[Ed. Note: This article won first place in the adult
technical/advanced article competition in the American Federation
of Mineralogical Societies in 1995.]
The Salton Sea is a relative newcomer to southern California. One hundred years ago, in the location currently occupied by the Salton Sea, you would have found the bottom of a dry lake. In fact, if you could climb into a time machine, you would discover that the area now containing the Salton Sea, known as the Salton Basin or Salton Trough, was alternately flooded and dry. During at least one period of time in its history, a significantly larger body of water occupied the Salton Trough. Evidence of this inundation still exists, and can be readily seen, in the surrounding hills.
During the early Pliocene age (about 8 million years ago), the Salton Trough constituted the northern portion of the Gulf of California. The former presence of marine (salt) water is recorded in fossil-bearing sandy shales which, today, can be found at the edges of the trough and in drill cores taken from the central part of the basin at depths of over 10,000 feet. Interestingly, the fossils found at both of these locations include oyster shells; however, oysters live only in relatively shallow water -- nothing more than a few hundred feet deep. So how did oyster beds, which form in shallow water, wind up nearly two miles beneath the present surface of the Salton Trough?
The answer can be found by examining the geology of the Salton Trough. The Salton Trough is an example of what geologists call a rift valley or graben. A rift valley is a strip of land bounded on opposite sides by roughly parallel faults. Through movement of the faults, the strip of land sinks in a process termed subsidence. In the case of the Salton Trough, the land is bounded on the northeastern side by the San Andreas, Sand Hills, and Calipatria Faults. The southwestern side of the trough is bounded by San Jacinto, Coyote Creek, and Superstition Hills Faults. Although the principal movement of these faults is lateral (parallel to the surface of the earth), there is also a vertical component to the movement. Thus, for example, while the land to the southwest of the San Andreas Fault moved about 200 miles northwest, it also dropped from three to four miles in the process.
As the Salton Trough subsided, erosion from the surrounding mountain ranges kept it filled with sediment. The overall effect was to keep the top of the sediment within a few hundred feet of sea level while the entire mass of sediments sank to a depth of several miles. This explains why shallow water oyster shells are found at depths of 10,000 feet in the central part of the basin.
All of this raises a puzzling question: If the Salton Trough was originally part of the Gulf of California, and if the basin is currently several hundred feet below sea level, then why isn't the Salton Trough and the Imperial Valley flooded with water from the Gulf? In fact, most of the sediments overlying the Pliocene rocks described earlier are non-marine (fresh water) sediments. This means that, early on, the Salton Trough was cut off from the Gulf of California. What happened?
What happened was the Colorado River. Flowing from higher ground, the fresh-water Colorado River carried sediment into the Gulf of California and the lower part of the Salton Trough. Over time, the sediment accumulated into a delta (also termed an alluvial plain) which eventually rose above the level of the waters in the Gulf. Today, this alluvial plain, which extends for a distance of 65 miles across the top of the present-day Gulf of California at a minimum elevation of 40 feet above sea level, has kept the Gulf waters from flooding the Salton Trough.
Geologic evidence shows that once the Colorado River's alluvial barrier was in place, fresh water flowed from the river into the Salton Trough filling it from time to time to varying depths. As the river changed course, as most rivers do, the flow would be diverted, in different proportions, to the Gulf and to the Salton Trough. All the while, the trough subsided from activity along the boundary faults. Erosional sediment poured in from the surrounding mountains and from the Colorado River to fill the resultant rift valley.
Many of the old shorelines are visible today in the mountains surrounding the Salton Sea. There is one very prominent shoreline at an elevation of 44 feet above sea level which records the presence of Lake Cahuilla which once filled the Salton Trough. Driving highway 86, along the southwestern edge of the Salton Sea, one can clearly see the horizontal line which separates the dark coating of calcareous tufa, which formed below the water level, from the lighter rocks above. Carbon-14 dating has shown that Lake Cahuilla existed up until just a few hundred years ago.
In the latter half of the 19th century, the Salton Trough was completely dry, or nearly so. Wallace Elliott, in his History of San Bernardino and San Diego Counties, dated 1883, mapped a "Dry Bed of Lake" and described it thusly:
"Desert Formerly Fresh-Water Lake: A portion of the Colorado Desert is many feet below the sea level, and there is evidence that since the retreat of the gulf much of it has been covered by a continuous sheet of fresh water. The evaporation of moisture has, however, in modern times, exceeded the precipitation, and most of the surface is now constantly dry; though at the lagoons there still remains a miniature representative of the wide-spread fresh-water lake which once occupied the area surrounding them.
