The Trans-Pecos is the region west of the Pecos River, bounded by the Rio Grande on the south and west, and on the north by the thirty-second parallel, which forms the boundary with the state of New Mexico. Most of the region's physical and cultural landscape has little in common with the rest of the state. Although it constitutes about 11 percent of the area of Texas, the Trans-Pecos has received less attention than the more populous east. It is about the same size as South Carolina. It also possesses the most varied, distinctive, and spectacular scenery in Texas. Most of the Trans-Pecos is typified by the Texas portion of the Basin and Range physiographic province. Toward the northeast, along the western margins of the Pecos River valley, the region grades into the Great Plains. Farther south the Edwards Plateau forms the eastern boundary. A western extension of the plateau, known as the Stockton Plateau, ends at the eastern boundary of the Basin and Range. The Edwards Plateau is sharply defined farther east by the Balcones Escarpment, but northward this plateau merges almost imperceptibly into the High Plains region known as the Llano Estacado. More than half of the entire United States-Mexico border is in the Basin and Range physiographic province, which reaches its eastern terminus in the Big Bend. Most of the area was once covered by vast seas that laid down great thicknesses of sedimentary rock. During a period of mountain building known as the Laramide Orogeny, the original horizontal structure of the sedimentary rocks was deformed into a series of folds. More recently, primarily during the Tertiary, volcanism and faulting resulted in upthrust ranges and downdropped basins. In the zone known as the Rio Grande Rift, which has been especially active during the last twenty-five million years, the boundary faults resulted in great vertical displacement. The Franklin Mountains of El Paso, for example, originally rose more than 25,000 feet from the floor of the original basin. In this area most of the uplifted ranges were formed along parallel faults and tilted to form what is known as rotated horst mountains. Most of the mountain ranges have a north-northwest trend. Today the basin fill is 2,000 to 8,000 feet thick. With the exception of the Rio Grande and two of its major tributaries, the Río Conchos (in Mexico) and the Pecos, the area is typified by internal drainage. Most water evaporates, and what is left seeps into the basin fill. These internal-draining basins provide significant recharge to aquifers. After the great ice age, the glacial melt filled freshwater lakes that formed remnant shorelines, terraces, large arroyos, and other features. Today the evaporation pans are associated with desert flats, playas alkaline deposits, and sandy surfaces. The Salt Basin, just west of the Guadalupe Mountains in the Dell City area, provides numerous and obvious examples of these landforms.
Basin and Range topography offers higher elevations and greater local relief than can be found anywhere else in Texas. Although most of the state is relatively flat and less than 2,500 feet above sea level, the Trans-Pecos basins are elevated about 4,000 feet and crossed by numerous widely spaced mountain ranges that rise an additional 2,000 to 3,000 feet. Some of the ranges, such as the Sierra Diablo and Delaware Mountains, have gentle slopes even at high elevations. Others, such as the Davis Mountains, are marked by steep slopes. The highest peak in Texas, Guadalupe Peak (elevation 8,749 feet), is located in the Trans-Pecos, as indeed are all Texas peaks higher than 5,000 feet. In fact, no peaks east of the Trans-Pecos in the United States or Canada rise above 6,700 feet. The lowest elevations in the Trans-Pecos occur in the narrow Rio Grande valley, where elevations fall below 2,000 feet in the Big Bend country, and on the plains that slope eastward to the Pecos River valley, where some elevations are below 3,000 feet. The mean altitude of the region (based on planimeter calculations) is 4,100 feet. Most of the Trans-Pecos is associated with the northern part of the Chihuahuan Desert, Mexico's largest arid zone. Only the highest elevations and the extreme eastern margin of the region receive sufficient precipitation in relation to temperature to be considered semiarid rather than true desert. In general, the average annual precipitation decreases with increasing distance from the Gulf of Mexico, the principal source of moisture for the region; the average annual precipitation is about fifteen inches at Marathon (altitude 4,043 ft.) and less than nine inches in El Paso (same altitude). In addition to the east-west variability, precipitation also increases with increasing elevation. Since the early 1970s, the region has received more precipitation-approximately two inches at most localities. About three-fourths of the annual precipitation falls during the six warmest months