Salton Sea, California

While it varies in dimensions and area with fluctuations in agricultural runoff and rainfall, the Salton Sea is about 24 wide by 56 km) long. It has an estimated surface area of 890 km2 making it is the largest perennial lake in California. Average annual inflow is less than 1.5 km3, which maintains a maximum depth of 13 m and a total volume of about 7.4 km3.

However, due to changes in water apportionments agreed upon for the Colorado River under the Quantification Settlement Agreement of 2003, the surface area of the sea is expected to decrease by 60% between 2013 and 2021. Current lake's salinity is ≈ 56 grams per litre (seawater is ≈ 35 g/l). Recently, the concentration of the main water mass has been increasing at a rate of about 3% per year. Some 3.6 million tonnes of salt is deposited in the trough each year. Until the advent of the modern sea in the early 1900s, the Salton Sink was the site of a significant salt-mining operation.

The Salton Trough is a continental rift zone at the head of the Gulf of California and adjacent to the San Andres Fault. Its position defines the transition from the offshore divergent tectonics of the East Pacific Rise to the onshore strike-slip dextral tectonics of the San Andreas Fault System. Infilling of the rift by the bi-polar Colorado River Delta for the past 4 Ma has isolated the northern part of the trough, forming the closed Salton Basin in an orographic desert.

The current Salton Sea was created anthropogenically in 1905 when a canal, designed to bring irrigation water from the Colorado River into the Imperial Valley, was breached during spring floods. The entire volume of this large river was then channelled into the Salton Sink, which at the time was an evaporitic mudflat/pan with a surface more than 80 m below sea level. It was two years before the breach was plugged and the river diverted back to its natural course, but, by that time the Salton Sink had become the Salton Sea, a vast saline inland lake extending over several hundred square kilometres. Much of the salinity was the result of the dissolution of the salt crust that previously occupied the lower portions of the sink.

Although this transformation was anthropogenic, the underlying Quaternary fill preserves a similar set of pan to saline lake alternations. High evaporation rates combined with an extremely variable hydrologic budget resulted in episodic infill by saline lake and lacustrine evaporite sediments. Evidence that the basin was occupied periodically by multiple lakes includes wave-cut shorelines at various elevations preserved on the hillsides of the east and west margins of the present-day Salton Sea. These indicate that the basin was occupied intermittently as recently as a few hundred years ago.

The last of the Pleistocene mega-lakes to occupy the basin was Lake Cahuilla. The history of Lake Cahuilla spans the late Pleistocene and the Holocene, with maximum lake extents occurring starting from 40,000 years ago. Pleistocene shorelines are found mainly on the western side at altitudes of 31–52 metres; an early 49–46-metre 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 more than 24 metres above sea level. Pleistocene lake levels are generally higher than the Holocene ones, which did not exceed 12 metres above sea level, probably due to natural erosion in the Colorado River delta.

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

Salton Sea

Salton Sea, California, with irrigated areas in the northwest and southeast.

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Fish kill washed into the strandzone of the Salton Sea

In the modern Salton Sea, there is a strong correlation between winds that overturn the stratified lake waters, and significant fish and barnacle kill due to hostile waters (low oxygen, high H2S and ammonia) rising into the upper water column. Following death, fish carcasses float to the lake surface and deposit and accumulate along the Salton shore, which retains wave-washed sediments that are rich in disarticulated fish and single bone and scale elements, along with a significant component of barnacle shells.

Hydrothermal hydrocarbon seeps and gryphons (mud volcanoes) in the Salton Sea sump

Within the Salton Sea geothermal system, rapid subsidence and rift-related magmatic intrusions at relatively shallow depth make up an arc of five Quaternary rhyolite domes known as the Salton Buttes. Related convective heating has flushed Plio-Pleistocene and younger sediments (fluvial and lacustrine deltaic matrices in a rift fill up to 1–4.5 km thick) with hypersaline chloride- and metal-rich brines, containing up to 26 wt% of dissolved solids. Seeps of mud, gas (CO2) and dominantly polyaromatic hydrocarbons are common in this geothermal field, where wells drilled for geothermal power record temperatures of up to 350°C at depths ranging from 1,500 to 2,500 m. Hydrocarbons appear to originate from hydrothermal heating rather than thermal maturation of organic compounds via burial (See Warren 2016, Chapter 14 for further discussion of the Salton Sea geothermal system and its importance in the improved understanding of base metal fluids).

Davis-Schrimpf seep field in the onshore Salton Sea, Southern California is an area of hydrothermal hydrocarbon seeps associated with the volcanic Salton Buttes (Svensen et al., 2007; Mazzini et al., 2011). There the gryphons and mud pots exhale carbon dioxide and steam, as well as ammonia and hydrogen sulfide (A). Some of these fumarolic vents reach heights of 2 metres (6 ft 7 in). Parts of the field have recently emerged from the Salton Sea, due to dropping water levels, which also caused the fumarolic vents to develop and grow noticeably, including an increased number of gryphons.

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Brines and hydrothermal oil in the Salton Sea, Southern California (after Svensen et al., 2007 and references therein). A) Davis-Schrimpf seep field is characterized by clusters of gryphons, local subsidence structures, and calderas. Generally, gryphon mud towers are as high as 3 m with a central conduit, where mud and water chambers are located below ground level. Mud pots and springs are open circular depressions as much as 30–50 cm below ground level, but may occasionally form positive structures to a few tens of centimetres high. B) Analyses of gryphon and spring water collected from Davis-Schrimpf field show a wide range in Cl and Br contents as do other seep and subsurface waters collected in the region. Compositions of waters define two trends in Cl−Br space related to salinity and origin of the waters. C) Organic geochemistry of CA-19 seep fluid. Gas chromatography shows the occurrence of homologous series of normal alkanes (n-Cxx) and isoalkanes (i-Cxx). Presence of unresolved complex mixture (UCM) and high concentrations of Pr and Ph suggests the influence of biodegradation. Biomarker hump (BMH) is typical for source rocks with low to moderate maturity.

