Lake Cahuilla

Name

The name "Lake Cahuilla" was used in 1907 by William Phipps Blake and As of 1961 is recognized by the US Geological Survey. It is named after the Cahuilla which refer to the lake in their oral tradition. A second name is "Blake Sea", after William Phipps Blake. The Cahuilla themselves named the lake paul, and their mythology states that when their creator paulnevolent was cremated, tears turned the lake salty.

"Lake LeConte" was coined in 1902 by Gilbert E. Bailey, it is occasionally used to refer specifically to the lake that existed during the Wisconsin glaciation or Pleistocene. M.R. Waters in 1980 applied the term to cover all lakes of Holocene age in the Salton Basin. This name is derived from Joseph Le Conte, a geography professor.

Presently, the name "Lake Cahuilla" applies to the reservoir at the northern end of the Coachella Canal, in the Coachella Valley. "Lake Cahuilla" is also the name of a seismic station in California.

Geography

Lake Cahuilla formed in the region of the present-day Salton Sea. It did extend over the southern Coachella Valley in the north and the Imperial Valley in the south, to the Cerro Prieto area in Baja California. This area was also known as the Colorado Desert. Presently, 5,400 square kilometres (2,100 sq mi) of the land are below sea level. The Salton Trough extends 225 kilometres (140 mi) northwest and has a width of 110 kilometres (68 mi) at the border.

Present-day towns in areas formerly covered by Lake Cahuilla are from north to south Indio, Thermal, Mecca, Mortmar, Niland, Calipatria, Brawley, Imperial and El Centro. Calexico and Mexicali may have been covered as well. To the southeast, the New River and the Alamo River now flow through the dry lakebed, while the Whitewater River and the San Felipe Creek come from northwest and southwest respectively.

Major shorelines are the 12 metres (39 ft) above North American Datum (NAD) shoreline and other 20–50 metres (66–164 ft) above NAD. With a southern shore south of the US-Mexico border, Lake Cahuilla had a length of 160 kilometres (100 mi), a maximum width of 56 kilometres (35 mi) and reached a depth of approximately 91 metres (300 ft) at a water elevation of 12 metres (39 ft). The maximum surface area was about 5,700 square kilometres (2,200 sq mi). The lake at maximum level held about 480 cubic kilometres (120 cu mi) of water. At maximum size, Lake Cahuilla was considerably larger than the Salton Sea and almost as large as the entire Salton Trough.

Bat Caves Butte and Obsidian Butte would have formed islands in the lake. Its relatively straight northwest-southeast trending eastern shores faced from northwest to southeast the Indio Hills, the Mecca Hills, the Orocopia Mountains, the Chocolate Mountains and the East Mesa. Its less regular western shore faced the Santa Rosa Mountains towards north and the Fish Creek Mountains and Vallecito Mountains farther south. Earlier lake stages may have extended into the Jacumba Mountains as well.

Inflow

Lake Cahuilla was formed by water from the Colorado River; groundwater and other inflows are negligible. Likewise, the precipitation (presently about 76 millimetres per year (3 in/year)) did not contribute much to the lake budget. The amount of water needed to sustain Lake Cahuilla at a level of 12 metres (39 ft) above sea level is possibly about half of the discharge of the Colorado River.

Sedimentation of the Colorado River Delta over time directed water into the Lake Cahuilla area. Distributaries in a river delta are inherently unstable and tend to change course often. Major floods may have triggered the change in river course, although most of the flood events in the prehistoric record do not appear to be associated with diversions to Lake Cahuilla. Given that the slope down towards Lake Cahuilla is steeper than the one towards the Gulf of California, once the river started entering the basin it likely stabilized in such a course. In fact, it is remarkable that this slope difference doesn't regularly cause the river to enter the Salton Trough. The diversion occurred close to the apex of the Colorado River Delta and would have discharged water directly through the Alamo River and indirectly through Volcano Lake and the New River. The infilling of the lake may have been a catastrophic flood, considering that native people fled the Imperial Valley to the mountains. Infilling to an altitude of 12 metres (39 ft) above sea level would have taken 20–12 years. When the lake was full, the Colorado River would have entered it at the southeastern side.

