Contributions from infrastructure-mediated flow including the prevalence of groundwater, wastewater, tap water and delayed rainfall-derived infiltration into sewer systems to streamflow are important to quantify. Processes such as leakage from urban water and wastewater systems, and infiltration into storm sewers may impact the hydrologic cycle of urban watersheds. This study, conducted in Toronto's Black Creek research catchment, aims to discern water sources from storm sewer outfalls into the creek during inter-event periods and assess their significance for catchment-scale streamflow. Two storm sewer outfalls servicing mixed land-use sub-catchments were chosen for a year-long sampling campaign. Stable isotope analysis of hydrogen and oxygen in water samples from the outfalls, Black Creek, and potential end-members revealed seasonal variations in water sources. Flow at Outfall 1 was predominantly sourced by municipal water, while flow at Outfall 2 was predominantly groundwater fed, aligning with expectations based on the pipe elevation of the servicing area relative to estimated water table elevations. Initial estimates suggest up to 30% of inter-event flow in the creek could originate from storm sewers, though uncertainties persist. Future research challenges include defining data needs and employing geospatial-based methods to characterize sewersheds. This study casts light on the complexity of urban hydrology, providing insights into the diverse sources impacting streamflow dynamics in the Black Creek catchment.