Hot Water in the Long Valley Caldera: Resource and Hazard

Boiling springs in Hot Creek are scenic but can erupt without warning, so the U.S. Forest Service closed parts of the area. U.S. Forest Service.

Long Valley Caldera, in eastern California, is a large oval depression formed 767,000 years ago in a massive volcanic eruption. It stretches northeast from Mammoth Mountain to Glass Mountain, nearly 20 miles (32 km).

• Later volcanism: a central highland, the resurgent dome, and smaller hills record later activity. Doe Ridge, near Mammoth Yosemite Airport, is built of lava erupted 300,000 years ago; the Forest Service's Hot Creek Geologic Site lies at its north end.
• Recent eruptions: in the past 10,000 years, eruptions built the Inyo Craters chain, running north from the town of Mammoth Lakes.

The hydrothermal system

Only part of a volcano's magma ever erupts; the rest slowly solidifies and gives off heat.

• Heating: snowmelt on the caldera's western rim sinks deep underground and absorbs heat and gases such as carbon dioxide from the leftover magma. A 1-mile-deep well (44-16) near the Inyo Craters hit water near 428 °F (220 °C).
• Flow: the buoyant hot water rises until permeable rock layers channel it east. As the ground surface drops, pressure falls and some water boils; steam and gas reach the surface near Shady Rest Park in Mammoth Lakes, creating about 12 acres of thermal ground where trees and plants have died.
• Where it comes out: the old hot springs at Casa Diablo have become steaming fumaroles as geothermal pumping lowered the water. A little warms the Hot Creek State Fish Hatchery. The main outlet is the Hot Creek gorge, which cuts across the flow path: its springs discharge 64 gallons (242 liters) a second — enough to fill eight Olympic pools a day. The rest emerges in springs as far east as Crowley Lake.

That volume sets Long Valley apart from most volcanic systems, where hot springs may be common but release far less water.

Simplified geologic map of Long Valley Caldera. USGS.

Cross section: Sierra snowmelt is heated to at least 428 °F near magma and flows east beneath the caldera (red arrows), reaching Hot Creek at 199 °F. USGS.

Benefits

• Electricity: the Casa Diablo plant pumps hot water up, uses its heat to make power, and injects the cooled water back underground. Running since 1985, it has grown to about 30 megawatts net — enough for 22,000 homes — and new wells near Shady Rest Park should add substantially more.
• Trout: the fish hatchery uses naturally warm water to keep its ponds from getting too cold in winter; its trout stock local lakes and streams, including Hot Creek.
• Recreation and heritage: the Hot Creek Geologic Site offers boiling springs, fumaroles, travertine aprons, hiking, wildlife and photography. Nearby springs have long been known to Native Americans, who hold them in special, spiritual regard.

The Casa Diablo geothermal plant, looking west toward Mammoth Mountain. James Howle, USGS.

Hazards

The danger is greatest at Hot Creek. Boiling springs line both banks, and others hide on the creek bed, mixing into cold water to make deceptively pleasant swimming. But sudden surges in spring flow can make the water hot enough to scald, killing fish and other life.

• May 2006: the creek-bed springs began to geyser intermittently, spraying very hot, muddy water up to 6 feet (2 m) high. The Forest Service closed parts of the site and put up fences and signs to keep people off unstable banks that can collapse into new boiling pools.
• Recurring: geysering has faded and not been seen in recent years, but vents keep shifting — obey all closures. Similar episodes occurred in the 1930s and early 1980s (the latter during intense earthquakes and ground deformation), and will likely happen again.

USGS monitoring

• Hot Creek: USGS tracks spring temperatures, total discharge, and temperatures in nearby well CH-10B. The 2006 geysering began with a small discharge spike, just as water at the top of CH-10B reached the local boiling point. The cause of the heating is unknown; earthquakes can reroute groundwater through hotter rock, and though May 2006 was quiet, a delayed response to earlier quakes west of the gorge is possible, since groundwater moves slowly.
• Geothermal effects: USGS assesses how power production affects the springs' temperature, flow and chemistry, and local groundwater, through the Long Valley Hydrologic Advisory Committee organized by Mono County.
• Volcanic unrest: the California Volcano Observatory (CalVO) watches for hydrothermal changes that could precede an eruption. The resurgent dome has risen nearly 3 feet (1 m) in the past four decades, and some Inyo lava is less than 700 years old. Rising magma releases heat and carbon dioxide ahead of itself, so CalVO monitors spring and well temperatures and chemistry, soil temperatures and CO₂ release.

Total hot-water discharge into Hot Creek has averaged 64 gallons (242 liters) per second since monitoring began; geysering in 2006 matched a small spike, but a similar spike 6 years later brought none. USGS.

Water temperatures in the upper 90 feet of well CH-10B rose gradually from 1988 to 2006, reaching boiling as geysering began — and stayed high after it stopped. USGS.

Over the years, well temperatures have risen and fallen and thermal ground has spread. These changes likely reflect earthquakes and geothermal pumping altering groundwater paths, not growing volcanic unrest. USGS keeps watching, both to understand events like the Hot Creek geysering and to give land managers accurate hazard and resource assessments.

Dying trees mark spreading thermal ground and carbon dioxide emissions near Shady Rest Park, 2017 — likely from changing groundwater levels, not rising volcanic activity. Deborah Bergfeld, USGS.

Sources

Based on Hot Water in the Long Valley Caldera—The Benefits and Hazards of This Large Natural Resource, by William C. Evans, Shaul Hurwitz, Deborah Bergfeld and James F. Howle, U.S. Geological Survey Fact Sheet 2018–3009 (publication page), California Volcano Observatory. Map, cross section, graphs and photographs are restored from the fact sheet's PDF; two photographs of the geysering, credited to an individual, are left out. A work of the United States government in the public domain.