Where the mapped heat actually sits
Start in Greenlee County, in the state's eastern mountains. The Clifton and Gillard hot springs there rank among the hottest natural springs in Arizona, reported well above 150 degrees Fahrenheit at the source in state survey descriptions. They sit along the Gila River and its tributaries, where faults let deep water rise. Smaller warm springs and warm wells show up across the southeast, near Safford and in the San Simon and Willcox basins.
Closer to home, the maps show warm groundwater under parts of the Salt River Valley, the basin that holds Phoenix, Tempe, and Mesa. Temperatures there run low, often just comfortably warm rather than hot, because the heat spreads through deep sediment instead of concentrating at a fault. That pattern, plenty of low and moderate temperature heat and only a handful of hot spots, is the single most important fact about Arizona geothermal, and it shapes everything that follows.
Why it heats greenhouses, not turbines
To make electricity from geothermal heat, a plant needs a resource hot enough to boil a working fluid and spin a turbine. Conventional plants want reservoirs well above the boiling point of water. Binary plants (which use the geothermal water to boil a second fluid that boils at a lower temperature, such as a refrigerant) can run cooler, on the order of 250 to 300 degrees Fahrenheit, but even that is hotter than most of what Arizona has mapped near the surface.
So the state's geothermal mostly does direct-use work, meaning the heat is used as heat. Warm water from Arizona wells and springs has fed aquaculture ponds raising tilapia and shrimp, greenhouses, and resort and spa pools, mostly in the Gila Valley and at scattered desert sites. It is real energy, and it displaces real gas and electricity, but it stays local to the resource. Warm water cannot travel down a transmission line the way electricity can.
What the data say about power someday
The prospects for utility-scale geothermal power in Arizona rest on two uncertain things. One is whether deeper, hotter resources exist where the old surface surveys could not reach; the heat-flow maps hint at warmer rock at depth in places, but drilling to confirm it is expensive and has not happened at scale here. The other is a newer approach, enhanced geothermal systems (fracturing hot dry rock deep underground and circulating water through it to pick up heat).
Enhanced systems are why the U.S. Department of Energy keeps funding geothermal research, including a test site in neighboring Utah. If that technology turns cheap and reliable, Arizona's hot rock at depth could start to matter to SRP and APS, the two large utilities that serve most of metro Phoenix and much of the state. That is a genuine maybe, not a plan. For now the resource assessments point to direct use, and the power question stays open.
Worth watching this month
1. The Arizona Geological Survey updates its hot springs and warm well records as new data come in, which is routine housekeeping rather than a sign of new projects.
2. Results from the U.S. Department of Energy enhanced-geothermal test site in Utah would show whether hot dry rock is getting cheaper to tap, and this is the item that actually matters.
3. SRP and APS resource planning filings so far rarely mention geothermal, so its continued absence is itself worth watching.
4. A new direct-use proposal in the Gila Valley, such as a greenhouse or fish farm tapping warm water, is where Arizona geothermal is most likely to grow next.