The Salton Sea, California's largest saltwater
lake (currently about 15 by 35 miles), is located in the Salton Trough, a
130-mile-long and 70-mile-wide below-sea level depression of concentrated
tectonic activity on which ride the Coachella and Imperial Valleys of Southern
California, and the western half of the Mexicali Valley and the Colorado River
delta in Mexico. (1,2) The Salton Trough is divided into two parts by the Salton Sea--the Imperial Valley is to the south and the Coachella Valley is to the north. The Coachella Valley includes the highly populated Palm Springs
area, and the Imperial Valley is the largest area of desert irrigation
development for growing crops in the US. Importation of water from the Colorado
River to Imperial Valley has transformed 500,000 acres of desert into a fertile
agricultural area. The Colorado River lies to the east of the Salton Trough.
The mouth of the Colorado River, which is now mostly dry, is in Mexico where the riverbed merges with the Gulf of California (Sea of Cortez).
I. Triple Plate Junction Beneath
the Salton Trough
The Salton Trough area, incredibly, is the place
where three moving tectonic plates meeta triple junction among
the Cocos, Pacific, and North American tectonic plates.
Cocos plate (gray-blue in above figure) is an
oceanic plate hugging the west coast of Mexico. Its name derives from Cocos Island, which rides on it. Seismic activity of the Cocos plate caused the 1985 Mexico City earthquake (8.1 on the Richter scale) described elsewhere. (3) Mexico City is located where the Cocos and North American plates rub up against one
South of the Cocos plate is a larger oceanic
plate called the Nazca plate (light blue in the figure above), named after
the Nazca region of Peru. The Galapagos archipelago, a group of volcanic
islands, signals the boundary between the Cocos and Nazca plates, which are
spreading apart along their shared margin. (4)
The Cocos and Nazca plates are remnants of the Farallon
plate, the ancient oceanic plate that broke up and has been subducting
under the North American plate since the Jurassic period 199 to 145 million
years ago. (5) The Farallon plate, named after the
Farallon islands off the coast of San Francisco that ride on it, has almost
completely subducted beneath the western portion of the North American Plate.
When the Farallon plate began its downward plunge beneath the North American
plate in the Jurassic period, it probably lifted up the Rocky Mountains. The
North American plate terminates at the San Andreas Fault system, which runs
down the length of California. The San Andreas Fault system is also the point
at which the North American plate rubs up against the Pacific plate.
The North American plate (brown in map
above) is made of both continental and oceanic (Atlantic Ocean) crust.
The Pacific plate (light yellow in figure
above) shares a boundary with the Cocos, Nazca, and North American plates.
Indeed, the point at which the Pacific, Cocos, and North American plates
meetthe triple junctionis beneath the Salton Trough.
II. East Pacific Rise and
the Gulf of California
The East Pacific Rise is a spreading rift valley
that extends northward from the middle of the equatorial Pacific Ocean and
disappears as a distinct bathymetric feature somewhere near the Gulf of California. (6,7) The East Pacific Rise connects with the San Andreas Fault
system under the Salton Trough.
How did the Gulf of California and Baja California form? "About five million years ago, the East Pacific Rise split the Baja Peninsula from the mainland of Mexico", explains Alles. (2) "The waters of the
Pacific then poured into the rift valley creating the Gulf of California. Since
then, like a giant door swinging open, plate tectonic activity along the East
Pacific Rise has moved the Baja Peninsula 162 miles (260 km) westward from the
mainland at the southern end of the gulf. The northern 'hinge' point of this
tectonic system is the Salton Trough". (2)
|“Computer-generated topographic map of the east Pacific Rise near 9 degrees north. Blues and greens represent lower elevations. Yellows and reds represent higher elevations. The narrow axis (red linear features) and narrow or absent axial rift, characteristics of fast-spreading mid-ocean ridges, are obvious. The spreading centers overlap near the middle of the image. The cones in the top-right corner of the image are a linear chain of near-axis seamounts on the Pacific Plate. The seamounts are greater than 200 m tall. View is to the north. Image courtesy of Stacey Tighe, University of Rhode Island. Image from U.S. Geological Survey’s “This Dynamic Earth Colorful bathymetry”. Source: http://volcano.und.edu/vwdocs/Submarine/plates/diverg/fast.html; accessed March 12, 2008.
