Boiling Water at Hot Creek—The Dangerous and Dynamic Thermal Springs 

in California’s Long Valley Caldera

along Hot Creek in east-central 

California have provided 

enjoyment to generations of 

visitors, but they have also been 

the cause of injury or death to 

some who disregarded warnings 

and fences. The springs and 

geysers in the stream bed and 

along its banks change location, 

temperature, and flow rates 

frequently and unpredictably. 

The hot springs and geysers of 

Hot Creek are visible signs of 

dynamic geologic processes 

in this volcanic region, where 

underground heat drives thermal 

spring activity.

he beautiful blue pools and

U.S. GEOLOGICAL SURVEY and the U.S. FOREST SERVICE—OUR VOLCANIC PUBLIC LANDS

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet 2007-3045

2007

T

Hot Creek flows through the Long Valley Caldera in a volcanically active region of east-central 

California. This stretch of the creek, looking upstream to the southwest, has long been a popular 

recreation area because of the warm waters from its thermal springs. These springs, however, are 

unpredictable and can suddenly erupt with violence and at boiling temperature. Because of this 

danger, the U.S. Forest Service has had to close parts of the Hot Creek Geologic Site to visitors.

(USGS photo by Chris Farrar.)

In the Hot Creek Geologic Site, located 

in a narrow gorge 8 miles (12 km) east of 

the town of Mammoth Lakes, numerous hot 

springs flow into a snowmelt-fed stream. The 

area is managed by the U.S. Forest Service 

as a geologic interpretive site and has been a 

popular recreational area for fishing, swimming, hiking, bird watching, and photography. The U.S. Geological Survey (USGS) 

has long monitored spring activity, water 

temperatures and chemistry, and stream 

flow as part of a program that seeks to better 

understand volcanic unrest and possible geologic hazards throughout the region.

The attractions of Hot Creek, however, 

also harbor danger. The locations, discharge rates, and temperatures of springs 

often change. The larger and more vigorous 

springs flow from fractures in the volcanic 

rock (altered rhyolite) in Hot Creek gorge. 

When fractures become sealed by mineral 

deposition, spring discharge and temperature decline. When new fractures develop or 

sealed fractures reopen, spring discharge and 

temperatures can increase suddenly. Rock 

fracturing happens because the thermal area 

lies within a region of frequent earthquakes 

and active ground uplift (deformation). The 

changes in the locations and vigor of springs 

can be sudden and dangerous to unprepared 

visitors, especially if they stray beyond 

walkways and fences.

Since May 2006, springs in and near the 

most popular swimming areas have been 

“geysering” or intermittently spurting very 

hot, sediment-laden water as high as 6 feet 

(2 m) above the stream surface. At times 

this geysering activity is vigorous enough 

to produce “popping” sounds audible from 

hundreds of feet away. The geysering usually 

lasts a few seconds and occurs at irregular 

intervals, with several minutes between eruptions. The unpredictability of this hazardous 

spring activity led the U.S. Forest Service to 

close parts of the Hot Creek Geologic Site in 

June 2006, and the closure has remained in 

effect to date (June 2007).

Conditions in Hot Creek can change very quickly. 

These fish—caught in a burst of high-temperature 

water—were cooked instantly.

(USGS photo by Chris Farrar.)

impressive boiling fountains
Hot Creek

Casa Diablo

Casa Diablo

Geothermal Plant

Owens River

Resurgent Dome

Doe Ridge

Hot Creek

Geologic Site

Owens River

Crowley

 Lake

37o 37' 30"

S I E R R A

N E V A D A

 Glass

Mountain

119o 00'

37o 45'

118o 45'

