The Tiger Creek Regulation Reservoir is part of PG&E’s Mokelumne Hydroelectric Project, located approximately 70 miles west of Sacramento, California. NHC conducted a Computational Fluid Dynamics (CFD) model study of the existing Tiger Creek Regulator Spillway to evaluate the hydraulic performance of the facility and identify options to increase capacity and mitigate the risk of dam overtopping. NHC developed a Flow-3D CFD model of the facility, validated the model using field observations, and used it to assess the hydraulic performance of different components of the facility, including the siphon spillways, side channel spillway, and the side weir. We identified performance issues related to the discharge capacity of the spillways and recommended design alternatives to increase capacity, and evaluated the flow patterns at the outlet channel in terms of the freeboard and capacity before overtopping of the dam. The CFD model was a valuable tool in evaluating the existing design of the facility, identifying hydraulic performance issues, and developing an optimized design to minimize or eliminate the issues.