This region is the onshore extension of the same ridge system that defines the Guaymas Basin more than 500 km to the south. As in the Guaymas Basin, saline water-, mud-, gas-, and petroleum-bearing seeps are part of the geothermal system in the Salton Sea. Seeps in the Davis-Schrimpf seep field (A; ≈14,000 m2) show considerable variations in water temperature, pH, density, and solute content. Water-rich springs tend to be hypersaline, have lower densities compared to gryphon outflows (<1.4 g/cm3), while Cl contents are as high as 45,000 ppm, and temperatures range between 15 and 34 °C (B). In contrast, nearby gryphons expel denser water-mud mixtures (to 1.7 g/cm3), have lower salinities (3600-5200 ppm Cl), and temperatures range between 23 and 63 °C. The main driver for this seep system is CO2 (>98 vol% of expelled gas). Halogen geochemistry of the waters shows two trends indicating that mixing of slightly deeper and shallow pore waters occurs and it seems that near-surface dissolution of halite may overprint the original fluid compositions, so creating the dichotomy in the isotope trends (B).

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δD and δ18O increase proportionally with salinity, and isotopic composition of the waters indicate a dominantly meteoric origin, modified by surface evaporation with little or no evidence of deep fossil or magmatic components (Mazzini et al., 2011). Very high Cl/Br (> 3,000) measured at many seep water pools suggests that increased salinities result from dissolution/recycling of halite crusts near the seep sites. Carbon isotopic analyses conducted by Svensen et al. (2007) suggest that hydrocarbon seep gases have a thermogenic origin. Their hypothesis of hydrocarbon generation via hydrothermal cracking is supported by the presence of petroleum in a water-dominated spring, composed of 53% saturated compounds, 35% aromatics, and 12% polar compounds (C). See Warren 2016 Chapter 9 for more details

The future of the Salton Sea and other continental saline sump lakes

For two years after the Salton Sea was created in 1905 by a breach in the levees of the Colorado River, river waters flowed in the Salton Sink desert Valley constructing a 15-mile by 35-mile freshwater lake some 45 miles outside of Palm Springs. In the 1950s and 60s, with its sandy beaches and year-round warm water, the Salton Sea was the perfect family vacation getaway. It attracted Hollywood's elite - Rock Hudson water-skied there, Frank Sinatra and Jerry Lewis visited their friend Guy Lombardo's yacht, which was moored there. The Beach Boys were members of the North Shore yacht club, Sonny Bono was a visitor, and President Dwight Eisenhower golfed there. By the 1970s, the lake area was in decline, and the salinity of the remaining lake waters was increasing.

There is minimal rainfall across its natural catchment (it does lie in a desert), and the original freshening of the saline valley sump was the product of an accidental river bank breach early last century. As the lake lies in a natural drainage sump (endorheic lake) there is no way for the waters to drain, so runoff waters contaminated with pesticides from nearby farms flows into the sump where toxic residues evaporatively concentrate in the sump sediments. As the lake shrinks, more and more of the lake floor sediment are exposed to wind action, salt levels in its waters continue to increase. Today its stratified waters are saltier than the Pacific Ocean, with turnover depleting oxygen levels in the upper water mass water. Thousands of fish die annually and are washed onto the shore where they wither and decay beneath the Valley's extreme heat.

Today, the Salton Sea is described in the popular press as an 'ecological nightmare and disaster' - a ghost town in the making. The once-bustling hotels are derelict, broken wooden frames of buildings stand in some spots as other structures are badly decaying with graffiti spray-painted over the boarded-up windows and doors. Where holidaymakers once sunbathed, the sand is littered with sun-bleached fish carcasses. On hot summer days when the temperature can reach up to 48°C), a pungent sulphurous pall hangs in the air that can be smelt 150 miles away in Los Angeles. In short, the Salton Sea today smells almost continuously of rotten eggs, not tourist dollars.

But it isn't just the smell and the ecological impact on the fish population that people are concerned about. The most pressing issue for residents near the Salton Sea is the continued threat of a toxic dust haze containing harmful pesticides, heavy metals and powdery-fine particles that desert winds sweep up from the drying lakebed. This dust is linked to asthma, respiratory diseases and perhaps an increased risk of cancer. One in five children in Imperial County have asthma and or other lung-related illnesses, compared with a national average of 1 in 12, according to the latest government data.

In 2018, Dr Tim Krantz, the former Salton Sea Database Program Director, said of the current conditions it is a 'public health disaster' in the making. He went on to say 'Our real concern changed from the ecology and the loss of that which is important, but our real focus became what's going to happen to these toxic dry lakebed sediments once they are exposed to the desert winds?'
'It will be an air quality disaster unparalleled in the world. Millions of people as far south as Mexi-Cali will be affected by this.'

In response to the developing dust problem, in mid-2018 the voters of California approved a bond measure that provides $200 million in funding to help control the alkaline toxic dust by building wetlands on the dry lakebed. But some residents don't think that's enough, as the wetlands cannot cover the whole of the exposed and enlarging mudflat area. 

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Salton Sea

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