A water and vegetation sampling program including peat coring was carried out within the McClelland Wetland, a boreal peatland complex situated within the rapidly developing oil sands mining region of northeastern Alberta, Canada. This study investigated both the current and past hydrology of the site to improve understanding of the spatiotemporal evolution of water sources, pathways, and vegetation succession prior to impending oil sands development in the western half of the watershed. The study used vegetation surveys and isotopic tracers (18O, 2H) for contemporary characterization of water sources and evaporation gradients, whereas flark orientation was used to identify general flow directions. Use of 18O preserved in α-cellulose within moss macrofossils and wood, combined with evidence from vegetation successions in several cores, provided detailed information on historical changes in the peatland as far back as 11,300 cal. years BP. Reliability of 18O archives from moss cellulose to reconstruct 18O in source water was confirmed in a sub-survey conducted at 227 sites where water and moss were both sampled and analyzed to quantify isotopic fractionation in dominant moss species. Overall, the study established existence of long-term groundwater source areas around the margins of the peatland, as well as contemporary permafrost thaw zones within the peatland itself. Both contemporary (spatial) and historical (temporal) 18O gradients reflect precipitation-derived origins for shallow groundwater sources with strong evaporative enrichment along the direction of flow. Historical reconstruction suggests persistence of open wet areas at the site amid several episodes of regional climatic and associated hydrologic changes.