of the year. The maximum precipitation occurs during July, August, and September when the northwestward circulation of moist unstable air around the western edge of the Bermuda subtropical anticyclone produces numerous convectional thunderstorms. More recently, the author has demonstrated that tropical storms in the eastern Pacific Ocean are frequently of sufficient size and intensity to supply the atmosphere of the interior desert with relatively large quantities of water vapor. Analysis of satellite imagery and surface observations for summer rainfall revealed that about 55 percent of the annual precipitation falling in El Paso was the result of tropical revolving storms, some of which reach hurricane intensity. Approximately 45 percent of the precipitated moisture was derived from the eastern, tropical Pacific and 55 percent from the Gulf of Mexico. Tropical revolving storms derived from either the Gulf of California or the Gulf of Mexico occasionally bring widespread heavy rain to the region and inflate the average annual precipitation figures. In September 1974 a series of such storms over a period of ten days helped to raise the annual precipitation at Toyah to 21.9 inches, versus an average of 9.92 inches. The period of maximum precipitation during the warmest months is followed by a mild, dry winter during which the flow of air is generally from the west, where topographic barriers inhibit the movement of any significant amounts of moisture into the Trans-Pecos.
The combination of clear skies, high altitudes, and southerly location enable the Trans-Pecos to receive the highest mean annual solar radiation of any location within the United States (data from faulty sensors had previously placed the maximum at China Lake, California, in the Mojave Desert). At lower elevations in the Rio Grande valley below Candelaria some of the highest temperatures in the United States are frequently recorded. In general, the Trans-Pecos exhibits some of the highest average maximum temperatures found in Texas during the winter (second only to the Gulf Coast) and some of the lowest average temperatures in summer (second only to the Panhandle). Large diurnal ranges are common in all seasons, and frosts occur frequently in winter, even at low elevations. Aridity imposes a major constraint on vegetation in the Trans-Pecos. Soils, topography, temperatures, and precipitation variations impose secondary controls that produce significant variations in vegetation from place to place within the region. Desert grassland, the most widespread vegetation type, occupies the lowlands between about 3,600 feet and 4,600 feet. Desert shrubs, particularly creosote bush, occupy the lowest elevations, and there are indications that this type of vegetation has been expanding upslope throughout the region as a result of grassland disturbance over the last 100 years. At higher elevations the desert grassland grades into open woodland. Dense woodland is generally restricted to the higher elevations of the Davis and Guadalupe ranges. Scattered throughout the region are smaller areas of riparian, holophytic, and other vegetation types adapted to specific site conditions. Upland soils are generally shallow and unproductive. Even the deeper alluvial soils of the basins are frequently too saline to be very productive. The Rio Grande is the world's twentieth largest river in volume, the fifth largest in North America, and the only significant surface stream in the Trans-Pecos. Runoff from the mountains of Colorado and northern New Mexico is controlled by major storage dams in southern New Mexico, from which water is released for heavy irrigation in the valleys above and below El Paso and Ciudad Juárez. The average annual flow of 370,000 acre-feet at El Paso is reduced by agricultural diversions and infiltration to 109,000 acre-feet at Fort Quitman. There are long stretches with virtually no water flow between the Quitman Mountains and Presidio. The Rio Grande is replenished at Presidio by inflow from the Río Conchos, which drains the eastern slopes of the Sierra Madre Occidental in Mexico. The Conchos, the most important tributary of the Rio Grande, supplies at least 17 percent of the latter's total flow. Through Big Bend the average annual flow is about 833,000 acre-feet. By way of comparison, the flow past Rio Grande City in southern Texas averages about three million acre-feet. Except for cities immediately along the Rio Grande, Trans-Pecos settlements must depend upon subsurface water. Because of the heavy water demands of El Paso, the Rio Grande provides less than 25 percent of that city's waters. Two major areas of groundwater are the basins of the Hueco Bolson, just east of El Paso, and the Salt Basin, which stretches for a considerable distance west of the Guadalupe, Delaware, and Davis mountains. The groundwater resources of the region are limited by low recharge rates and high mineral contents.
