Hot Water in the Long Valley Caldera—The Benefits and 

Hazards of this Large Natural Resource

The Long Valley Caldera is a large, 

oval-shaped depression formed 767,000 

years ago during a massive volcanic 

eruption. The caldera stretches northeastward from Mammoth Mountain to 

Glass Mountain, a distance of nearly 

20 miles (32 kilometers). A central 

highland known as the resurgent dome 

and numerous smaller hills within the 

caldera reflect subsequent volcanic 

activity; for example, Doe Ridge near 

the Mammoth Yosemite Airport, is built 

from lava erupted 300,000 years ago. 

The Hot Creek Geologic Site, a U.S. 

Forest Service managed recreation area, 

is located at the north end of Doe Ridge. 

Volcanic eruptions in the past 10,000 

years have also created the visually 

impressive Inyo Craters chain that trends 

north from the town of Mammoth Lakes.

The Hydrothermal System in Long 

Valley Caldera

In any volcanic system, only a part 

of the magma (molten rock) erupts at 

the land surface. What remains underground slowly solidifies and loses heat 

to surrounding rocks. At Long Valley, 

snowmelt on the western caldera rim 

infiltrates deep underground and absorbs 

a large amount of heat and gases (like 

carbon dioxide) released from the 

residual magma. A 1-mile-deep well 

The boiling springs in Hot Creek are scenic but can erupt unpredictably. 

Because of this danger, the U.S. Forest Service closed parts of the area 

to visitors. Photograph courtesy of U.S. Forest Service.

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2018–3009

March 2018

The volcanic processes that have shaped the Long Valley Caldera in eastern California 

have also created an abundant supply of natural hot water. This natural resource provides 

benefits to many users, including power generation at the Casa Diablo Geothermal Plant, 

warm water for a state fish hatchery, and beautiful scenic areas such as Hot Creek 

gorge for visitors. However, some features can be dangerous because of sudden and 

unpredictable changes in the location and flow rate of boiling water. The U.S. Geological 

Survey monitors several aspects of the hydrothermal system in the Long Valley Caldera 

including temperature, flow rate, and water chemistry.

(44-16, see map on page 2) drilled near 

the Inyo Craters chain encountered water 

at temperatures near 428° F (220° C). 

The heated water is naturally buoyant 

and rises from depth, but before it can 

reach the surface, it encounters highly 

permeable rock layers that channel the 

flow eastward. Along the way, the elevation of the land surface and thus the 

pressure on the water drops, allowing 

the water to boil. Some steam and gas 

reaches the land surface in the vicinity of 

Shady Rest Park, at the east edge of the 

town of Mammoth Lakes. Steam upflow 

has created approximately 12 acres 

(nearly 50,000 square meters) of thermal 

ground near the park, where trees and 

other vegetation have died.

Before geothermal power production 

began, the hot water discharged from hot 

springs at Casa Diablo, but these have 

turned to steaming gas vents (fumaroles) 

as the water level has been drawn down. 

A small amount of hot water warms the 

spring-fed ponds at the Hot Creek State 
Fish Hatchery. However, the main point 

of discharge is the Hot Creek Geologic 

Site, where Hot Creek has cut a gorge 

that intersects the underground hot water 

flowpath. The hot springs in the gorge 

discharge a total of 64 gallons (242 liters) 

of water per second—enough to fill eight 

Olympic-size swimming pools each day. 

Hot water that does not discharge in 

Hot Creek gorge comes out at a number 

of hot and warm springs as far east as 

the shores of Crowley Lake. This large 

volume of hot water distinguishes Long 

Valley Caldera from most other volcanic systems, where hot springs may be 

common but release much less water.

Benefits of Natural Hot Water

The geothermal plant at Casa Diablo 

operates production wells that bring 

the hot water to the surface, where the 

heat is used to generate electricity. The 

cooled water is then injected back into 

the ground. The success of this operation, 

which began in 1985, has led to expansion of the plant’s power production 

capabilities to the present net capacity 

of about 30 megawatts, enough to power 

22,000 homes. Increased production 

from new wells near Shady Rest Park is 

expected to substantially boost generating 

capacity.

The Hot Creek State Fish Hatchery 

relies on the naturally heated water to 

maintain fish growth rates throughout the 

winter months, when low air temperatures would otherwise make the ponds 

too cold. The trout from these geothermally heated ponds are then released to 

local lakes and streams, including Hot 

Creek, where they provide recreational 

enjoyment for fishing enthusiasts.