When the Colorado River drained into Lake Cahuilla, the entire sediment flow (c. 150,000,000 tonnes per year (4,700,000 long ton/Ms)) of the Colorado would have entered the lake. Sedimentation of the inlet during highstands and resulting river course changes away from Lake Cahuilla would have resulted in the Colorado River changing its course back to the Gulf of California.

Other major streams that drained into Lake Cahuilla are the Whitewater River from north and San Felipe Creek and Carrizo Creek from southwest. More minor drainages came from Arroyo Salado on the western shore and Salt Creek and Mammoth Wash on the eastern shore. Some further unnamed drainages did exist. Further drainages from the Chocolate Mountains and the Cargo Muchacho Mountains may have reached the lake but are now buried by the Algodones Dunes. All these water systems are ephemeral.

Presently the only major streams entering the basin come from mountains west and northwest of it, but during the Pleistocene they likely transported more water. In fact, when lower sea levels entrenched a more southerly course of the Colorado River, Lake Cahuilla may have been nourished solely by local runoff during the Wisconsin glaciation.

Shorelines

Shorelines lie at altitudes of 7.6–18.3 metres (25–60 ft) above sea level, the variation is probably caused by slumping, measurement problems and different wave cut and beach deposit thicknesses. The latest highstand lasted long enough to allow the formation of well developed shorelines. Fish fossils found off the coastline suggest that lagoons connected to the lake formed there. Fluctuations of the lake level caused the deposition of beach berms. Based on recessional shorelines with distances of slightly over 1.5 to 1.23 metres (4 ft 11 in to 4 ft 0 in) from each other, 96 metres (315 ft) of depth would have evaporated in about 70 years.

The shoreline is particularly well visible at Travertine Point at the Santa Rosa Mountains, where the colour contrast between the dark desert varnish above the shoreline and the travertine below is recognizable from US highway 99.

On Lake Cahuilla's eastern shore, the nature of the shoreline ranges from 7.6 metres (25 ft) high wavecut cliffs beneath the Mecca Hills over baymouth bars farther south, one of which reaches a length of 5.6 kilometres (3.5 mi) at the Orocopia Mountains. Even farther south shingle beaches are found, showing evidence of vigorous wave activity. At East Mesa, a c. 50 kilometres (31 mi) long barrier beach may have formed from sediments deposed by flash floods. Often, material eroded from the eastern and southwestern shores was deposited in the form of gravel and sand bars off the coast. As lake levels rose, at least one tributary stream had its valley filled in with Lake Cahuilla sediments. Tufas formed along shorelines, reaching maximum thicknesses of 1 metre (3 ft 3 in). They are found especially on the northwestern shores. At the Fish Creek Mountains, beaches made up of gravel and a travertine layer on the mountain front mark the shore.

Water composition

As deduced from the presence of freshwater molluscs, Lake Cahuilla during its highstand was a freshwater lake, while lower lake level stages show fossil evidence of increased salinity. Alternatively the lake may have been brackish. The exact salinity may have been lower where the Colorado entered the lake and higher farther north.

Water currents

High cliffs, sandbars and piles of pebbles testify to the existence of strong wave action on the northeastern shore, which was influenced by strong northwesterly winds. To the contrary, the gentle southern slopes of the lake bed probably reduced wave action on the lake's southern shores.

Strong northwesterly winds probably created southbound lake currents on the eastern shores, forming beach structures from sediment imported from north into the lake.

Outflow

Only about half of the discharge of the Colorado River was needed to sustain Lake Cahuilla; the rest drained across the delta into the Gulf of California. A 12 metres (39 ft) high above sea level outflow sill close to Cerro Prieto formed the likely spillway for the lake. Other data point to a sill height of 10 ± 0.299 metres (32.81 ± 0.98 ft) but topographic maps of the area are not very precise. The present day sill is about 2 kilometres (1.2 mi) long and Cerro Prieto lies on the drainage divide between the New River and Rio Hardy watersheets. Water reached the Gulf of California through the present day Rio Hardy channel. Oxygen-18 isotope data from tufas suggest that the lake was closed or mostly closed for much of its time, that is the outflow did contribute little to the water balance. In addition, some water may have been trapped in aquifers.

