The San Francisco Public Utilities Commission (SFPUC) serves 2.3 million wholesale and retail customers in the San Francisco Bay area. The SFPUC's primary source of supply (about 85% of the annual supply) is the Hetch Hetchy Reservoir located in Yosemite National Park. The Hetch Hetchy watershed is of such pristine quality that the SFPUC was granted and maintains filtration avoidance for this supply. Currently the Hetch Hetchy supply is treated with lime for pH adjustment and disinfected with sodium hypochlorite as it travels 130 miles to the Bay Area. SFPUC is interested in alternative disinfectants for compliance with the Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR) and is currently considering ultraviolet (UV) disinfection at its Hetch Hetchy supply. Thus, knowing how transmittance varies with raw water quality will be essential in the evaluation of UV as an alternative disinfectant. Transmittance data are also critical to the selection, design, and operation of UV disinfection equipment, and will continue to be used in the day-to-day operation and control of the UV disinfection system. The findings of this analysis will form the basis for selecting a transmittance monitoring sampling program and for determining how to incorporate on-line measurements with grab samples. Includes tables, figures.