Ten water purveyors at Lake Tahoe in Nevada and California formed a partnership to seek filtration avoidance under the Surface Water Treatment Rule of the Safe Drinking Water Act. Watershed sanitary surveys were prepared addressing watershed characteristics, sources of potential pollution, water quality monitoring with emphasis on microbial constituents, and best management practices. Sampling for coliforms, Giardia, Cryptosporidium, and enteric viruses in raw water intakes began in 1991, and indicate a level of purity considered to be equivalent to that of a controlled watershed. Factors affecting water supply protection are addressed, as is the potential of total trihalomethane formation with use of ozone as a disinfectant. The extreme clarity of Lake Tahoe in conjunction with existing watershed controls appears to form an effective first barrier in a multi-barrier treatment chain to assure a safe and cost-effective water supply for Lake Tahoe water purveyors utilizing surface water intakes.