The present-day sill to the Gulf of California lies at an altitude of 9 metres (30 ft) above sea level; the sill was probably higher in the past seeing as the highest shorelines of Lake Cahuilla are 18 metres (59 ft) above sea level. During the Pleistocene, the sill was higher and thus lake levels could reach higher elevations. A rejuvenation of the river triggered by decreasing sea levels or tectonic subsidence at Cerro Prieto caused the levels of the various lakes to progressively decrease. Dacitic lava flows from the Cerro Prieto volcano may have stabilized the overflow sill against erosion; it is otherwise difficult to explain why the overflow sill over fairly easily eroded material was stable against downcutting by overflow.

Once cut off from the Colorado River by changes in the latter's course, Lake Cahuilla would have evaporated at a rate of 1.8 metres per year (0.19 ft/Ms), eventually drying in 53 years. Data taken from fossil Mugil cephalus suggest that during the recession of the lake, Colorado River water still occasionally reached the lake.

Climate

The present day climate of the Lake Cahuilla area is dry and hot during summer. Temperatures range 10–35 °C (50–95 °F) with maxima of 51 °C (124 °F). Precipitation amounts to 64 millimetres per year (0.080 in/Ms). The mountains west of the Cahuilla area are considerably wetter. Evaporation rates can reach 1,800 millimetres per year (2.2 in/Ms).

Winds on the lake probably occurred in two patterns, northwesterly winds with speeds of 50 kilometres per hour (31 mph) and more persistently westerly winds with speeds of 24 kilometres per hour (15 mph). These winds did form substantial waves in the lake and created longshore currents along the eastern shores of Lake Cahuilla.

Pleistocene climate is less well known although it was probably not much wetter than today, except in the mountains where precipitation increased. Drainage changes in the Colorado River Delta probably account for most of the water budget changes responsible for the formation of Lake Cahuilla. Conversely, in the Mojave Desert large lakes formed during that time. In the early Holocene the North American Monsoon strongly influenced the local climate and then progressively weakened.

A colder climate was accompanied by cold-limited animal species appearing at lower altitudes and glaciers forming on the San Bernardino Mountains. A probable southward shift of the storm belts led to windier weather. According to data obtained from tufa in Lake Cahuilla, after the end of a wet period 9,000 years before present, between 6,200 and 3,000-2,000 years before present extended droughts occurred.

Geology

Tectonically, Lake Cahuilla formed in a region where the Gulf of California tectonic zone meets the San Andreas fault tectonic system. Volcanic activity and earthquakes occur as a consequence to this tectonic structure. The San Andreas Fault runs roughly parallel to the northeastern margin of Lake Cahuilla, where it moved at a rate of 9–15 millimetres per year (0.011–0.019 in/Ms) over the last 45,000-50,000 years, with earthquakes documented in sediments from Lake Cahuilla, but this southern segment has not ruptured in historical time. Tectonic extension occurs at the points where the fault forms stepovers, although the extensional structures are still relatively immature.

The Cahuilla Basin, also known as the Salton Sink, is part of the through that is occupied by the Gulf of California. The basin structure is surrounded by various crystalline rocks that were formed from the Precambrian era forwards to the Tertiary. About 10–16 kilometres (6.2–9.9 mi) of sediment fill the basin since the Miocene, testifying to rapid tectonic subsidence. Four million years ago, the Colorado River started to enter into the area. The formation of the Colorado River Delta separated the Salton Trough during the Pleistocene from the Gulf of California; during the Pliocene the connection still existed. Another basin in the region is formed by the Laguna Salada, with yet smaller basins such as the Mesquite Basin also reported. Approximately 6 kilometres (3.7 mi) of sediment have accumulated in the Salton Trough, masking the underlying crust. Heat flow analysis suggests that active extension is underway in the trough.

Faults and earthquakes

When Lake Cahuilla existed, individual earthquakes caused as much as 1 metre (3 ft 3 in) displacement. Sediments of Lake Cahuilla have shown deformation structures similar to these formed by the 1971 San Fernando earthquake in the Van Norman Reservoir of the Los Angeles Aqueduct. These were formed by soil liquefaction. Sediments of the lake at Coachella have yielded evidence of eight earthquakes, between 906 – 961, 1090 – 1152, 1275 – 1347, 1588 – 1662, and 1657 – 1713. Less certain is the timing of events between 959 – 1015 and 1320 – 1489.