The Hot Creek Geologic Site, 

managed by the U.S. Forest Service, is a 

scenic stop for visitors to see the boiling 

hot springs and fumaroles along Hot 

Creek gorge. The site offers excellent 

opportunities for hiking, wildlife viewing, 

photography, and of course, exploring 

the local geology. Some hot springs and 

pools have beautiful aprons of travertine (calcium carbonate) deposited by 

the mineralized waters. Additional hot 

springs can be found within a few miles 

of the Hot Creek Geologic Site. Many are 

frequently visited by the public and have 

long been known to Native Americans, 

who have accorded them special status 

and spiritual significance.

Resurgent

Dome 

Inyo

craters

and

domes

Magmatic 

Intrusions

Well CH-10B

Hot Creek

gorge 

Mammoth

Lakes 

Caldera fill deposits

LONG VALLEY CALDERA

Earthquakes

Casa Diablo

Geothermal

Plant

**

*

*

*

Basement rocks

WEST EAST

2 

4 

Glass

Mountain

SIERRA

NEVADA

Depth, in kilometers

Well 44-16

428oF

(220oC)

338oF

(170oC)

199oF

(93oC)

HHoott WWaatteerr

Cold

Water

Cold

Water

Simplified geologic map (top) and cross section (bottom) of Long Valley Caldera. The 

resurgent dome, Doe Ridge, and the Inyo craters and domes all reflect volcanic activity 

since 767,000 years ago, when a giant eruption formed the caldera. The hot springs in 

Hot Creek are fed by Sierra Nevada snowmelt that seeps underground, is heated to at 

least 428° F (220° C) in the vicinity of molten rock (magma), and flows eastward beneath 

the ground (red arrows). The temperature of the water reaching the springs at Hot Creek 

could be affected by earthquake activity, geothermal heat extraction, mixing with 

cooler water, and other factors.

0 5 MILES

0 5 KILOMETERS

119° 118° 45’

37° 45’

37° 37’ 30”

203

395

395

Inyo craters and domes

CALIFORNIA

 Mammoth

 Mountain

 Resurgent Dome

 Shady Rest Park

Mammoth

 Lakes

Map

Area

 Glass

Mountain

Crowley

 Lake

Mammoth

Yosemite 

airport

Hot Creek State

Fish Hatchery

Well CH-10B

Well 44-16

LONG VALLEY CALDERA

Owens River

Owens

Mammoth

Hot

Creek

Creek

River

Casa Diablo

Geothermal Plant

Doe

Ridge

Hot Creek

Geologic

Site

Hazards of Natural Hot Water

The presence of natural hot water 

in Long Valley Caldera is most apparent to the public at the Hot Creek 

Geologic Site. This is also where the 

hazard to the public is greatest. Boiling 

springs line both sides of the creek, 

and additional springs are concealed on 

the creek bottom. The hot water from 

these submerged springs mixes into 

the cold creek and creates deceptively 
The total discharge of hot water into Hot Creek shows small variations 

over time but has averaged 64 gallons (242 liters) per second (dashed 

line) since U.S. Geological Survey monitoring began. The onset of 

geysering in May 2006 corresponded to a small spike in discharge, but 

geysering was not linked to a similar spike 6 years later.

The temperature in Hot Creek can change rapidly as shown in these photos taken 5 seconds apart. Water at 199° F (93° C), the 

boiling point at this elevation, erupted above the creek during the violent geysering event. Any swimmer caught in this part of the 

creek would have been severely scalded. Photographs courtesy of Alix Ginter.

1988 1992 1996 2000 2004 2008 2012 2016

0

15

30

45

60

75

90

0

50

100

150

200

250

300

350

Period of geysering 

Average discharge:

64 gallons per second

Hot spring discharge, in liters per second

Hot spring discharge, in gallons per second

Approximate range of hot spring discharge at volcanoes in the western United 

States (Mount Rainier, Wash.; Mount Hood, Oreg.; Lassen Volcanic Center, Calif.)

comfortable swimming conditions. 

However, very rapid increases in spring 

discharge can sometimes raise water 

temperatures high enough to scald, 

killing unlucky fish and other organisms. 

Be careful that you do not fall victim to 

such a fate!

In May 2006, the stream bottom 

springs began to geyser intermittently, 

spraying very hot, sediment-laden water 

as much as 6 feet (2 meters) above the 

stream surface. The unpredictability 

of the geysering and the clear hazard 

to swimmers and streamside spectators soon led the U.S. Forest Service to 

close parts of the Hot Creek Geologic 

Site to protect public safety. Fences and 

The geothermal power plant at Casa Diablo, looking west 

toward Mammoth Mountain. Photograph by James Howle, U.S. 