Crowley

 Lake

395

395

 Mammoth

 Mountain

Mammoth

203

Creek

Mammoth

 Lakes

Inyo Craters and Domes

0 5 MILES

5 10 KILOMETERS 0

Airport

Fish hatchery

CH-10B

44-16

Well

Well

Long Valley Caldera

Area

of 

Map

C a l i f o r n i a

The thermal springs in Long Valley Caldera have long been known to Native Americans. Many of the hot springs have special 

status with Native American tribes and have 

been used for spiritual and medicinal purposes. Early written records of the springs 

came from visits by pioneers and scientists— USGS scientist G.A. Waring visited 

thermal springs in Long Valley in 1908 and 

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

resurgent dome, Doe Ridge, Mammoth Mountain, and the Inyo Craters and Domes all reflect volcanic 

activity since 760,000 years ago, when a giant eruption formed the caldera. The thermal springs in Hot 

Creek are fed by Sierra Nevada snowmelt that seeps underground and migrates eastward, becoming heated to temperatures as high as 428°F (220°C) in the vicinity of partially molten rock (magma) 

beneath the western part of the caldera. The water cools as it migrates eastward beneath the ground 

(red arrows). The temperature at which water emerges in the springs at Hot Creek could be affected 

by changes in seismic activity, heat extraction, mixing with cooler water, and other factors.

Resurgent

Dome 

Inyo

Craters

and

Domes

Magmatic 

Intrusions

Well

CH-10B

Hot Creek

Gorge 

Town of

Mammoth

Lakes 

Caldera fill deposits

Long Valley Caldera

1997 Earthquake swarm

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

later wrote a paper describing hundreds of 

springs throughout California.

Volcanic History of the Hot Creek Area

The Hot Creek Geologic Site lies within 

the Long Valley Caldera, a large topographic depression formed 760,000 years ago 

during a massive volcanic eruption that produced extensive and thick deposits of ash 

and pumice. A reservoir of partially molten 

rock (magma) beneath the caldera has produced subsequent volcanic eruptions. This 

activity formed many hills within the caldera, such as the 300,000 year-old Hot Creek 

rhyolite flow, known locally as Doe Ridge. 

The toe of that slow-moving lava entered a 

lake, where interaction with water altered 

the rhyolite and formed clay and perlite, a 

gray to black, glassy material with a pearly 

luster now exposed along the path from the 

parking area down into the gorge. After the 

lake receded, a stream cut the steep-sided 

gorge through the toe of the solidified lava.

This stream, known here as Hot Creek, 

begins its winding course some 11 miles 

(17 km) to the west as Mammoth Creek, 

flowing through a series of small lakes west 

of the town of Mammoth Lakes. The stream 

water is derived primarily from melting 

snow as it leaves Twin Lakes, 8,500 feet 

(2,600 m) above sea level. It is quite cold, 

rarely above 50°F (10°C). About 1.5 miles 

(2.5 km) upstream from the thermal area, 

Mammoth Creek is joined by warmer water from thermal springs in the Hot Creek 

State Fish Hatchery. From this point on, 

the stream is named Hot Creek even though 

water temperature seldom exceeds 68°F 

(20°C) until it reaches the main thermal 

springs in the gorge.

Why is Hot Creek Hot?

In hydrothermal (“hot water”) systems 

the circulation of ground water is driven 

by a combination of topography and heat 

sources. The system in Long Valley Caldera is recharged primarily from snowmelt 

in the highlands around the western and 

southern rims of the caldera. The meltwater 

infiltrates to considerable depths, where 

some is heated to at least 430°F (220°C) by 

hot rock near cooling magma beneath the 

Inyo Craters and Domes, 10 miles (16 km) 

west of Hot Creek. This volcanic chain extends from the western part of Long Valley 

Caldera northward to Mono Lake and has 

produced numerous eruptions over the past 

40,000 years, the latest only a few hundred 

years ago. The heated water, kept from 

boiling by high pressure, still has lower 

density than cold water, and it rises along 

steeply inclined fractures to depths of 0.3-

1.25 miles (0.5-2 km). It then flows eastward through rock layers to discharge points 

at the surface along Hot Creek and around 

Crowley Lake. The water temperature de-
200

210

220

230

240

250

260

270

280

290

300

1988 1997 2007

Geysering begins

THERMAL SPRING DISCHARGE, IN LITERS PER SECOND

The U.S. Geological Survey monitors the thermal water discharge in Hot 

Creek gorge by measuring chloride concentration in the stream above 

and below the hot springs. The onset of geysering in 2006 was accompanied by a small spike in discharge. (1 liter = 0.264 gallon.)

creases eastward because of heat loss and 

mixing with cold water, and in the springs 

near Crowley Lake temperatures are at only 

about 125°F (50°C).

