Playas are enclosed shallow depressions in desert basins, tectonic lows, interdune flats, wadis, and abandoned channels, which contain deposits and evaporites from the impoundment of episodic stream flow or near-surface ground water. Playas can be entirely dry or seasonally filled with water. The sediment load carried into a playa from streams or blown in by wind characteristically includes clay, silt, and fine-grained sand. The evaporites, which come from runoff as well as from ground water, commonly contain chlorides, sulfates, nitrates, carbonates, borates, or other salts, including toxic ones such as cyanates or arsenates. Evaporation of water from playas leaves exposed surfaces of the resident material, which can be any mix of clay, silt, fine sand, and salt.
Playa surfaces change seasonally with addition or loss of water, and with wind activity. They can be smooth to rough, wet to dry, and hard to soft, puffy, flaky, cracked, ridged, and friable, and they can have hummocky relief of 1 to 2 m. Although dust generation by wind erosion of fine particles is common (see Summary for Depressions - Deflation Hollows/Basins/Blowouts), not all playas are equally susceptible to wind erosion. Hard, smooth, and dry playas with a high clay/low salt content seem to be more frequent along ephemeral, intermittent, and dry desert water courses (wadis). Soft, rough, wet playas with high salt/low clay content tend to occur in depressions whose floors intersect the water table (see Summarys for Depressions and for Sabkhahs). Desiccation cracks, which are indicative of a high clay content, are common on playa surfaces. Such cracks are repeated in networks that break the surface into rectangular or polygonal-like blocks that range from millimeters to tens of meters across. Larger cracks can be 2 to 3 m deep and more than a meter wide at the surface. Large, long, relatively straight cracks also occur, presumably the result of subsurface sapping and subsequent collapse. Under other conditions of moisture and composition, the surface can be broken into relatively smooth polygonal pans, one to several meters wide, separated by sharp, hard ridges 10 to 40 cm high. The causes of this pattern are not known. Although vegetation is absent from the playa surface, it sometimes rings the playa in the form of shrubs, grasses, and reeds.
The downwind margins of many playas are bounded by low mounds of windblown sand, silt, and clay. These mounds are called "lunette dunes" and are built of fine particles and clay aggregates deflated from the playa sediments by wind. They have low relief and no slip faces. Wind erosion of the semiconsolidated playa sediments, as well as of the lunette dunes, can produce deflation hollows, and wind-aligned hillocks known as yardangs, which can be a few meters high. These are common erosional remnants on playa surfaces in North Africa, Iran, and China, and at isolated localities on lunettes in the U.S., such as Rogers Lake, California.
Episodic streams may enter the playa from runoff during or shortly after rainfall in distant watersheds. Water may also appear due to rising ground water in localities where the basin floor or a channel intercepts the water table.
Water in evaporitic playas can be highly mineralized, and generally needs treatment to be potable. In Saudi Arabia, silt and clay playas can often hold rain water as pools floating on the brine below. This water remains very sweet for weeks as the pool evaporates--in fact until the last liquid is gone--and such sites are favorite places for Bedouins to collect drinking water and water live stock.
Because of various surface conditions on playas (wet, dry, slippery, soft, sticky, hard, puffy, friable, ridged, cracked, etc.), their use for foot and vehicular traffic and aircraft operation ranges from easy to impossible, and must be evaluated according to both composition and moisture. Dry playas produce extreme glare and are typically hot in the summer season.
Playas that are dominantly clay rich and dry will support foot and vehicular traffic as well as aircraft operations, and can generate dust. In photos, most of these playas usually have very light and relatively uniform tones. If wet, the same playas will not support traffic--foot, vehicular, or aircraft and will also show darker tones in an image. If but slightly wet, the surface can become slippery or sticky, and can seriously diminish the take-off capability of fixed-wing aircraft. Under some conditions of composition and moisture content not yet known, a playa crust can have a plasticity that affects fixed-wing aircraft operations, and can make it impossible for heavy aircraft to take off. At a speed well below take-off speed, the plastic deformation rate of the playa crust can be exceeded and, consequently, the crust continually fractures, preventing the airplane from accelerating to its take-off speed (Holt[1]). Both the desiccation cracks and the pan and ridge topography typical of some playas can impede vehicular traffic or prevent fixed-wing aircraft operations. The conditions responsible for these various characteristics are not yet known.
In general, image tones suggest the condition of a playa: bright-toned areas will probably be dry and the darker-toned areas will probably be wet. A very careful check should be made, however, because although the surface might indeed be dry, the material beneath the crust might be a wet, gumbo-like mass, or quicksand. Vehicles breaking through such a crust can become immobile. Playas that contain a mixture of mostly silt and salts present an irregular puffy surface, as do crust surfaced flats that intersect the water table (see Summary for Sabkhahs). If dry, the surface can be crossed with wide-tired vehicles, although they will sink several centimeters into this layer. Any traffic, land or air, on these playas will generate a large amount of airborne dust. Aircraft operations should be avoided. If wet, these playas cannot be traversed. The factors that encourage or suppress dust generation and the relations between these factors and image pattern elements are not yet well established, nor are spectral reflectance characteristics that might serve as indicators of these conditions.
(common names are in bold) Dry lakes, pans, salt lakes, vlei (South Africa), faydah (Saudi Arabia), sabkhahs or chotts (North Africa), nor, nur, or bahir (China), takyrs (Soviet Central Asia), billabongs (Australia), kavirs (Iran), and salinas or salars (Central and South America).
1. Holt, H., U.S. Geological Survey. Personal communication, August 1986.
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