This study relates to the construction and application of a water quality model for the Upper Blackfoot River basin in southeastern Idaho to serve as a management tool for future phosphate mining and processing operations in the area. The model was developed by making suitable modifications to an existing model which had been used for the Boise River study. The Blackfoot River model is steady state, deterministic and onedimensional. It has been calibrated and verified and has been found satisfactory under the limitations of the observed data and the complexity of the ecosystem. The technique of model application has been illustrated with the help of arbitrary waste discharges at selected points in the basin. Two hypothetical combinations of inputs have been assumed. The effects of these discharges on the water quality of the streams have been simulated by the model. The model can be useful in deciding the management policies for phosphate mining and processing operations in order to mitigate or avoid their adverse impacts on water quality of the streams. The model can be adapted to the planning of these operations as well as to the planning and management of other human activities in the area with regard to their water quality impacts. Proposals for improving the data collection system in the basin include: (i) coordination of data collection programs of the various agencies, (ii) hydrologic and water quality data gathering on ungauged major tributaries and on the main stems of the Diamond Creek and the Blackfoot River, and (iii) collection of essential weather data in the basin