The springs in Hot Creek all emerge along 

a stream section between two faults and discharge a total of about 8.5 cubic feet per second (about 240 liters per second) of hot water. 

This water flow represents nearly 70 percent 

of the total heat discharged by all thermal 

springs in Long Valley Caldera. The thermal 

springs farther east all discharge less water 

and at lower temperatures.

Possible Causes of the Recent Changes

Geysering activity similar to that which started in May 2006 also occurred along Hot Creek 

in 1980. That activity was during a period of 

greatly increased seismic (earthquake) activity 

and ground deformation. In 2006, however, 

local seismic activity and ground deformation were at the lowest levels in years, and the 

reason for this recent geysering is unclear. The 

change seems to be related to increased temperature in the shallow thermal ground water that 

supplies the springs. This increase was measured in the USGS monitoring well CH-10B, 

330 feet (100 m) deep and located 0.6 mile (1 

km) south of the gorge. Water level in this well 

is at the same altitude as the Hot Creek springs, 

and the temperature at that level reached the 

boiling point at about the time when geysering 

began in Hot Creek. What caused the temperature increase in the aquifer is not known, but 

it may be a delayed response to an earthquake 

swarm in 1997 that could have opened new 

flow pathways for hot water.

An alternative explanation for the onset 

of geysering is increased pressure in the 

aquifer leading to increased flow rate at 

Before the present episode of unpredictable and dangerous geysering at Hot Creek, similar activity occurred here in the the 1930s and again in the early 

1980s. A pool (outlined) where people used to soak in the 1970s (left photo) was near boiling temperature in the 1980s. It was then fenced off by the U.S. 

Forest Service in the interest of visitor safety. As you can see in the recent photo (right), Hot Creek Geologic Site continues to change, and the fence now 

goes through a pool instead of around it. (Left: USFS file photo; right: USGS photo by Dina Y. Venezky.)

The temperature in Hot Creek can change in seconds. These photos were 

taken 5 seconds before and then during a violent geysering event, in which 

boiling water (at 199°F or 93°C, the boiling point at this elevation) erupted 

above the surface. Any swimmer caught in this part of the creek would have 

been severely scalded. (Photos courtesy of Alix Ginter.)

10:30:20 10:30:25

The Hot Creek GEOLOGIC SITE is always changing

THERMAL WATER INPUT TO HOT CREEK DANGER COMES QUICKLY

93oC

199oF

1978

20oC

68oF

20oC

68oF

30oC

86oF

2007
Printed on recycled paper

Christopher D. Farrar, William C. Evans, 

Dina Y. Venezky, Shaul Hurwitz, and Lynn K.Oliver

the springs. In 2006, following a winter of 

heavy snow, there was abundant snowmelt 

that would have increased pressure in the 

aquifer. USGS measurements show that 

the total hot-spring discharge fluctuated 

strongly in June 2006, but it later returned 

to values near the long-term average.

What’s in the Water?

For several decades, the USGS has collected and analyzed water and gas samples 

from most of the streams, springs, and fumaroles (steam vents) in Long Valley Caldera, 

including those in the Hot Creek Geologic 

Site. The hot-spring water is naturally enriched in dissolved minerals. It is rich in 

sodium bicarbonate and contains high concentrations of arsenic, boron, and fluoride, all 

in excess of safe drinking-water standards. 

Stream water entering the thermal area is still 

relatively pure, but it leaves the area higher 

in dissolved substances because the mineralized hot-spring water mixes into the stream.

The springs and some areas of soil on the 

banks also discharge steam and other gases. 

