[′lik·wəd ¦däm·ə‚nād·əd ‚hī·drə¦thər·məl ′rez·əv‚wär] (geology) Any geothermal system mainly producing superheated water (often termed brines); hot springs, fumaroles, and geysers are the surface expressions of hydrothermal reservoirs; an example is the hot-brine region in the Imperial Valley-Salton Sea area of southern California.