Occur in many thermal areas where the surface of the Earth intersects the water table. The temperature and rate of discharge of hot springs depend on factors such as the rate at which water circulates through the system of underground channelways, the amount of heat supplied at depth, and the extent of dilution of the heated water by cool ground water near the surface.

USGS - http://vulcan.wr.usgs.gov/Glossary/ThermalActivity/description_thermal_activity.html