Instead of using conventional storage units, e.g., reinforced concrete tanks and lined earthen basins, which are relatively expensive and require a lot of urban land area, the in-receiving water flow balance method (FBM) facilities use the receiving water body itself for storage volume. The FBM facilities have been operating successfully for approximately ten years for control of separate stormwater entering relatively quiescent freshwater lakes in Sweden, and are able to take ice and wind loads without adverse impact. The objective of the project which the paper discusses is to demonstrate a facility for Combined Sewer Overflow (CSO) storage in a harsh estuarine/marine site having tidal exchange, freezing, and coastal storm phenomena. The prototype demonstration facility located in Fresh Creek, a tributary of Jamaica Bay in Brooklyn, New York, U.S.A. started operation in November 1988. The evaluation includes CSO capturing efficiency under the impediments and flow saltwater and freshwater density differences and curtain leakage; structural ability to endure the harsh coastal marine environment; and floatables and settleable solids removal effectiveness. Interim data (including that from salinity profiling) from several storm-flow occurrences indicates that the saltwater-freshwater stratification phenomenon is enabling the facility to operate effectively and detailed results of the evaluation are presented.