Patterns of seismic activity detected by paleoseismology suggest that the filling of Lake Cahuilla might have triggered stress changes that caused earthquakes along the San Andreas Fault and other faults when they were already close to rupture. Such lake-induced seismicity is known from reservoirs and referred to as induced seismicity. Alternatively, earthquakes could have caused course changes in the Colorado River that then caused the lake to flood or to dry up; paleoseismology at Coachella is consistent with this hypothesis. Some earthquakes such as the 1892 Laguna Salada earthquake caused large vertical displacements that could have triggered flooding. Conversely, tectonically driven uplift of the northern side of the Colorado River Delta tends to stabilize the present southward course of the river against diversions to the north.

The San Andreas Fault has offset Indian stone rings, its path is buried by sediments from Lake Cahuilla. During the Pleistocene, this fault was relatively inactive compared to the Imperial Fault and the San Jacinto Fault. Other faults that crossed the shores of Lake Cahuilla are the Extra fault zone, which divides a northern more stable basin from a southern basin that underwent tectonic extension and slightly slower sedimentation, the Coyote Creek Fault (whose movement rate has been estimated from displacement of Lake Cahuilla sediments and probably accelerated during the time of Cahuilla's highstand), the Superstition Mountain Fault which extends from the Coyote Creek fault, the San Jacinto Fault which runs parallel to part of Cahuilla's western shore, was last active in 820-1280, 1280, 1440-1637 and 1440-1640 and whose fault trace could be buried beneath sediments from Lake Cahuilla, and the Elmore Ranch fault which displays evidence of after-lake activity in the Superstition Hills. Faults on the lake floor include the Brawley Seismic Zone, potentially the Cerro Prieto Fault, the Imperial Fault, and the Kane Springs Faults. The Imperial Fault may have ruptured together with a rupture of the San Andreas Fault during a highstand of Lake Cahuilla, and was last active during the 1940 Imperial Valley earthquake.

Volcanoes

Several volcanoes existed on the floor of Lake Cahuilla, now emergent at the southeastern margin of the Salton Sea. The presence of volcanism there may have been faciliated by extensional faults, which would have provided pathways for magma ascent. These include the Cerro Prieto and the Salton Buttes. Cerro Prieto is formed by two c. 200 metres (660 ft) high lava domes that coalesce into a volume of about 0.6 cubic kilometres (0.14 cu mi) and a 200 metres (660 ft) wide crater on the northeastern dome. In addition, mud pots and mud volcanoes exist on the floor of the Cahuilla Basin. Geothermal energy is obtained in some parts of the region.

The Salton Buttes are five lava domes that form a 7 kilometres (4.3 mi) long chain. Each dome is less than 1 kilometre (0.62 mi) wide. They are formed by rhyolite, which contains xenoliths. These domes are known as Mullet Hill, Obsidian Butte, Red Island and Rock Hill. Obsidian Butte formed subaerially but tufas and wavecut forms show that Lake Cahuilla submerged the dome. Red Island erupted within Lake Cahuilla, forming pyroclastic flow deposits. Wave action removed pumice and probably formed beach bars from this volcano. Pumice rafts are found attached to local shorelines.

Potassium-argon dating has yielded ages of 16,000 years ago for the Salton Buttes, later superseded by an age estimate of 33,000 ± 35,000 years ago and finally with a date of 2,480 ± 470 years before present on the basis of uranium-thorium dating, but some of them still release steam. Cerro Prieto appears to be 108,000 ± 46,000 years old based on potassium-argon dating, but legends of native Cucupah people may indicate Holocene activity.

Obsidian from Obsidian Butte has been found as far as 500 kilometres (310 mi) away from Obsidian Butte. It started being used between 510 BC-640 AD, which led to the theory that the Obsidian Butte could only be used as a source of obsidian once it was no longer covered by Lake Cahuilla. Obsidian Butte was underwater during the highstands, but at lower water levels it would have formed an island in Lake Cahuilla. During the late historical period it was a source of obsidian for southernmost California.

