Long Valley Caldera hosts a large hydrothermal system, with hot groundwater, boiling springs, and fumaroles.
The hydrothermal system is recharged with water primarily from snow-melt in the Sierra Nevada highlands along the western rim of the caldera. This water infiltrates to depths of a few kilometers where it is heated to at least 220 °C (428 °F) by hot rock near geologically young intrusions of magma. The thermal water rises along fractures to depths of 1-2 km in the western part of the caldera, just north of the town of Mammoth Lakes, and then flows eastward within permeable rock layers to discharge points along Hot Creek and around Crowley Lake in the eastern half of the caldera.
Changes in the hydrothermal system at Long Valley occur irregularly. To characterize these changes, the USGS monitors the discharge of hydrothermal water through Hot Creek and other surface features, water level and temperature in wells, and water and gas compositions of thermal features across the caldera. Carbon dioxide emissions are regularly measured at a number of areas within the caldera. These measurements are used to help scientists more clearly understand other geophysical signals (such as ground movement and earthquakes), and also to characterize the baseline conditions in the caldera so that even small changes, which could signal the onset of volcanic unrest, are more easily perceptible.
The Casa Diablo geothermal power plant on the flanks of the resurgent dome of the caldera uses hot water from the Long Valley hydrothermal system for power production. The USGS collects water samples and measures discharge at springs within the caldera and conducts yearly measurements of gas discharge and shallow soil temperatures at locations near the power plant. These monitoring efforts are used to provide background information needed to discriminate between potential geothermal-induced signals from ones related to volcanic and tectonic processes.