alkali flats

A dry lake bed, also known as a playa, is a basin or depression that formerly contained a standing surface water body, which disappears when evaporation processes exceeds recharge. If the floor of a dry lake is covered by deposits of alkaline compounds, it is known as an alkali flat. If covered with salt, it is known as a '' salt flat.''

Terminology

If its basin is primarily salt, then a dry lake bed is called a '' salt pan'', ''pan'', or ''salt flat'' (the latter being a remnant of a salt lake). ''Hardpan'' is the dry terminus of an internally drained basin in a dry climate, a designation typically used in the Great Basin of the western United States.

Another term for dry lake bed is ''playa''. The Spanish word ''playa'' () literally means "beach". Dry lakes are known by this name in some parts of Mexico and the western United States. This term is used e.g. on the Llano Estacado and other parts of the Southern High Plains and is commonly used to address paleolake sediments in the Sahara like Lake Ptolemy.

In South America, the usual term for a dry lake bed is ''salar'' or ''salina'', Spanish for ''salt pan''.

''Pan'' is the term used in most of South Africa. These may include the small round highveld pans, typical of the Chrissiesmeer area, to the extensive pans of the Northern Cape province.

Terms used in Australia include ''salt pans'' (where evaporite minerals are present) and ''clay pans''.

In Arabic, a salt flat is called a ''sabkha'' (also spelled ''sabkhah'', ''subkha'' or ''sebkha'') or ''shott'' ('' chott''). In Central Asia, a similar "cracked mud" salt flat is known as a takyr. In Iran salt flats are called ''kavir''.

Formation

A dry lake is formed when water from rain or other sources, like intersection with a water table, flows into a dry depression in the landscape, creating a pond or lake. If the total annual evaporation rate exceeds the total annual inflow, the depression will eventually become dry again, forming a dry lake. Salts originally dissolved in the water precipitate out and are left behind, gradually building up over time. A dry lake appears as a flat bed of clay, generally encrusted with precipitated salts. These evaporite minerals are a concentration of weathering products such as sodium carbonate, borax, and other salts. In deserts, a dry lake may be found in an area ringed by bajadas.

Dry lakes are typically formed in semi-arid to arid regions of the world. The largest concentration of dry lakes (nearly 22,000) is in the southern High Plains of Texas and eastern New Mexico. Most dry lakes are small. However, Salar de Uyuni in Bolivia, near Potosí, the largest salt flat in the world, comprises 4,085 square miles (10,582 square km).

Many dry lakes contain shallow water during the rainy season, especially during wet years. If the layer of water is thin and is moved around the dry lake bed by wind, an exceedingly hard and smooth surface may develop. Thicker layers of water may result in a "cracked-mud" surface and teepee structure desiccation features. If there is very little water, dunes can form.

The Racetrack Playa, located in Death Valley, California, features a geological phenomenon known as " sailing stones" that leave linear "racetrack" imprints as they slowly move across the surface without human or animal intervention. These rocks have been recently filmed in motion by the Scripps Institution of Oceanography at UC San Diego and are due to a perfect coincidence of events. First, the playa has to fill with water, which must be deep enough to form floating ice during winter, but still shallow enough that the rocks are exposed. When the temperature drops at night, this pond freezes into thin sheets of "windowpane" ice, which then must be thick enough to maintain strength, but thin enough to move freely. Finally, when the sun comes out, the ice melts and cracks into floating panels; these are blown across the playa by light winds, propelling the rocks in front of them. The stones only move once every two or three years and most tracks last for three or four years.

Ecology

While a dry lake bed is itself typically devoid of vegetation, they are commonly ringed by shadscale, saltbrush and other salt-tolerant plants that provide critical winter fodder for livestock and other herbivores.

In southwest Idaho and parts of Nevada and Utah there are a number of rare species that occur nowhere else but in the inhospitable environment of seasonally flooded playas. A new species of giant fairy shrimp was found in 2006. Although a large predatory species, it evaded detection because of the murkiness of the playa's water caused by winds and a fine clay load. This shrimp species is able to regenerate using tiny undetectable cysts that can remain in a dry lake bed for years until conditions are optimum for hatching.

''Lepidium davisii

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