Changing hydroclimatic conditions are the primary source of risk to hydroelectric power generation. The research described in this dissertation investigates hydrological and drought variability in the Winnipeg River basin, Canada, during the last several hundred years using instrumental hydroclimate data and paleoclimatic records derived from tree rings. The basin drains parts of northwestern Ontario, northern Minnesota and southeastern Manitoba, and is the most important component of the hydrological system used to generate power in Manitoba. Extreme low annual flows in the Winnipeg River are associated with enhanced meridional flow across western Canada during summer and autumn, which suppresses precipitation over the watershed and reduces runoff from spring snowmelt. In contrast to the declining flows observed for other regional rivers, mean annual discharge in the Winnipeg River basin has increased substantially since the early 1920s. For a longer perspective, fifty-four ringwidth chronologies (mainly Pinus resinosa and P. strobus) were used to assess changes in summer climate in the Winnipeg River region since AD 1783. Tree growth in this region is significantly, but weakly, correlated with both temperature and precipitation during summer. Synthetic tree-ring records produced by the Vaganov-Shashkin model of tree-ring formation are consistent with these relationships with climate, and suggest that the primary factor limiting tree growth switches from temperature to moisture in mid-summer. The Winnipeg River tree-ring record indicates that summer droughts were more persistent in the 19^th and late 18^th century than during the last 100 years, but there is no evidence that drought was more extreme prior to the onset of direct monitoring.

This dissertation also examines past changes in summer drought over the broader region using 138 ringwidth records from the Canadian Prairies provinces and adjacent areas. Regional ringwidth signals are primarily related to summer moisture and drought conditions. These summer-sensitive records are not linearly related to major modes of climate variability, including ENSO and the PDO, which mainly affect the climate of western Canada during winter. Extended drought records inferred from regional tree-ring series indicate that drought on the Canadian Prairies has exhibited considerable spatial heterogeneity over the last several centuries.