Stable isotope tracers of δ 18 O and δ 2 H are increasingly being applied in the study of water cycling in regional-scale watersheds in which human activities, like river regulation, are important influences. In 2015, δ 18 O and δ 2 H were integrated into a water quality survey in the Muskoka River Watershed with the aim to provide new regional-scale characterization of isotope hydrology in the 5,100-km 2 watershed located on the Canadian Shield in central Ontario, Canada. The forest dominated region includes~78,000 ha of lakes, 42 water control structures, and 11 generating stations, categorized as "run of river." Within the watershed, stable isotope tracers have long been integrated into hydrologic process studies of both headwater catchments and lakes. Here, monthly surveys of δ 18 O and δ 2 H in river flow were con-