The Santa Clara Valley Water District (SCVWD) receives approximately 150,000 acre-feet of water per year from the Central Valley Project through the San Felipe Division Unit of the San Luis Reservoir. When water levels in the San Luis Reservoir fall below 300,000 acre-feet, algae-rich waters in the upper 20 to 30 feet of the reservoir can be drawn into the intakes. In the future, the San Luis Reservoir may be drawn down below the "low point" as much as 50 percent of the time due to proposed CALFED changes in water management. These conditions can cause taste, odor and filter clogging problems for SCVWD as well as line and emitter clogging problems for San Felipe agricultural customers using drip irrigation systems. In order to better quantify the treatment and supply problems associated with reservoir drawdown to "low point" conditions, SCVWD conducted a pilot study of the impacts of high algae feedwater on flocculation, sedimentation, ozonation, and filtration processes. The pilot study included collection of algae from the reservoir surface water during the peak algae growth season. This collection allowed for testing of feedwaters with high algae concentrations (on order of 5,000 to 10,000 cells per mL and up to 1,000 fluorescence Rhod-equivalents). Blue-green algae (i.e. Aphanizomenon, Microcystis, Anabaena) are the dominant species in the San Luis Reservoir. Two pilot treatment trains were run side-by-side to compare water treatment performances for: algae-laden surface water collected from the top 3 ft., containing algae concentrations that may be anticipated under "low point" conditions; and, intake water withdrawn from the reservoir bottom, representing current withdrawal practices from San Luis reservoir. Side-by-side pilot testing of these two waters allowed evaluation of treatment performances including: filter clogging, filter run length, headloss development, unit filter run volume (UFRV), filtered water turbidity, filtered water particle counts, water production capabilities, taste and odor control, and impact on intermediate ozonation. Pilot-scale evaluation compared filtration performance for the two water sources and two different filter media designs: anthracite and sand dual-media design; and, granular activated carbon. This paper focuses on the pilot study results that demonstrate and correlate the impact of various algae count feedwaters with shorter filter runs. Includes tables, figures.