Temporary detention of stormwater runoff can improve water quality. The detention system study was constructed for the “Demonstration of Urban Stormwater Treatment” Marsh in Fremont, California. The wetland system covers approximately 22 ha and is divided into three separate subsystems. Each system was designed to allow testing of soil/vegetation/flow configurations for comparative effectiveness at removing water pollutants. System A simulates pretreatment at a wastewater treatment plant by reducing suspended particles. System B provides a combination overland flow and pond system divided into four cells. Secondary treatment is accomplished by System C, which provides storage, detention between storms, and a place for biological activity to reduce suspended and dissolved constituents. Electrical conductivity and chlorides in the groundwater zone directly below the marsh indicate that the groundwater is saline and is probably contiguous with the marsh at System C. There is a gradation from freshwater conditions at the System A and B inlets to brackish-water conditions at the System C outlet.