The Arctic is warming quickly and influencing lake water balances. We used water isotope concentrations taken from samples of 25 lakes in the Canadian Arctic and estimated the average ratio of evaporation to inflow (E / I) for each lake. The ratio of watershed area (the area that flows into the lake) to lake area (WA / LA) strongly predicted E / I, as lakes with relatively smaller watersheds received less inflow. The WA / LA could be used to predict the vulnerability of Arctic lakes to future change.