Biology

Bivalves developed at the shores of Lake Cahuilla. These include Anodonta californiensis and possibly Pisidium casertanum. Anodonta is sometimes found associated with its own tunnels. It was probably used by inhabitants. Gastropods identified include Amnicola longinqua, Gyraulus parvus, Helisoma trivolvis, Physella ampullacea, Physella humerosa and Tryonia protea. These taxa were relatively abundant at the shores of the lake. Ostracods include Cypridopsis vidua, Cyprinotus torosa and Limnocythere ceriotuberosa. Sponges have been identified in fossil deposits as well. One mammal found in the lake was the muskrat, Ondatra zibethicus.

The shores of Lake Cahuilla developed arrowweed, tules and willowweed, with mesquite at distance to the shoreline. Land plants identified in Lake Cahuilla sediments include evening primroses, pine, Polypodiaceae, ragweed, saltbushes, Selaginella sinuites and sunflower. Many of these are represented by pollen. The Pleistocene lake and adjacent lagoons featured charophytes of the genus Chara.

The bird species that populated Lake Cahuilla resembled these around the present-day Salton Sea and may have contained species from the Gulf of California as well. They include Aechmophorus grebes, American coot, American white pelican, Anas and Aythya ducks, black-crowned night heron, eared grebes, pied-billed grebes and most likely shorebirds.

Fish species that have been identified to have existed in Lake Cahuilla include Cyprinodon macularius, Elops affinis, Gila elegans, Gila cypha, Gila robusta, Mugil cephalus, Ptychocheilus lucius, and Xyrauchen texanus. Lake Cahuilla featured similar fish species as the lower Colorado River.

Diatom species identified in sediments left by Lake Cahuilla include Cocconeis placentula, Epithermia argus, Epithermia turgida, Mastogloia elliptica, Navicula palpebralis, Pinnularia viridis, Rhopalodia gibba, Surirella striatula, Terpsinoei musica and Tetracyclus lacustris. Other species whose identification is less clear are Campylodiscus clypeus, Cyclotella kuetzingiana, Hantzschia taenia, Navicula clementis, Navicula ergadensis, Nitzschia etchegoinia, Nitzschia granulata and Synedra ulna.

During periods where the level in the lake rose, vegetation in the flooded areas drowned and organic material coming from it was washed ashore and later buried in coastal sediments. Five fish species and waterfowl populated the lake, and evidence exists of marshes on its shore. The flora and fauna along the seashores was probably robust enough to tolerate lake level drops for a while before increased salinity resulted in their disappearance.

Chronology

The history of Lake Cahuilla spans the late Pleistocene and the Holocene, with maximum lake extents commencing 40,000 years ago. Pleistocene shorelines are found mainly on the western side at altitudes of 31–52 metres (102–171 ft); an early 49–46 metres (161–151 ft) high shoreline was dated at 37,400 ± 2,000 years before present. At Travertine Point, evidence of a lake going back to 13,000 ± 200 years ago has been found. According to dates obtained from tufas, between 20,350 and 1,300 years before present water levels were always above −24 metres (−79 ft) above sea level. In the northeastern section of the lake, Pleistocene shorelines lie close to the path of the Coachella Canal. Pleistocene water levels are generally higher than Holocene ones which did not exceed 12 metres (39 ft) above sea level, probably due to erosion in the delta.

The latest highstand of Cahuilla was 400–550 years before present. Water levels of 12 metres (39 ft) above sea level occurred between 200 BC and 1580. The well preserved shorelines, lack of desert pavements and desert varnish on shore features, a relative lack of soil and archeological evidence suggest that Lake Cahuilla reached its maximum in the late Holocene.

While it was assumed at first that the lake existed in a single long interval between 1000-1500, later a succession of wet and dry phases was determined from radiocarbon dating. Each phase was stable for prolonged times, however. About three or four highstands were identified in the lake, one theory assumes four highstands between 695-1580. One chronology assumes these highstands occurred 100 BC - 600 AD, 900-1250 and 1300-1500. Six or five different cycles are documented at Coachella. At Superstition Mountain five lake cycles from 817-964, 1290-1330, 1440-1640, 1480-1660, 1638-1689 and 1675-1687 are documented; the 1440-1640 cycle may have consisted of four sub-cycles that occurred within short time distances from each other. An older highstand was observed at East Mesa and dated to 3,850 years before present. At least 12 different cycles of lake growth and lake shrinkage occurred over the last 2,000 - 3,000 years. Radiocarbon dates of the highstands range 300 ± 100 to 1,580 ± 200 before present. The basin probably was not entirely dry between the last three highstands. The Colorado River Delta shows evidence of reduced sedimentation during the times in which the river drained into Lake Cahuilla.