A similar interpretation of the formation of the
Gulf of California by Elders, et al., is:
Pacific plate appears to be a flank of the East Pacific Rise modified by the
westward boundary of the North American plateMarine geophysical studies both
in the Gulf of California and in the adjacent ocean strongly support the
interpretation that the gulf originated in the spreading apart of the
continental crust. It appears that the Gulf of California is part of the active
boundary between the North American and Pacific plates". (8)
Hydrothermal vents jetting water at 380 degrees
plus or minus 30 degrees Celsius exist on the axis of the East Pacific rise.
(9) Indeed, "the axis of the East Pacific Rise is marked by a zone of
recent volcanism approximately 1000 meters wide. Near the center of the
volcanic zone, there is a very narrow band of active hydrothermal ventsat
least 25 vents along a strip 7 km long and only 200 to 300 meters wide",
according to one group of researchers. (9)
III. Salton Trough
The Mexicali-Imperial Valley is the northern
extension of the Gulf of California. "It is a broad structural trough
partly filled with lacustrine [lake] and deltaic [Colorado River delta] silts,
sands, and gravels of late Tertiary age, and by great thicknesses of Quaternary
alluvium and lake sediments", notes Elders, et al. (8) They continue:
"The trough has "steep,
step-faulted margins and a broad, relatively flat basement floorThis trough is
comparable in shape and size with the deeper submarine basins of the southern
part of the Gulf of California, but it is partly filled [in the Imperial
Valley] by the vast accumulation of sediments of the Colorado River Delta".
Elders, et al., believe that "the Salton
trough formed by a combination of tensional and right-lateral strike-slip
movements associated with the opening of the gulf as Baja California was
transferred from the North American to the Pacific lithospheric platesThe
details of how Baja California was transformed from the American to the Pacific
plate remain enigmatic", they opined. (9)
IV. Brief History of the Colorado River and its Relation to the Salton Trough
Upper mantle processes caused a broad regional
uplift of the Colorado plateau about 20 million years, according to Atkinson
and Leeder. (10) The uplift of the Colorado plateau also initiated the Grand
Wash Fault at the western edge of the Colorado Plateau where the Colorado River of 20 million years ago poured over this growing step on its way westward.
(10) Recent work by Polyak, et al., shows that the Colorado River cut a deep
gorge as it propagated upstream to create the Grand Canyon. (11)
The Colorado River drains the western slope of
the Rocky Mountains. Its natural course is to flow to the Gulf of California. A
diversion canal built (the "All American Canal") from the Colorado
River to the Imperial Valley to irrigate crops has resulted in desiccation of
the river before it reaches the gulf.
V. Inadvertent Creation of the
The current Salton Sea is manmade that was a
catastrophe by some accounts. (12) It formed over two years between 1905 and
1907, while an anxious Teddy Roosevelt was president of the United States. What happened? President Roosevelt said it well:
the conditions are these: The Imperial Valley, so called, in San Diego County, Cal., includes a large tract of country below sea level. Southeast of the valley and
considerably above its level is the Colorado River, which flows on a broad,
slightly elevated plane, upon which the river pursues a tortuous course,
finally entering the Gulf of California. The lands in Imperial Valley are 200
feet or more below the level of Colorado River. Down as far as the
international border they are protected from inundation by low-lying hills.
