ABSTRACT:The Montana Department of Natural Resources and Conservation developed a hydrologic model to help analyze the effects of allocating water for consumptive and instream uses in the upper Missouri River basin of Montana. The model, a PC‐based FORTRAN program, uses a mass‐balance approach to compute monthly streamflows, reservoir operations, hydropower production, and irrigation and municipal water uses throughout the 54,000 square mile basin for a 59‐year base period. Simulation results are presented as monthly mean and percentile‐exceedence values. The model was run for baseline conditions and six hypothetical water‐allocation alternatives. Results were used by staff resource area specialists to assess potential impacts to water quantity and distribution, water rights, water quality, stream channel form, fisheries, wildlife, recreation, hydropower production, and economics. These analyses were presented to the public and the decision‐making board in an environmental impact statement (EIS). Though, in many instances, the model did not allow for detailed, site‐specific analyses, the model was an important tool and its simulation results formed the hydrologic basi