"[...] fresh-water shells -- anodonta, planorbis, physa, and amnicola -- [show] plainly the bed of a former lake. These shells are very abundant, the Amnicoloe being sometimes drifted by the wind till they cover and whiten the ground, and look like miniature snow wreaths. They have generally lost their epidermis, and the Anodontas are considerably broken and decayed, but on the whole are so well preserved that it seems impossible that many years have elapsed since they were inhabited by living animals."
So what happened to cause the formation of the Salton Sea in this "Dry Bed of Lake" within the last one hundred years? Wallace Elliott provided the first hint:
"Dr. O. M. Wozencraft, of San Bernardino, has for years entertained the idea of turning the waters of the Colorado River, or a portion of them, into a system of canals for the purpose of irrigating that portion of the Colorado Desert lying below the level of the sea."
In 1891, Mother Nature started what Wozencraft proposed. E. B. Preston, wrote in the Eleventh Report of the [California] State Mineralogist (1893), that "In the month of June, 1891, a steady flow of water entered the depression from the southeast and continued to the northwest uninterruptedly until an area 30 miles long and averaging 10 miles in width was covered to a depth of 6 feet."
To this body of water was given the name "Salton Lake." The name was derived from the high saline content of the lake. At first, an underground connection to the Gulf of California was suspected to explain the salinity. Eventually it was determined that the water entering Salton Lake was fresh, from the Colorado River. The saltiness was due to the enormous rate of evaporation which concentrated the tiny sodium chloride content of the river. The salinity of Salton Lake was so great that the New Liverpool Salt Company established a facility to produce a fine quality of table salt from the water.
In 1901, the California Development Company built the first canal system to divert water from the Colorado River to the Imperial Valley for the purposes of irrigation. The canal system functioned well for several years until, in 1905, unusually high floods overwhelmed the canal system and destroyed the regulating machinery. For a period of time the Salton Sea received the full, uncontrolled flow of the Colorado River.
Over the next two years, the Southern Pacific Railroad Company attempted to repair the breach. Underestimating the power of the Colorado River, their control structures were repeatedly washed away. Finally, in 1907, the river was sealed, but only after 350,000 acres of land had been flooded. Today some of that land has been reclaimed by the lowering of the Salton Sea through evaporation. Presently, the Salton Sea, fed by runoff from irrigation, has reached equilibrium with the rate of evaporation and stands at 235 feet below sea level.
For a spectacular, first-hand look at some of the non-marine sediments which make up the "filling" of the Salton Trough, take the automobile trip shown on the map below. Start by heading east from Indio on Interstate-10 to the Cottonwood Spring Road exit, then follow the road signs to the town of Mecca. The 20-mile drive descends from 1600 feet above sea level to 190 feet below sea level in Mecca.
Initially, the drive takes you past the Orocopia Mountain foothills and Buried Mountain (when you see it, you'll know why it was given that name). At the point where the road enters the bottom of Box Canyon Wash, you'll get a clear view of the fresh-water sediments which make up the 100-foot, near-vertical walls of the canyon. Looking at the folded layers, tilted in every direction, it is clear that significant geological activity has occurred in the area. One of the causes of this activity is located just ahead on the road. Box Canyon and the surrounding hills terminate abruptly where the road crosses the San Andreas Fault. This point, nearly at sea level, marks the edge of the Salton Trough. Take highway 86 south, along the western edge of the Salton Sea, to get a good look at the ancient shoreline in the mountains to the west.
To the Editor:
I wholeheartedly agree with Richard's saying that Box Canyon Road, east of Mecca, California, is very worthwhile viewing ["Descent to Mecca;" Lithosphere; June 1995]. It's nice to get off the freeway and look at a rock up close. I've driven both ways four or five times trying to make sense of how the San Andreas Fault has contorted these formations. For an equally spectacular, but different, view of the erosive power of running water, take a look at the next canyon north -- "Painted Canyon."
Just east of the orchards, vineyards, and Coachella Canal, turn
north onto a graded gravel road. The first three miles parallel
the Mecca Hills; then the road deteriorates as the next mile
and a half goes up the
"golly-gee-whiz-never-seen-anything-like-it" canyon. This
description is based upon a twenty-year-old memory of a
National Association of Geology Teachers (NAGT) field trip.
I plan to check it out in a standard highway vehicle this
September. The AAA maps of both Riverside and Imperial
Counties illustrate this location.
Copyright © 1995 by Fallbrook Gem and Mineral Society, Inc.
The preceding articles were originally published in the June and September 1995 issues of Lithosphere, the official bulletin of the Fallbrook [California] Gem and Mineral Society, Inc; Richard Busch (Editor).
Permission to reproduce and distribute this material, in
whole or in part, for non-commercial purposes, is hereby granted
provided the sense or meaning of the material is not changed and
the author's notice of copyright is retained.
Last updated: 18 September 2002