Boreal water resources face increasing stress due to changing land use, population growth, and climate change. The complex dynamics of catchment water retention and discharge profoundly impact the flow of water, transporting dissolved and particulate substances to surface water. Effective water resource management necessitates an understanding of the time and pathways that precipitation is transformed into streamflow. Fraction of young water (Fyw) which is the ratio of precipitation introduced within 2-3 months to streamflow. It is an alternative indicator for mean transit time (MTT), the average duration water spend traversing to the stream in a catchment and it has been recently proposed for precise understanding of hydrological dynamics in diverse catchments. Despite the importance of estimating the Fyw, limited efforts have been made in mesoscale Precambrian shield catchments, which is heterogeneously characterized by varying overburden thickness and extensive bedrock outcrop areas, with lakes and wetlands. This study determines Fyw over 3 to 5 years of precipitation and streamflow water isotope (δ18O) records for 13 catchments in the Sturgeon-Nipissing-French basin in Northeastern Ontario, CA, with drainage areas ranging from 27 km2 to 6,875 km2. The study aims to define the range of estimated Fyw values and identify landscape and hydrometric variables explaining their variation across catchments. For seven monitored stations, Fyw is estimated using both unweighted and flow-weighted δ18O. Preliminary results indicate Fyw values ranging from 18% to 51%, with no significant difference between flow-weighted and unweighted values. Further analyses found surface storage (wetlands) presence correlates positively with Fyw variability.