These dams serve to decrease peak flows during spring runoff and summer rainstorm events by retaining water for a short period of time to reduce flow rates. These structures are often incorporated into existing municipal roads.
 

The purpose of a dry dam is to slow any runoff that occurs. Upon filling the dam, water is "metered out" through a culvert located at the base of the dam. Essentially, the dam removes the "peak flow" of water within a particular watercourse. To view an animation of how this works, look at the Steppler Dam rain event.

 

	This picture is of a typical flood dam that is full. Notice that the vegetation has really grown in around the water and on the banks.