Geological Survey.

signs were installed or relocated to keep 

visitors away from unstable banks that 

can collapse without warning into newly 

formed pools of boiling water. Although 

the geysering activity gradually declined 

with time and has not been observed in 

recent years, the constant and unpredictable shifting of hot spring vents makes it 

imperative that visitors observe all safety 

guidelines and closures posted by the 

U.S. Forest Service. Similar episodes of 

geysering in Hot Creek occurred in the 

1930s and early 1980s, the latter of which 

coincided with a period of intense earthquake activity and deformation of the 

ground surface, and will likely happen 

again in the future.

USGS Monitoring

USGS scientists monitor water 

temperatures in the hot springs and 

in nearby observation well CH-10B 

(see map). They also monitor the total 

discharge from all the hot springs in the 

gorge. The onset of geysering in May 

2006 was accompanied by a small spike 

in total hot spring discharge and occurred 

as the temperature at the water surface in 

well CH-10B reached the local boiling 

point. The ultimate cause of the temperature increase is not known. Earthquake 

activity is known to alter groundwater 

flow paths, and diversion of flow through 

hotter rocks could have raised water 

10:30:20 10:30:25

86° F

30° C

199° F

93° C

68° F

20° C

68° F

20° C
Temperatures in the upper 90 feet (below water level) in well CH-10B showed a 

gradual increase between 1988 and 2006. The top of the water column reached 

the local boiling point about the time that geysering began in Hot Creek (May 

2006), suggesting a direct connection. However, temperatures in well CH-10B 

have remained high since 2006, even as geysering has ceased.

Background photograph. Dying trees show continuing expansion of an area of thermal ground 

and diffuse carbon dioxide gas emissions near Shady Rest Park in 2017. The expansion likely 

results from changing groundwater levels and does not indicate increased volcanic activity. 

Photograph by Deborah Bergfeld, U.S. Geological Survey. Inset photograph. USGS scientist 

measuring the flow of carbon dioxide in an area of thermal ground.

ISSN 2327-6916 (print)

ISSN 2327-6932 (online)

https://doi.org/10.3133/fs20183009

temperatures. Although May 2006 was a 

time of low earthquake activity, a delayed 

response to earthquakes that occurred 

west of Hot Creek gorge in previous 

years is possible, given that the underground flow of water is slow.

Additionally, the USGS assesses how 

geothermal power production affects 

the hot springs—their temperature, 

flow rates, and chemistry—as well as 

the local environment and groundwater 

resources. This part of the monitoring 

program is coordinated through the Long 

Valley Hydrologic Advisory Committee (LVHAC) organized by Mono 

County. The USGS must also determine 

if changes in hydrothermal 

activity could be a prelude 

to volcanic activity; this part 

of the monitoring program 

is coordinated within the 

USGS California Volcano 

Observatory (CalVO), which 

continuously assesses the 

hazard at the many volcanoes 

in California and Nevada. 

Renewed volcanic activity is a 

concern at Long Valley, where 

the resurgent dome inside 

the caldera has been pushed 

upward nearly 3 feet (1 meter) 

in the past four decades, and some eruptions of lava along the Inyo Craters chain 

are less than 700 years old.

Magma must rise from great depth to 

the surface before it can erupt, and rising 

magma is an abundant source of heat 

and gases like carbon dioxide, which can 

ascend in advance of the magma itself. 

If the hydrothermal system intercepts the 

rising heat and gas, it could respond by 

getting hotter and more gas rich; therefore, CalVO monitors the temperature 

as well as the water and gas chemistry 

in various springs and wells, and soil 

temperatures and release of carbon 

dioxide in areas of thermal ground.

Over the many years of monitoring, 

both increases and decreases in well 

temperatures have been observed and the 

areas of thermal ground have expanded. 

However, these changes likely do not 

signify increased volcanic unrest and can 

be attributed to the impacts of earthquakes and geothermal energy production 

on groundwater flowpaths. The USGS 

continues to monitor the hydrothermal 

system, not only to better understand 

the cause of anomalous events like the 

Hot Creek geysering, but also to provide 

accurate hazard and resource assessments 

to land managers. 

By William C. Evans, Shaul Hurwitz, 

Deborah Bergfeld, and James F. Howle 

Edit and layout by Katherine Jacques

Figures by Linda Masonic

For more information, contact:

U.S. Geological Survey 

California Volcano Observatory 

345 Middlefield Road, MS-910

Menlo Park, CA 94025 

Water level

86° 90° 94° 98° 102°

160

140

120

100

80

Water temperature, in degrees Celsius

Depth, in feet

Local boiling temperature

Warming

1988–2006

July 1988

November 1997

May 2004

July 2006

July 2016

EXPLANATION