Van Lake basin

Lake Van ( tr, Van Gölü; hy, Վանա լիճ, translit=Vana lič̣; ku, Gola Wanê) is the largest lake in Turkey. It lies in the far east of Turkey, in the provinces of Van and Bitlis in the Armenian highlands. It is a saline soda lake, receiving water from many small streams that descend from the surrounding mountains. It is one of the world's few endorheic lakes (a lake having no outlet) of size greater than and has 38% of the country's surface water (including rivers). A volcanic eruption blocked its original outlet in prehistoric times. It is situated at above sea level. Despite the high altitude and winter highs below , high salinity usually prevents it from freezing; the shallow northern section can freeze, but rarely.

Hydrology and chemistry

Lake Van is across at its widest point. It averages deep. Its greatest known depth is . The surface lies above sea level and the shore length is . It covers and contains (has volume of) .

The western portion of the lake is deepest, with a large basin deeper than lying northeast of Tatvan and south of Ahlat. The eastern arms of the lake are shallower. The Van-Ahtamar portion shelves gradually, with a maximum depth of about on its northwest side where it joins the rest of the lake. The Erciş arm is much shallower, mostly less than , with a maximum depth of about .

The lake water is strongly alkaline ( pH 9.7–9.8) and rich in sodium carbonate and other salts. Some is extracted in salt evaporation ponds alongside, used in or as detergents.

Geology

Lake Van is primarily a tectonic lake, formed more than 600,000 years ago by the gradual subsidence of a large block of the earth's crust due to movement on several major faults that run through this portion of Eastern Anatolia. The lake's southern margin demarcates: a metamorphic rock zone of the Bitlis Massif and volcanic strata of the Neogene and Quaternary periods. The deep, western portion of the lake is an antidome basin in a tectonic depression. This was formed by normal and strike-slip faulting and thrusting.

The lake's proximity to the Karlıova Triple Junction has led to molten fluids of the Earth's mantle accumulating in the strata beneath, still driving gradual change. Dominating the lake's northern shore is the stratovolcano Mount Süphan. The broad crater of a second, dormant volcano, Mount Nemrut, is close to the western tip of the lake. There is hydrothermal activity throughout the region.

For much of its history, until the Pleistocene, Lake Van has had an outlet towards the southwest. However, the level of this threshold has varied over time, as the lake has been blocked by successive lava flows from Nemrut volcano westward towards the Muş Plain. This threshold has then been lowered at times by erosion.

Bathymetry

The first acoustic survey of Lake Van was performed in 1974.

Kempe and Degens later identified three physiographic provinces comprising the lake:
*a lacustrine shelf (27% of the lake) from the shore to a clear gradient change
*a steeper lacustrine slope (63%)
*a deep, relatively flat basin province (10%) in the western center of the lake.

The deepest part of the lake is the Tatvan basin, which is almost completely bounded by faults.

Prehistoric lake levels

Land terraces (remnant dry, upper banks from previous shorelines) above the present shore have long been recognized. On a visit in 1898, geologist Felix Oswald noted three raised beaches at 15, 50 and 100 feet (5, 15 and 30 meters) above the lake then, as well as recently drowned trees. Research in the past century has identified many similar terraces, and the lake's level has fluctuated significantly during that time.

As the lake has no outlet, the level over recent millennia rests on inflow and evaporation.

The water level has vacillated greatly. Investigation by a team including Degens in the early 1980s determined that the highest lake levels ( above the current height) had been during the last ice age, about 18,000 years ago. Approximately 9,500 years ago there was a dramatic drop to more than below the present level. This was followed by an equally-dramatic rise around 6,500 years ago.

As a deep lake with no outlet, Lake Van has accumulated great amounts of sediment washed in from surrounding plains and valleys, and occasionally deposited as ash from eruptions of nearby volcanoes.
This layer of sediment is estimated to be up to thick in places, and has attracted climatologists and vulcanologists interested in drilling cores to examine the layered sediments.

In 1989 and 1990, an international team of geologists led by Stephan Kempe from the University of Hamburg retrieved ten sediment cores from depths up to . Although these cores only penetrated the first few meters of sediment, they provided sufficient varves to give proxy climate data for up to 14,570 years BP.

A team of scientists headed by palaeontologist Professor Thomas Litt at the University of Bonn has applied for funding from the International Continental Scientific Drilling Program (ICDP) for an akin deeper-drilling project. This expects to find it "stores the climate history of the last 800,000 years—an incomparable treasure house of data which we want to tap for at least the last 500,000 years." A test drilling in 2004 detected evidence of 15 volcanic eruptions in the past 20,000 years.

Recent lake level change

Similar but smaller fluctuations have been seen recently. The level of the lake rose by at least during the 1990s, drowning much agricultural land, and (after a brief period of stability and then retreat) seems to be rising again. The level rose approximately in the 10 years immediately prior to 2004.

Climate

It is in the highest and largest region of Turkey, which has a Mediterranean-influenced humid continental climate. Average temperatures in July are between 22 and 25 °C, and in January between −3 °C to −12 °C. On some cold winter nights the temperature has reached −30 °C.

The lake, particularly on urban townscape shore tempers the climate, in the city of Van, where the average temperature in July is 22.5 °C, and in January −3.5 °C. The average annual rainfall in the basin, ranges from 400 to 700 mm.Матвеев: Турция �то значительно ниже установленной позже корректной цифры в 161,2 метра

Ecology

Prior to 2018, the only fish known to live in the brackish water of Lake Van was ''Chalcalburnus tarichi'' or Pearl Mullet ( tr, inci kefali), a Cyprinid fish related to chub and dace, which is caught during the spring floods. In May and June, these fish migrate from the lake to less alkaline water, spawning either near the mouths of the rivers feeding the lake or in the rivers themselves. After spawning season it returns to the lake. In 2018, a new species of fish, which is deemed as '' Oxynoemacheilus ercisianus'', has been discovered inside a microbialite.

103 species of phytoplankton have been recorded in the lake including cyanobacteria, flagellates, diatoms, green algae, and brown algae. 36 species of zooplankton have also been recorded including Rotatoria, Cladocera, and Copepoda in the lake.

In 1991, researchers reported the discovery of tall microbialites

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