A study of the geothermal resource at Klamath Falls, Oregon, has shown that thermal water occurs in an extensive, heterogeneous aquifer at depths of a few hundred to nearly 2,000 feet over an area of nearly 2 square miles. The highest measured water temperatures are more than 130°C. Chemical and isotopic analyses suggest that the aquifer is supplied from a deeper zone in which meteoric recharge water having low chloride and silica concentrations mixes with high-temperature water (about 190°C) having a moderately high chloride concentration (120 milligrams/kilogram). The probable temperature of the hot-reservoir water is estimated on the basis of consistent results from the sulfate-water isotope and silica geothermometers and calculated mixing fractions of 40 and 44 percent thermal water derived from chloride and silica mixing models