The impact of climate change on water resources in Northeastern Ontario watersheds is expected to be highly complex due to the variation in land cover and geology within the Precambrian Shield. Our study focuses on estimating source water partitioning to streamflow, given the limited regional studies on anticipated climate change impacts. We predict streamflow and changes to streamflow contributions for the period of 2020-2082 using a distributed hydrological model (isoWATFLOOD) in the Sturgeon River-Lake Nipissing watershed (12,000 km2) (headwater to Georgian Bay, Lake Huron). Results show that isoWATFLOOD can accurately simulate the timing and pattern of streamflow and ?18O signatures across the SN watershed during calibration and validation periods. Using eight different GCMs (RCPs 4.5&8.5), our findings indicate that climate change anticipated for the period of 2020-2082 will have a significant impact on streamflow rates and patterns throughout the SN watershed. On average, there will be an increase in annual discharge of 4.8% -11.5% across the watershed due to warmer fall and winter seasons, fewer days with freezing temperatures, more annual precipitation, and an increase in the number of days with extreme precipitation. Our study reveals predicted changes in hydrological partitioning due to climate change, such as a decrease in the annual average daily lower-soil-zone water contribution to streamflow (-21% to +15%) and an increase in average annual daily direct runoff. Collaboration between hydrologic modellers and regional water resources managers is essential to evaluate current management plans and ensure that municipalities are prepared for the challenges of climate change.