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Excerpted from
Geology of the Imperial Valley

a Monograph by
Eugene Singer

FOR THREE MILLION YEARS, at least through all the years of the Pleistocene glacial age, the Colorado River worked to build its delta. By then, the delta had reached the western shore of the Gulf of California (the Sea of Cortez) creating a massive dam which excluded the sea from the northern reaches of the Gulf. Meandering at random across the ever-growing fan-shaped mass, the river changed its course constantly. For a while, the course would shift to the north, and the stream flowed into the isolated Salton basin, filling it with a large freshwater lake. Eventually, a river shift to the south to the Gulf of California would abandon the inland lake to evaporation and extinction.

As a result, the Salton basin has had a long history of alternately being occupied by a fresh water lake and being a dry, empty desert basin, all according to the random river flow, and the balance between inflow and evaporative loss. A lake would exist only when it was replenished by the river, a cycle that repeated itself countless times over hundreds of thousands of years.

There is abundant evidence that the basin was occupied by multiple lakes during this period. Wave-cut shorelines at various elevations are still preserved on the hillsides of the east and west margins of the present lake, the Salton Sea, showing that the basin was occupied intermittently as recently as a few hundred years ago. The last of the Pleistocene lakes to occupy the basin was Lake Cahuilla, identified on older maps as Lake Leconte.

Lake Cahuilla

Lake Cahuilla was possibly one of the largest lakes of the past. It was a huge freshwater body covering over 2,000 square miles to a depth of more than 300 feet. The lake was almost 100 miles long by 35 miles across at its widest point, extending from the delta in Mexico north to the vicinity of Indio. It was six times the size of the present Salton Sea. This ancient freshwater lake completely filled the Salton Basin to overflowing behind the natural delta-dam.

The muddy water of the Colorado River flowed into Lake Cahuilla for centuries. The rich soil of the Imperial and lower Coachella Valleys was built up from river silt deposited on the floor of the old lake. The thick accumulation of lakebed deposits is evidence of a long period of deposition.

The shoreline of the old lake is still visible at the base of the surrounding mountains. It averages about 40 feet above sea level, but varies from 25 to 50 feet elevation. The variability of elevation is thought to be due to subsidence of the basin floor.

Radiocarbon age-dating of charcoal and fish bones found interstratified in the lagoonal silts behind gravel bars suggests that the lake existed since before the year 1200. Further evidence discloses that about 900 years ago, while Lake Cahuilla was a young, vigorous freshwater lake, the Cahuilla Indians, generally thought to be connected to the Aztecs by language, appeared from the northeast.

With the first Spanish explorations in the 16th century, they found no lake in the Salton Basin. This suggests that Lake Cahuilla had evaporated completely by 1600, or about 400 years ago. Yet, these early Spanish records allude to Indian legends of the existence of a large body of water to the west. The Indians now living in the Coachella Valley have distinct legends to the effect that at some time in the past the valley was occupied by a large body of water.

Prof. Blake in his 1854 exploration report notes that the Indians told him of a time when a great body of water existed in which there were many fish and of the manner in which the water disappeared 'poco a poco' (little by little) until the lake became dry. The Indians now living in the desert put this event as far back as the lives of four or five very old men. (The year 1900 less four or five times 60 years would place the approximate time of its end at about the year 1600.)

Lake Cahuilla's end must have been rapid when it came. The lake had been sustained for centuries as the net inflow of river water balanced the loss by evaporation. But again, the river changed its course. Possibly there was an ancient flood caused by a surplus of melt water, or perhaps the river's own natural levees became so high to be unstable. Whatever the cause, the river changed its course to flow once again south into the Gulf of California, and the lake was abandoned.

When fresh river water was no longer supplied, evaporation became the dominant factor. The lake quickly wasted away, leaving beach deposits, travertine deposits, wave-cut cliffs, sand bars and other shoreline features as proof of its existence. In its final stages, the lake level appeared to have retreated in steps, as more than a dozen separate shorelines still appear in aerial photos of the western shore and the Coachella Canal between Niland and Mecca.

So it was that Lake Cahuilla disappeared, leaving a playa, a flat, extensive salt-encrusted mud flat, desolate and without vegetation. Typical of playas, the lake bed was a dry, smooth hard packed surface. When supplied with a little water from an infrequent rain, or perhaps some random inflow via the New River, the playa would temporarily become a huge pond, sometimes miles across but filled with only a few inches to a foot of water.

Lake Cahuilla left abundant evidence of its existence. Foremost among these is the old shoreline representing the high water level of the old lake. This is evident from Indio to Cerro Prieto in Mexico at a height of about 40 feet above sea level. Where the shore abutted the bedrock of the mountain slopes, it left a whitish encrustation, called travertine. The travertine deposits destroyed or covered the original desert varnish.

The travertine appears as a showy light-colored deposit along the base of the Santa Rosa Mountains, from La Quinta for several miles southeast along the shore of the present Salton Sea. In places, the travertine is several feet thick on the rock face. The sharp contrast of the light colored travertine against the reddish brown desert varnish causes the old shoreline to be highly visible from a considerable distance.

Travertine, or tufa, is a freshwater lime deposit. It is derived from fresh waters that have a high concentration of calcium carbonate, CaCO3, the material of sea shells. Freshwater algae use carbon dioxide in photosynthesis precipitating the lime. This usually occurs in shallow water where algae can grow in abundance on resistant rock surfaces.

Travertine Rock is a vivid reminder of old Lake Cahuilla. It is along Highway 86, near Desert Shores, on the northwest shore of the sea. Travertine Rock was a small islet of bedrock that projected above the lake's high water line. Below this line, the boulders are heavily crusted with pale brown travertine, from a few inches to three feet thick and appearing sponge-like.

Travertine Rock is connected to the Santa Rosa Mountain mass by a conspicuous saddle, or tombolo, rising 150 feet. Successive Lake Cahuilla shorelines were once visible on the saddle, but they have been destroyed by recent cultivation of the land.

Shell fossils from a brackish water environment are abundant on the valley floor. They are arranged in linear accumulations parallel to the old shoreline. As the shorelines retreated, enormous numbers of Pleistocene gastropods and pelecypods (mollusks) became stranded, leaving their shells in windrows that stretch for miles. These beaches and their shells are most pronounced along the northwest and eastern margins of the Salton Sea. They may be reached by several side roads west from Highway 86, in some places less than a quarter of a mile from the main highway.



Wave-cut shore lines and sand and gravel bars are found near Niland. These are left from the ancient beaches and strand lines. In most places, the beach line has a sand ridge a few feet high, covered with abundant well-preserved freshwater shells.

Fish Traps

West of Valerie Jean, and along the lower mountain slope, is a valuable archeological site.

These are stone structures considered to be ancient Indian fish traps. There are three rows of shallow pits excavated in the talus slope. Each row has about 40 circular pits 10 feet in diameter and three feet deep. This photo, taken in 1929 clearly shows the circular pits as well as the stratified levels of the retreating shoreline.

These artifacts are thought to have been used by the Cahuilla Indians for fishing purposes, as they lie just below the high-water mark of Lake Cahuilla. The arrangement of the pits suggests they might have been constructed to keep up with the receding shoreline. With the high evaporation rate in the arid climate, each row of traps was probably used for only a few seasons before it was replaced. So, the traps are likely to be about 400 years old.