Although the El Paso area was partially explored by Europeans beginning late in the 1500s, the Trans-Pecos saw no real settlement until Texas became a part of the United States. Before the Civil War a small settlement was established on the Texas side of the river at El Paso, and some mining activity was taking place in Presidio County. The combination of aridity, isolation, and roving bands of Apache and Comanche Indians discouraged rural settlements in the region. The rural counties of the Trans-Pecos (away from El Paso) are still the least populated areas in Texas, and they are not growing. Population in them amounts to one inhabitant per square mile, whereas the Texas state average is slightly more than fifty per square mile. About three-fourths of the rural Trans-Pecos population lives in eight or nine small towns. Only the growth of El Paso County, with about 600,000 residents in 1993, differs from the pattern of sparse population and minuscule growth. In general, the cultural landscape of the Trans-Pecos, like the physical one, has more in common with the Southwest than with the rest of Texas. Extensive cattle grazing is the dominant commercial land use in rural areas of the Trans-Pecos. Only small scattered areas have sufficient water for irrigation. Aridity and long-standing land-tenure practices have combined to produce unusually large ranches and low animal densities in this region of predominantly private land ownership. The average working ranch is larger than 20,000 acres, compared to the state average of less than 1,200 acres. Although cattle grazing is the dominant land use in the Trans-Pecos, the region supports only about 2 percent of the cattle in Texas. They are more often worked by the stereotypical cowboy here than elsewhere. There is virtually no rain-fed agriculture in the region, and even irrigated agriculture is confined to a small fraction of the land area of the lowest basins. Floodplain irrigated agriculture is found along the Pecos River valley, along the Rio Grande above and below El Paso, and in a much smaller strip near Presidio, where water from the Río Conchos replenishes the depleted Rio Grande. Irrigated agriculture based upon groundwater pumping is essentially limited to the Salt Basin around Dell City and its southward extension through Van Horn. High-quality cotton, pecans, alfalfa, and vegetables such as tomatoes, onions, and chiles are the major crops of the region. As cotton production has declined in the face of mounting costs and increased competition from other areas, pecans and vegetables have increased in importance. Water may become even less abundant in the future because of the increasing depths from which it must be pumped and the increasing cost of energy to run the pumps. Agriculture in the region is also threatened by the ongoing conversion of farmland to urban uses in the Rio Grande valley above and below El Paso. Because of the presumed lack of significant untapped groundwater reserves of acceptable quality, no new major agricultural developments are anticipated.
Although the Trans-Pecos is not especially noted for its mineral production, economically valuable deposits of various minerals are scattered throughout the region. The oilfields of West Texas extend across the Pecos River, and sulfur, talc, fluorspar, and silver are among the minerals that either are being worked at the present time or have been produced in significant quantities in the recent past. Tourism is important in such sites Big Bend and Guadalupe Mountains national parks and the nearby Carlsbad Caverns in New Mexico. The Trans-Pecos has several other historical and recreational areas, including Hueco Tanks State Historical Park, Fort Davis National Historic Site, and Big Bend Ranch State Natural Area. The Trans-Pecos is on major transportation routes. The region sits astride the lowest, most southernmost, and generally ice-free east-west corridor across the United States, a fact that brings large volumes of traffic. Historically, El Paso del Norte was on a strategic north-south trail connecting Mexico City with Santa Fe, New Mexico. Today, more than forty million people a year cross the border at this locality. The east-west route was established with the coming of the railroad in 1879; additional rail routes followed and were later supplemented by a number of major energy pipelines connecting the Midland-Odessa region with the West Coast. Most recently Interstate highways 10 and 20 were built through the region. On the negative side, the ease of transportation to the region, coupled with its isolation, small population, and lack of political clout, has made the Trans-Pecos and adjoining southern New Mexico the target for dumping of everything from radioactive waste to New York fecal sludge. See also AGRICULTURE, GEOLOGY, GRASSLANDS, MINERAL RESOURCES AND MINING, and RANCHING.