Although Tom Harrison Maps marks the road to Hot Creek Geothermal Area as a 4WD road, it is actually easily accessible to most vehicles.
Hot Creek Geothermal Area is one of the most amazing examples of the region's turbulent volcanic history. The entire area, from Mammoth Mountain to Crowley Lake, forms the 20 mile long and 10 mile wide Long Valley caldera. It was created about 750,000 years ago with the eruption of a giant volcano/magma chamber 2,500 times larger than the eruption of Mt. St. Helens in 1980 that spread a thick layer of ash over all of the western US. This catastrophic eruption emptied the magma chamber and caused it to collapse on itself, forming a depression that today is the Long Valley caldera. Hot Creek is a reminder of that time period; water from the Sierra Nevada seeps down into the ground, where it is heated by the currently rising magma, and boils back up to the surface at Hot Creek.
From the parking area, you can walk down the gorge to view a fascinating variety of mud pots, hot springs/pools, and fumaroles. At times, temporary geysers have even been known to shoot up here. Although swimming and soaking in the creek was once popular here, it was prohibited starting June 2006 by the Forest Service due to increased unstableness in the area.
Hot Creek Fish Hatchery is an interesting stop on the way to the Hot Creek Geothermal Area. I've never seen more trout in my life than here. The hatchery raises 3 million cutthroat and rainbow trout each year to stock the streams and lakes throughout the Sierra Nevada. There are two main areas of interest at the hatchery; a pool with the older fish that are generally used for eggs, and a larger set of pools for newly hatched trout. In the first set of pools, you'll find rainbow trout up to 10 pounds swimming about. These fish are generally raised for three years to produce eggs; after that, to prevent the gene pool from degrading, they are released into the wild. The second and larger set of pools features millions of young trout who are released into the Sierra. It's a fascinating place.
Geothermal activity and water level in the creek vary from year to year. These hot springs were almost dry for several years, but can return the next season. You may not want to bathe in these pools, but several other spots can be found that are more conducive.
Mammoth Creek flows eastward from the Mammoth Lakes towards its mixture with the Owens River near the head of Lake Crowley. This entire region of the Sierra is rife with volcanic activity, cinder cones, pumice flats and geothermal activity. The creek tumbles near the small Mammoth Lakes airport and then goes by a large California state fish hatchery. Downstream from this, the creek serves as a magnet for fly fishermen as the creek enters a small canyon. The canyon twist and turns and runs directly through one of those areas of geothermal activity with hot springs pumping water directly into the lake. The creek runs through a couple of deep pools which have heat emanating from the stream bed directly. Atop the canyon's south walls, there is a large parking lot with changing rooms and a trail that takes people down to the creek pools. You can then swim around in the pools to find the spot with the 'correct' temperature for you. The pools do get crowded during the day, but if you go early - or late - enough, the whole setting is magical. Also, the scene tends to change from year to year, depending on what is happening geothermally or what kind of a snow year it was.