The Colorado River Basin faces serious water shortage risks that threaten the environment and create major financial risks for cities, agriculture, and other essential services. Traditional water allocation models often cannot help decision-makers because they oversimplify water rights, assume unchanging demand, and ignore real-world behavior and policies. This comprehensive model addresses these shortcomings using newly available detailed water rights data to provide fine-grained analysis of water distribution and economic impacts at the individual level. Unlike previous models, this tool accounts for uncertainties in climate, demand, and policy to calculate the probability of shortages and associated costs under different scenarios. The model is applied to the Colorado River Basin's growing water crisis to assess user-specific water supply and financial outcomes. The open-source Python framework can be extended to analyze other institutionally complex river basins.