Beaver Creek Reservoir was formed as a result of diverting the natural flow of Beaver Creek away from mine and plant areas and southward to the Athabasca River via Poplar Creek. The diversion was initiated in the fall of 1975 with the closure of the Beaver Creek Dam; filling of the reservoir was completed in the spring of 1976. When it became necessary to remove mine depressurization water from the mining area, Syncrude was granted permission by the Government of Alberta to discharge this effluent into Beaver Creek Reservoir, on the condition that chloride levels in water entering Poplar Creek did not exceed 400 mg/L above ambient levels. The present study was designed to investigate the survival of selected organisms in Beaver Creek Reservoir during 1979. The primary objective was to determine the response of selected species of aquatic biota to saline mine depressurization water after average dilution in the Beaver Creek Reservoir. More specific requirements of the study were: a) the study was to be carried out entirely within the Beaver Creek Reservoir using test organisms held in situ; b) the study must include three sampling locations and three replicates of each test organism at each station; c) test organisms must include: periphyton (on artificial substrates), native species of fish (white sucker and fathead minnow), and native species of invertebrates (either Gammarus or Hyalella); and d) field studies were to be conducted between June and October, 1979 and were to examine both short and long term effects.