South of the boundary in the Republic of Mexico the hills cease abruptly, and
only the broad, low mud banks of the river protect the valley from being
converted into an inland sea or lake. In order to protect it from too much
water, works of supply or of protection must be built in Mexico, even though they may tap the river in the United States. The United States can neither aid
nor protect the interests of its citizens without going upon foreign soil
"Many schemes have
been discussed either to convert the Salton sink area into a lake or to
irrigate the desert lands below sea level by making a cut in Mexico through the
west bank of the Colorado River. It waswell understood that if the cut in the
bank was not carefully guarded the river would quickly get beyond control.
Finally, after many plans had been tentatively tried, the California
Development Company, a New Jersey corporation, actively undertook the work.
"To insure the
safety of Imperial Valley, the head of the canal on the river was first placed
on United States territory near where the river was bounded by hills. The canal
then swung southwest and west away from the river through Mexican territory to
connect with natural depressions leading to the valley and back into the United States. The organizers of this company, in order to carry on the work in Mexico, caused to be created a subsidiary company in Mexico, acting under Mexican laws
entered upon its construction work with large plans, but with inadequate
capital. All of its structures for the control and distribution of water were
temporary in character, being built of wood and of the smallest possible
dimensions. Through the efforts thus made a large amount of land was brought
under cultivation, and at one time it was reported that over 100,000 acres were
being more or less irrigated.
"The first heading
of the canal of the California Development Company was in the United States immediately north of the Mexican border. It was found, however, after a time
that the heading on the United States side of the line did not give a grade to
furnish sufficient flow or water, and after headings had been opened at other
points without successful results a cut in the river bank was made four miles
further south in Mexican territory. This gave the water a shorter and steeper
course toward the valley.
"The making of this
cut in a bank composed of light alluvial soil above a depression such as this
without controlling devices was criminal negligence. This short cut on Mexican
soil was made in the Fall of 1904. It was gradually eroded by the passage of
the water, and in the Spring of 1905, the floods of the Colorado River entering
the artificial cut rapidly widened and deepened it until the entire flow of the
river was turned westerly down the relatively steep slope into the Imperial
Valley and then into what is known as Salton Sin, or Salton Sea.
"After the mischief
became apparent strenuous efforts were made by the California Development
Company to close the break, but these were without success. Finally the
Southern Pacific Company, finding its tracks imperiled and traffic seriously interfered
with, advanced money to the California Development Company, received as
security a majority of the shares of the company, and thus took charge of the
"By means of the
facilities available to the Southern Pacific Company the break in the west bank
of the Colorado River was closed on Nov. 4, 1906. A month later, however, a
sudden rise in the river undermined the poorly constructed levees immediately
south of the former break, and the water again resumed its course into the Salton Sea". (13)
Eventually, the Southern Pacific Company was
able to put the Colorado River back into its pre-levee-break channel. The
damage, however, was done. The Salton Trough was filled with watera sea 45
miles long and 20 miles wide, equaling 110 miles of shoreline.
The Salton Trough has been filled with water
before its accidental filling in 1905. Prehistoric "Lake Cahuilla" formed when the Colorado River shifted its course within its delta, and flowed
northwest instead of south around 300 years ago. The area was also briefly
flooded in 1891. Both of these collections of water eventually evaporated.
VI. Activity of the San Andreas Fault in Salton Trough
"The part of the San Andreas [fault system]
called the Coachella Valley segment has always been one of the most worrisome
along the entire fault", notes Kerr. (15)
"Indeed, the first
comprehensive evaluation of earthquake hazard in California, released in 1988
by a working group of the USGS, rated it the most dangerous of all [italics
added]. The geologic record of this segment, which runs southward from near Palm Springs 100 kilometers to the eastern shore of the Salton Sea, shows that it
experienced repeated earthquakes, at intervals of 200 to 300 years, until about
300 years ago. Since then, it has been eerily quiet, steadily accumulating
strain that must eventually be released. The USGS working group gave it a 40%
probability of producing a magnitude 7.5 quake by 2018 that would rip as far
north as Palm Springs". (15)
Salton Sea Trough area is an exceedingly complex geological, riverine and
lacustrine area underlain by the junction of three giant moving tectonic
plates. Historically, the Imperial Valley and the Coachella Valley are one of the most, if not the most, seismically active regions in the State of California. (16) Twelve significant seismic events have occurred in Imperial County during the period April 1906 to October 1987. (16) The Salton Sea Trough is overdue for
a big earthquake.