Some legends of the Kami and Cahuilla tribes probably refer to Lake Cahuilla, often stating that the lake bed tended to be dry but occasionally flooded; then the tribes would have to relocate to the mountains. Evidence for the lake's existence in the historical record, however, is unclear.

It is not clear whether the highstand of Lake Cahuilla occurred before or after 1540, year in which the Coronado expedition went through the area, although some transverses have been interpreted as to imply that it was not. It is possible that at that time, the Colorado River was draining into both the Gulf of California and Lake Cahuilla. Juan de Oñate in 1605 and Eusebio Kino in 1702 report that natives told them of the existence of a lake. Likewise a map by John Rocque c. 1762 shows a lake that the Colorado River drains into. Williams Blake in 1853 reported of a legend of the Cahuilla of a lake extending "from mountain to mountain" and evaporating "little by little", interrupted by a flood without warning. Based on observations made by Juan Bautista de Anza during his 1774 trip through the region, Lake Cahuilla did not exist anymore at that point. A short refilling between 1680-1825 is possible, however.

Some overly old radiocarbon dates of Lake Cahuilla deposits may be the consequence of the Colorado River transporting ancient carbonates into the lake, and discrepancies between shell and other organic material ages can reach 400–800 years owing to old carbon in Lake Cahuilla. In addition, shells can absorb carbon-14 from the air. Other research has documented no substantial old carbon effects.

It is likely that ephemeral lakes formed in the Lake Cahuilla basin during floods of the Colorado River, such as in 1828, 1840, 1849, 1852, 1862, 1867, and 1891. Joseph Widney in 1873 proposed to recreate the whole sea, in the hope of increasing precipitation over southern California and thus to enhance agricultural productivity; this was known as the "Widney Sea". Since 1905-1907, a new lake exists where Lake Cahuilla once stood, the Salton Sea. This lake formed when heavier than average spring melt runoff in the Colorado River breached an irrigation canal. The Salton Sea might have grown to the size of Lake Cahuilla if human efforts had not stopped the flood that gave rise to that lake.

Research history

In 1853, William Phipps Blake suggested that the Colorado River Delta cut off the basin from the sea and formed a playa; later two freshwater stages and one marine stage were identified in the basin. One year later he reported the existence of the 12 metres (39 ft) shoreline. Sykes in 1914 postulated that between 1706-1760 the Colorado River flooded the Lake Cahuilla basin, but there is no historical evidence for this. E.E.Free in 1914 on the basis of a wavecut terrace estimated the existence of only one lake cycle. Hubbs and Miller (1948) assumed two freshwater stages.

Originally it was believed that Lake Cahuilla formed around 900 AD and existed until 1500 but with fluctuations as the Colorado River changed its course. In 1978, Philip J. Wilke proposed that two highstands occurred, one between 900 and 1250 and another between 1300-1500. Another proposal by Waters in 1983 suggested highstands 700-900, 940-1210 and after 1250, the latter with some brief recessions to lower lake levels. Both proposals were criticized on the grounds that they came to definite conclusions with insufficient information.

Malcolm J. Rogers suggested that early highstands of Lake Cahuilla had strong effects on the spread of ceramics in the region of California and Baja California, although this is considered untenable today.

Products and significance

The Algodones Dunes, which border old Cahuilla shorelines, were formed by sand blown from Lake Cahuilla. This theory was first formulated in 1923. The process occurred either immediately after the lake reached modern highstands, or during earlier higher stands. Most likely, sand was transported to the dune field during times where the lake receded and its bed was exposed to wind. Various stages of Lake Cahuilla may correspond to waves of migrating dunes.

At first the Whitewater River and local washes were considered the primary source of these sands, which would have been transported to the Algodones area by longshore drift. This would imply a minimum age of 160,000 years ago. Later the Colorado River was later identified to be the main source of these sediments, potentially with some contribution from local drainages. At prevailing winds, most of the sediments from the Colorado would have been transported to the Cerro Prieto area and possibly carried by wind to the Gran Desierto de Altar.

