Active subglacial water influences ice flow dynamics and glacier mass balance, but direct observations are scarce due to the inaccessibility of fieldwork. Here, we stack the ArcticDEM strip elevations and produce the first-ever subglacial water inventory that covers almost the entire Canadian Arctic, a vast region without many existing records of these events. This new inventory identifies 37 active water bodies beneath glacier ice, which consist of three types: classic subglacial lakes, terminus subglacial lakes at the glacier convergence zone, and ice-marginal lakes extending beneath the ice. Over 80% of the lakes had at least one rapid hydrological event between 2011 and 2021, with the most significant one draining over 4 Gt in 2020 and decreasing the ice surface elevation by 148 meters. Our findings suggest subglacial draining needs to be considered while assessing past and future glacier mass balance, and the new inventory will help improve our understanding of subglacial hydrology mechanisms across the Arctic under global warming.