Houston Engineering developed a water balance model in GoldSim for a large oxbow lake in the midwestern US that was experiencing historically significant water level declines. The model accounted for direct precipitation, runoff, evaporation, seepage/groundwater in and out using the Dupuit equation and measured groundwater and lake elevations. Outflow over two outlet structures (one at each end of the lake) was simulated using weir equations and rating curves. Inflows from a supplemental water source were taken from pumping records and measured inflow to the lake. The project revealed that aging infrastructure had experienced maintenance issues that unknowingly decreased supplemental flows to the lake – a major discovery of the modeling effort. Data collection (groundwater levels, lake levels, etc.) is ongoing to provide a more robust data set, which it is hoped will eventually be used to improve model validation and its ability to forecast future scenarios. The model was useful for determining the water deficit that had accumulated and how much pumping of supplemental water would be needed. Additionally, the model was helpful in vetting the potential benefits of relocating the supplemental water discharge location to reduce infiltration losses.