The purpose of storm water detention is to slow the rate of storm water runoff. When land is developed less water is absorbed into the ground. The ponds slow the rate of runoff to approximate the rate of the undeveloped property.
Ponds can have a wet or dry bottom. If some water remains they are called retention ponds. If the pond is dry they are detention ponds The photo below shows a detention pond. During a major storm it impounds water and releases it through the drain in the lower right corner.
The photo above and below show a storm water retention pond. It fills up during flood stage and then releases water slowly. But some water is retained as a permanent ponds.
More info on the links below:
I wrote this post since a friend asked me what the difference was between detention and retention ponds.
Here was his humorous response:
"I now understand. Like being detained vs retained in Jail!"
Exactly!! Only different.