Retention basins, or wet ponds, retain a deep, permanent pool of water that can collect stormwater and release it slowly to maintain a desired water level, after which the excess stormwater is released slowly via an outlet (drawdown orifice). Retention basins should always have a baseline level of water present and may be vegetated. Retention basins provide a higher level of pollutant retention (up to 80 percent) and a lower chance of sediment resuspension than detention basins (dry ponds).