Clay and fine silt, dominated by lutite, were deposited in the lake. Closer to the shore, sand was also emplaced. Deltaic deposits have been found as well. Minerals identified include biotite, chlorite, illite, kaolinite, montmorillonite and muscovite, with colours varying depending on the region of origin of the sediments. The material deposited by Lake Cahuilla is also known as the Cahuilla formation. The Borrego and Pleistocene Brawley formations may also be linked to Lake Cahuilla. These lacustrine materials bury the northern part of the Colorado River Delta, and they give the ground a grayish colour. The clays left by the lake were used for the production of ceramic by the inhabitants of the region; likewise Lake Cahuilla is responsible for the fertile soils of the Coachella Valley and Imperial Valley, an important agricultural province of the United States. Halite deposits left by the lake were mined in the 19-20th century.

The weight of the water in Lake Cahuilla caused the surface beneath the lake to sink by about 0.4 metres (1 ft 4 in). Such ground depression has been observed at the ancient lakes Lake Bonneville, Lake Lahontan, Lake Minchin and the modern reservoirs of Lake Mead, Three Gorges Reservoir in China and La Grande in Quebec.

The genus Cahuillus of helminthoglyptid land snails is named after the lake. It contains the species Cahuillus indioensis with two subspecies indioensis and cathedralis, Cahuillus greggi and Cahuillus mexicanus.

Archeology

Numerous archeological sites made by the Cahuilla have been found on the shores of the lake, including a number of campsites. On the northwest shore of Lake Cahuilla, remains of fish, shell middens and fishing weirs have been identified, indicating that early inhabitants of the region had relationships with Lake Cahuilla. Likewise, its recession probably influenced the local inhabitants. Patayan pottery and stone artifacts are among the archeological finds made at the Lake Cahuilla highstand shoreline, along with petroglyphs in the travertine. Four onshore campsites have been found at Bat Caves Butte, Myoma Dunes, Travertine Rock and Wadi Beadmaker. The nature of the so-called "Ancient Fish Traps"at the foot of the Santa Rosa Mountains is questionable as it appears to postdate the lake periods.

Based on research on findings made there, the lake did support a substantial population that relied mostly on resources from the lake, including aquaculture and fishing. Estimated populations range from 20,000 to 100,000 people. When the lake dried up, the inhabitants switched to other economic activities. Agriculture did not play a major role in food supply.

About 650 fish weirs were found at the lake shores. They were probably built on an annual basis. This "industry" declined as waters receded, probably because of declining numbers of fish in the shrinking lake.

The Elmore Site, discovered in 1990 during the course of an archeological survey that accompanied work to improve State Route 86, lies close to the southwestern coast of Lake Cahuilla, about 67 metres (220 ft) beneath the highstand level. Archeological features found there include bones mostly of birds, ceramics, charcoal from fires, pits from wood posts or storage pits, sandstone slabs, and shells of mostly marine origin. This archeological site was active after the waters of Lake Cahuilla had receded from the site, probably for a short time 1660-1680 AD.

It is likely that the repeated fillings and dryings of the lake did have substantial effects on the communities around the lake. Further, the relatively large size of Lake Cahuilla meant that large "international" communities were affected by the lake. Indeed, evidence indicates that at least three different ethnic groups - Cahuilla, Kumeyaay and Cucapa - existed around the later history of the lake in its area. The effects of the lake's expansion most likely were dominantly positive on the communities concerned, unlike in the Colorado River Delta which lost part of its water supply. The distribution of the languages in the region may reflect the effects of fluctuations of Lake Cahuilla; population shifts caused by the drying and flooding of Lake Cahuilla may have favoured exchanges between the Tepiman and River Yuman languages and the propagation of B2a mitochondrial haplogroups in the native people.

When Lake Cahuilla filled, it may have induced Quechan people to migrate into the area. This migration is considered to be a possible source for the spread of agriculture to the Peninsular Ranges. Legends have it that lost ships, described for example as pirate ships or galleons, sailed Lake Cahuilla and are now buried somewhere in the Colorado Desert.