention pond that has emptied out over the summer, and you can have a Detention pond designed to have a permanent pool.