1. The Salton Trough is
also called the Salton Sink, the Salton Basin, and the Salton Sea Trough.
2. David L. Alles: "Geology
of the Salton Trough", August 6, 2007, p. 1. Available at http://fire.biol.wwu.edu/trent/alles/GeologySaltonTrough.pdf; accessed March 12,
3. Michael A. Cassaro and
Enrique Martinez Romero: The Mexico Earthquakes-1895: Factors Involved and
Lessons Learned. American Society of Civil Engineers, February 1986.
4. Martin Mesched and Udo
Barckhausen: "Plate Tectonic Evolution of the Cocas-Nazca Spreading Center".
In EA Silver, G. Kumra and TH Shipley (eds.) Proceedings of the Ocean
Drilling Program, Scientific Results, January 2000, Volume 170. Available
at http://www-odp.tamu.edu/publications/170_SR/VOLUME/CHAPTERS/SR170_07.PDF; accessed March 12,
5. "Farallon plate
remnants". NASA/Goddard Space Flight Center Scientific Visualization
Studio. Available at http://svs.gsfc.nasa.gov/vis/a000000/a002400/a002410/; accessed March 12,
6. Roger L. Larson, HW
Menard, and SM Smith: "Gulf of California: A result of ocean-floor
spreading and transform faulting". Science, August 23, Volume 161,
1968, p. 781.
7. Ronald L. Larson and
Fred N. Spiess: "East Pacific Rise Crest: A near-bottom geophysical
profile". Science, January 3, 1969, Volume 163, p. 68.
8. Wilfred A. Elders,
Robert W. Rex, Tsvi Meidav, et al.: "Crustal spreading in Southern
California". Science, October 6, 1972, Volume 178, Number 4056, p.
9. RISE Project Group: "East
Pacific Rise: Hot springs and geophysical experiments". Science, March 28,
1980, Volume 207, Number 4438, p. 1421-1433.
10. Tim Atkinson and Mike
Leeder: "Canyon cutting on a grand time scale". Science, March
7, 2008, Volume 319, pp. 1343-44.
11. Victor Polyak, Carol
Hill, and Yemane Asmerom: "Age and evolution of the Grand Canyon revealed
by U-Pb dating of water table-type speleothems". Science, March 12,
2008, Volume 319, pp. 1377-1380.
12. Doug Sharp: "The
Salton Sea Catastrophe". Available at http://www.rae.org/salton.html; accessed March 12,
13. "President asks
action to curb the Colorado. Tells Congress that 700,000 acres are menaced by
river". The New York Times, January 12, 1907. Available at http://query.nytimes.com/mem/archive-free/pdf?_r=1&res=9C00E3D61130E233A25750C1A9679C946697D6CF&oref=slogin; accessed March 12,
2008. See also "The problem of the Salton Sea". The New York Times,
January 20, 2907. Available at http://query.nytimes.com/gst/abstract.html?res=9D00E0D9153EE033A25753C2A9679C946697D6CF&scp=1&sq=problem+of+the+salton+sea&st=p; accessed March 12,
Laylander: "The regional consequences of Lake Cahuilla". The San
Diego State University Occasional Archaeological Papers. Available at http://soap.sdsu.edu/Volume1/LakeCahuilla/cahuilla.htm; accessed March 12,
Kerr: "Ominous pattern looms in California earthquakes". Science,
July 10, 1992, Volume 247, p. 155.
County Emergency Management Plan, revised July 2007, pp. 80-81. Available at http://www.co.imperial.ca.us/EmergencyPlans/EmergencyPlanOperationsRevJuly2007.pdf; accessed March 12,