A Hydrodynamic Reservoir Eutrophication Model (HREM) was developed to determine the potential for various water system management practices to enable New York City's Croton water supply system to improve its water quality and meet specific quality goals. Following an in-depth investigation of water quality problems and their probable causes, a set of five water quality goals were established, focusing on meeting current and future regulatory standards for unfiltered potable water, as well as achieving parity between the Croton system and other New York City water supplies, with respect to source water quality. Twenty-three management practices were initially screened; those showing most promise were assembled into Water System Management Plans. The hydrodynamic model was then developed, calibrated and used to test the effectiveness of each plan in meeting the water quality goals. The plan that offered the greatest potential to meet the five goals was identified as the Optimal Water System Management Plan, to be further evaluated through pilot testing and additional studies. The entire effort was completed in under eighteen months.