Apart from steam, most of the gas is carbon 

dioxide, with minor amounts of hydrogen 

The captivating blue 

color of some of the 

spring pools in the 

Hot Creek area is 

caused mainly by 

two conditions—

(1) the deposition 

of white to light-tan 

calcium carbonate 

and clay minerals 

on the bottoms and 

Why Are the hottest

pools blue?

sides of the pools provides a bright background to reflect light, and (2) the properties of water that 

absorb red wavelengths of light but reflect blue wavelengths give the reflected light a blue tinge. 

This is the same effect seen in plaster-lined swimming pools. The bluest pools at Hot Creek gorge 

also have fine particles of calcium carbonate or clay kept in suspension by the boiling action. In 

general, the blue pools are the hottest because in water cooler than about 160°F (70°C) bacterial 

and algal masses give a greenish or brownish cast to the water. (USGS photo by Rick Iwatsubo.)

Visiting Hot Creek Safely

Visiting Hot Creek can be 

an enjoyable and rewarding 

experience, but you should 

be aware of the dangers and 

take them seriously. Know the 

hazards—boiling or scalding 

water, steam vents, unstable 

ground and boulders, hot ground 

or mud, swiftly flowing water, 

a stream with unpredictable 

currents, and water unfit for 

human consumption.

Follow these tips for your safety:

•	 Keep a clear head and be observant— conditions can 

change quickly.

•	 Keep careful watch on

children and pets (always keep 

pets on a leash).

•	 Follow directions and

warnings on signs.

•	 Don’t cross over fences or

barriers, and stay on walkways.

sulfide (which smells like rotten eggs) and 

other gases. Gas bubbles can be seen rapidly 

rising through the water in places along the 

edge of the stream. Areas of steam venting are 

always present at Hot Creek, but their visibility 

increases as the air temperature drops or the 

humidity rises. The seasonal variations in air 

temperature and humidity can lead to a false 

appearance of big changes in steam discharge.

The quality and temperature of the stream 

water in Hot Creek is generally acceptable for 

sustaining aquatic organisms, including a robust 

population of wild trout. However, very rapid 

changes in thermal spring discharge can sometimes raise water temperatures high enough to 

kill unlucky fish and other organisms. Be careful that you do not fall victim to such a fate!

The work of USGS scientists, in cooperation 

with the U.S. Forest Service, investigating the 

hydrothermal features of Hot Creek and the geology of Long Valley Caldera is shedding new 

light on this fascinating area and its geologic 

hazards. This work is only part of the efforts of 

the USGS to study and monitor the Nation’s 

volcanic regions and water resources.

For more information contact:

U.S. Geological Survey

(530) 546–0187

http://volcanoes.usgs.gov/

or

See also: Living with a restless caldera – 

Long Valley, California (USGS Fact Sheet 108-96); 

Invisible CO2 gas killing trees at Mammoth Mountain, 

California (USGS Fact Sheet 172-96); 

and Future eruptions in California’s Long Valley areawhat’s likely? (USGS Fact Sheet 073-97)

This Fact Sheet and any updates to it are available online at http://pubs.usgs.gov/fs/2007/3045

Edited by Peter H. Stauffer 

Graphic design by Judy Weathers

Banner design by Bobbie Myers

COOPERATING ORGANIZATIONS

 U.S. Depatement of Agriculture, Forest Service

°F °C

0 -18

428 220 

199 93 Water boils at 

Hot Creek

Hottest well in 

Long Valley

212 100 Water boils at sea level

152 70 

122 50 Upper limit for mosses

100 38 

 

68 20 

32 0 Water freezes to ice

Upper limit for 

blue-green algae

Typical hot-tub 

temperature

Comfortable 

(”room”) temperature

How to get there:

The Hot Creek Geologic Site 

is located northeast of the 

Mammoth-Yosemite Airport 

and is accessed from U.S. 

Highway 395 by the Hot Creek 

Hatchery Road.

A USGS scientist measures temperature in 

a near-boiling pool.