A flow through basin with discrete inlets and outlets designed to retain some volume stormwater runoff in a persistent pool of surface water. Designs may include additional detainment storage of stormwater for some minimum time to reduce peak flows. Wet pool capacity, treatment capacity and draw down time of treated volumes will vary across specific BMPs. One or more outflow offices may exist at different elevations. Lowest outlet elevation sets wet pool capacity. Dense vegetation is common. Dominant vegetation is wetland species and can be supplemented with riparian species with very high densities. Substrate is typically fine organic matter and silt, making volume reductions via infiltration negligible. Volume reductions occur primarily by evapotranspiration. Moderate to large sized centralized BMPs that accept runoff from mixed land use drainage areas.