The Rouge River Watershed lies in TRCA’s jurisdiction. The Watershed’s drainage area is 336 km2, and encompasses numerous urban areas including the City of Markham, , Richmond Hill, Whitchurch Stouffville, and the City of Toronto. In 2012, TRCA initiated the Rouge River Watershed Hydrology Update Study, to update the hydrologic modelling previously completed in 2001, re-assess and re-establish the stormwater management criteria (specific to flood control over a wide range of events) for the whole of the Watershed. The hydrologic analyses for this Study have applied the PCSWMM hydrologic model. The model was calibrated using local flow data, and the model performance evaluated using criteria developed by the Wastewater Planning User Group (WaPUG). The hydrologic analyses applied synthetic design storms as a surrogate for continuous simulation and frequency analyses, hence the design storm selection included comparison to frequency flows determined from continuous flow data collected within the watershed. The study analyzed hydrologic conditions for existing, approved and future development per the current Official Plans within the watershed. The study recommendations establish unitary storage and discharge criteria for sizing of end-of-pipe facilities for quantity (flood) control for new development per the current Official Plans, and evaluated performance for the 2 – 100 year storm events, as well as the 350 year storm and the Regional Storm event. The study also provides recommended unitary sizing for distributed source controls to reduce runoff volume, based upon assessments of spatially-varied catchment-scale application and watershed-scale performance. The study recommendations and PCSWMM modelling will be used by TRCA to guide the planning and design of stormwater management within the watershed for years to come.