 If runoff enters directly into a swale, perpendicular to the main flow direction, the edge of the swale acts as a buffer and provides pre-treatment for the water entering the swale.Ponds and Lakes are artificial bodies of open water that are usually created by constructing a dam wall with a weir outlet structure.Similar to constructed wetlands, they can be used to treat runoff by providing extended detention and allowing sedimentation, absorption of nutrients and UV disinfection to occur.