From the Proceedings of the 1976 Meetings of the Arizona Section - American Water Resources Assn. and the Hydrology Section - Arizona Academy of Science - April 29-May 1, 1976, Tucson, Arizona / An environmental impact statement (EIS) is meant to be a predictor of the consequences of actions on the environment. However, uncertainties in the statements make it difficult to determine the reliability of the predictions and thus the consequences of the actions. Hence, use of an EIS could be counter-productive if the inherent uncertainties are not recognized and considered in its evaluation. Examination of several EIS's from a systems viewpoint is used to expose the following sources of uncertainty: (1) the identification of the components of the system, (2) the natural uncertainty of the inputs to the system and of the transformation functions producing the output, (3) uncertainties in the modeling of the system due to limitations of sample, economic and technological data. The above viewpoint is used to analyze the Colorado River Salinity Control Project EIS. Uncertainties are identified and classified and means for assessing and incorporating their effect on the environmental impact assessment are discussed.