The Attawapiskat River catchment makes up a ~57,000 km2 area in Ontario?s Far North extending from Precambrian Shield headwaters through the Hudson Bay Lowlands (HBL) ecozone to the coast. The region is peatland dominated and the low gradient, large expanses require further analysis and study to address uncertainties about their variations in hydrologic response. Recent hydrologic or catchment classification studies aim to assess broad-scale hydrologic systems in terms of smaller ?building blocks? to help develop hypotheses of how hydrologic systems function within specific terrains, but few if any have focused on low gradient peatland dominated systems. This study applies Principal Component Analysis (PCA) to representative catchments within the HBL ecozone, the Boreal Shield and the transition between the two in the Attawapiskat River watershed to assess hydrologic similarity based on physical, climatic and hydrologic characteristics. Different assessments of hydrologic similarity between catchments were made based on the combination of metrics/characteristics included in seven scenarios. Physical and terrain-based characteristics grouped catchments by physiographic region (HBL, transition zone and Shield), while hydrologic characteristics (i.e. tracer and flow-based metrics) grouped catchments both by physiographic region and partly by groundwater influence. Physical and terrain-based characteristics were found to exhibit the most control on the PC-space while hydrologic characteristics provided additionally important details about source water contributions to overall catchment hydrology. This study illustrates the importance of tracer-based/flow metrics in hydrologic similarity analyses.