The occurrence of nature of thermal springs in the Idaho batholith and adjacent areas suggest a relation between structural controls and deeply circulating hot-water systems. Springs issuing from granitic rocks are associated mostly with major regional fault structures. Springs issuing from other rocks probably are related to local faulting. Individual spring flows and water temperatures are variable and range from less than 1 gallon per minute to 2,710 gallons per minute and from 20.5 degrees to 94.0 degrees Celsius. Annual spring discharge is at least 27,000 acre-feet; heat discharges convectively is estimated to be 5.0 x 107 calories per second. Thermal springs discharge relatively dilute water; dissolved solids range from 103 to 839 milligrams per liter. The chemical quality of the water suggests deep circulation of meteoric water. Estimated reservoir temperatures are generally less than 100 degrees Celsius, but temperatures for several springs exceed 150 degrees Celsius. Stable-isotope data suggest that most of the thermal water is not derived from current precipitation. Carbon-14 values indicate that thermal waters are old; apparent residence times range from 9,000 to more than 40,000 years.