Clackamas County is proposing to divert flow from its Kellogg Creek wastewater treatment plant, which is overloaded, to the nearby Tri-City plant, which is organically underloaded. The performance of wastewater treatment plants in the Pacific Northwest is heavily influenced by storm water flows. Therefore, the County decided to determine how the Tri-City plant effluent quality might be impacted by not only this diversion but also by storm water flows. The process simulator, BioWin, was calibrated for Clackamas County's Tri-City wastewater treatment plant. The calibration made extensive use of a comprehensive historical performance database, which was supplemented by two periods of intensive daily composite and diurnal sampling. Analysis of the summer and winter sampling campaigns allowed the stormwater and I/I component of flow to be segregated from the diurnal pattern of sewage flow. The simulator was used to forecast the impact of storm water flows on plant performance in concert with a desired transfer of flow from the Kellogg Creek treatment plant.