Groundwater-surface water interactions are complex phenomena varying naturally over space and time that are influenced by land-use changes. The Whitson River watershed is a mixed land-use Precambrian shield watershed located in Northeastern Ontario, Canada in which groundwater is a source of municipal water supply experiencing increasing chloride concentrations, ongoing municipal drainage projects are being implemented supporting increased development, and periodic flooding, and climate change are long-term concerns. This study used watershed-scale synoptic surveys of stable isotopes (δ18O and δ2H), geochemistry (Cl-), and a corresponding mixing-model approach to quantify seasonal and spatial variation in surface water, ambient and urban groundwater contributions to streamflow. Mixing-model analyses showed groundwater contributes a critical, sustaining source to streamflow (> 50%) during summer baseflow conditions, and ~15% in fall when connection to shield lakes and wetlands dominate. Three-component mixing-models showed similar results to two-component models refining groundwater contributions into ambient and urban groundwater sources. Estimated urban groundwater (high Cl-) contributed ~30% to summer baseflow, dropping to ~4% in the fall. Baseflow estimates from graphical hydrograph separation were similar to mixing-model groundwater estimates in the summer but overestimates groundwater contributions during the fall. This discrepancy indicates the complexity of source water contributions to streamflow in Precambrian shield landscapes with significant wetlands, lakes, and in many instances limited overburden storage. Evaluating source water contributions to streamflow across the Whitson River watershed has generated a region-specific conceptualization for a basin undergoing change, and landscape-scale focus under-represented in scientific literature, providing information that will be of value for long-term management.