Uranium-thorium disequilibrium dates and stable isotope analyses of travertine deposits at Soda Dam, New Mexico, have been used to determine the age and to document the evolution of the Valles caldera hydrothermal system. Soda Dam discharges from Paleozoic and Precambrian rocks in San Diego Canyon southwest of the caldera, and the canyon was filled with several hundred metres of Upper Bandelier Tuff during formation of Valles caldera, 1.12 Ma. According to the dates, the Valles hydrothermal system is at least 106 yr old or nearly as old as the caldera. The basic hydrology of the system has thus remained virtually unchanged during this time. Travertine ages indicate pulses in travertine deposition from ∼1.0 to 0.48 Ma, from 0.107 to 0.058 Ma, and from 0.005 Ma to present. The volume of various travertine deposits suggests that the hot-spring system at Soda Dam was once larger than it is today, possibly discharging as much as 5 to 10 times more fluid of Na-HCO3-Cl composition. Stable isotopes of the travertines indicate, however, that the hot-spring system has probably never been more than about 10 °C hotter than the present maximum temperature of 48 °C. Stable isotopes also show that the δ13C of CO2 and δ18O of H2O in hot-spring waters has remained relatively constant for 106 yr. The similar ages between caldera formation and the onset of large-scale hydrothermal circulation and travertine deposition on pre-Bandelier Tuff rocks imply that incision rates by the Jemez River in Bandelier Tuff were relatively rapid. As much as 400 m of tuff were cut by the ancestral Jemez River in 105 yr or less.