After the filing of the Final Supplement to the Environmental Impact Statement on the O'Neill Unit, the Environmental Protection Agency and the Nebraska Department of Environmental Control raised questions concerning oxygen depletion in Norden Reservoir. To fully respond to those questions, a combination of nutrient loading models and a temperature simulation model were used to project the trophic state, productivity, and duration of stratification of the reservoir. The trophic state index indicates that the reservoir would be eutrophic; the estimated mean annual chlorophyll-a concentration indicated a high likelihood that algal blooms would occur. Dissolved oxygen curves for the reservoir were then developed for the period immediately prior to fall overturn. Curves were developed based on a reservoir with similar nutrient loadings and on theoretical curves from the literature. From these and the temperature model, oxygen concentrations in the reservoir discharge were estimated and compared to water quality standards and aquatic life criteria. It was concluded that the dissolved oxygen would be sufficient to support aquatic life but may fall below the State standard under certain circumstances. Alternatives for meeting the standard are analyzed. The reservoir and the river downstream are expected to support a cold-water fishery; most species now present in the river will be eliminated. The methods developed for this study may have application to other impoundments where an analysis is required to answer questions concerning the potential for such problems. While the results are only rough estimates, they can be used in environmental impact assessment and management planning.