Playas are shallow, disc-shaped basins etched in the southern Great Plains. Lakes are formed when surface runoff fills the basins. In Spanish, playa means "beach," and the word was perhaps chosen because the lakes represent essentially all of the naturally occurring surface water in a vast semiarid region-the "only beach in town." About 19,000 playas dot the prairie landscape on the southern High Plains of Texas; another 6,000 basins are located in adjacent states. Some Texas counties, such as Floyd, contain more than 1,700 playas, but the average is nearer 600 basins per county. Playas are ephemeral lakes that can reach a size of more than 200 acres, although the average is about seventeen acres. Most of them are dry for much of the year and fill naturally only from the runoff of rainstorms. Playas are unlike most lakes in that water is not carried to or from them by streams or rivers. Instead, each drains a relatively small, closed watershed surrounding its basin. Nonetheless, because they are so numerous, playas collect between two million and four million acre-feet of water each year, on an aggregate of about 250,000 surface acres. Playa basins are lined with lenses of nearly impermeable clay, often the Randall type that prevents the surface water from percolating downward into the underlying Ogallala Aquifer. Thus, without large watersheds, stream flow, or frequent precipitation, the shallow waters soon evaporate, leaving the basins dry until the next rainstorm renews the cycle. Most playa basins originated from the deflating action of strong prairie winds sweeping across the landscape. Older notions attribute formation of the basins to the wallowing of bison (see BUFFALO) that once roamed the plains, but few if any playas were actually so formed.
Some of the earliest inhabitants of North America and later the Plains Indians may have camped near playas, but these associations are largely lost to history. Water-filled playas attracted wildlife and hunters. Spearheads attributed to the Folsom (or a Folsom-like) culture have been recovered from a bed of elephant bones discovered in an ancient playa in Roberts County. Some of the larger, more permanent playas presumably served as campsites for nomadic Indians. The first mention of the playas appears in the records of the sixteenth-century trek of Francisco Vásquez de Coronado and his conquistadors as they searched fruitlessly for the legendary Seven Cities of Gold; the men were often forced to drink from what they called buffalo wallows. In 1844 an expedition of the Republic of Texas subsisted on ducks, cranes, and antelope shot near a playa as the group advanced toward Santa Fe. Although the irregularity with which water accumulates in playas made them unreliable sources for explorers and settlers, during wet years the chronicles of some expeditions cursed the playas as impediments that required frequent detours from the line of travel.
Because the heavy clay soils hold some moisture even in dry years, playa basins often were important sources of hay at the turn of the century; tons of wild hay were sold to cattlemen, who generally considered playa-grown hay superior. Playas still contribute to agriculture on the semiarid southern High Plains of Texas, where irrigation is expensive and the Ogallala Aquifer is being depleted. About 70 percent of the playas larger than ten acres have been excavated into deep pits for storing irrigation water and natural runoff; once in the pits, the water remains available for a second irrigation cycle. The pits are excavated with steep sides that create a favorable ratio of surface area to volume and minimize evaporation. Though this practice vastly reduces the natural littoral zones-those shallow areas of high biological productivity that represent the foundation for aquatic food webs-playas that might usually be dry collect irrigation runoff that supports a rich community of plant life and provides surface water for wildlife. In years of normal rainfall the playas are second in importance only to the Gulf Coast of Texas as wintering grounds for waterfowl in the Central Flyway; more than a million mallards, wigeons, pintails, and green-winged teals winter on the playas. Vegetation growing in playa basins also provides cover for large numbers of ring-necked pheasants. Playas supply unique wildlife habitats in an intensively cultivated region.