Heat flow measures how much warmth works its way up from deep inside the earth, in milliwatts per square meter. Across the United States it averages about 65.
Most of the West runs well above that. The crust there has been stretched thin and broken by faults, which leaves hot rock closer to the surface. Rain and snowmelt sink down through those cracks and come back up hot.
The cool blue East has only a scattering of springs. Most of them, like the ones in Hot Springs, Arkansas, and Warm Springs, Virginia, come from water that circulates unusually deep before it surfaces.