The research problem addressed by this project was the long-term variability and reliability of the flow of the Athabasca River, which is the major source of water for the extraction and processing of bitumen from the oil sands in north-eastern Alberta. The corresponding research questions were 1) to what extent do gauged flows, which are the basis for surface water allocation, fail to capture the full range of hydroclimatic variability and extremes evident in a longer proxy record, and 2) what are the implications for oil sands extraction and processing of this paleohydrology. The primary objective was achieved by reconstructing the annual and seasonal flow of the Athabasca River for the past 900 years using annual growth increments from old trees at a network of sites in the upper reaches of the river basin and in adjacent watersheds. The industrial partner, Canada’s Oil Sands Innovation Alliance (COSIA) is a collaboration of oil sands producers focused on accelerating the pace of improvement in environmental performance in Canada’s oil sands through collaborative action and innovation. COSIA shared the research findings with the oil sands producers that comprise their alliance. The project provided the oil sands industry will an expanded perspective on the variability and reliability of their major surface water supply, a perspective that captures extremes of climate and water levels that are outside the range of recorded historical flows.