The Pecos River, one of the major tributaries of the Rio Grande, rises on the western slope of the Santa Fe mountain range in Mora County, New Mexico (at 35°59' N, 105°33' W), and runs south through San Miguel, Guadalupe, De Baca, Chaves, and Eddy counties in New Mexico before it enters Texas just east of the 104th meridian. In Texas the river flows southeast, forming the boundary between Loving and Reeves, Reeves and Ward, Ward and Pecos, Pecos and Crane, Pecos and Crockett, and Crockett and Terrell counties. It then enters Val Verde County at its northwestern corner and angles across that county to its mouth (at 29°42' N, 101°22' W) on the Rio Grande in the Amistad Reservoir, between Comstock and Langtry some thirty-eight miles northwest of Del Rio. Through most of its more than 900-mile-long course, the Pecos River parallels the Rio Grande. The total drainage area of the Pecos in New Mexico and Texas is about 44,000 square miles. Most of its tributaries flow from the west; these include the Gallinas River, the Rio Hondo, the Rio Felix, the Rio Penasco, the Delaware River, Independence Creek, Toyah Creek, and Comanche Creek. Entering the Pecos from the east are the tributaries Alamogordo, Taiban, Live Oak, and Howard. The topography of the river valley ranges from mountain pastures in the north, with an elevation of more than 13,000 feet above sea level, to grasslands, semiarid irrigated farmlands, desert with sparse vegetation, and, in the lowermost reaches of the river, deep canyons. The principal cities along the river in New Mexico are Santa Rosa, Fort Sumner, Roswell, Artesia, and Carlsbad; in Texas, the main city on the river is Pecos, the Reeves county seat. In the early 1990s none of these places had a population of more than 40,000. Oil is produced in the eastern portion of the Pecos river valley, part of the Permian basin, and sulfur and potash are also important products.
In addition to being a county boundary stream, the Pecos serves as the eastern boundary of the most mountainous and arid region of Texas, generally known as the Trans-Pecos. From below Sheffield in eastern Pecos County to the river's confluence with the Rio Grande, it passes through a deep gorge, which has long constituted a barrier to transportation and which has prevented irrigation from this part of the lower Pecos. Elsewhere along the course of the river, however, the diversion and impoundment of its water has radically altered its appearance in Texas. Early-day travelers described the river as generally sixty-five to a hundred feet wide and seven to ten feet deep, with a fast current. It was fordable at only a few places, the most famous of which was the Horsehead Crossing. Irrigation from the upper section of the Pecos (between Girvin in northeastern Pecos County and the New Mexico boundary) began in 1877. Appreciable development of irrigation in this area, however, did not come until after 1888. Completion of the Red Bluff Reservoir (in Reeves and Loving counties, forty-five miles north of Pecos) and a hydroelectric power plant in 1936 made possible the creation of water-improvement districts in the lower valley. By the mid-1980s there were more than 400,000 acres under irrigation, using both surface and underground water; crops produced in the Pecos valley included cotton, alfalfa, forage, grain sorghums, vegetables, and fruits, especially cantaloupes. By that time the river was usually a small, shallow, narrow stream with a sluggish current; it was bordered by desert shrubs. Except during floods, its flow for a considerable distance downstream from the Red Bluff Reservoir consisted principally of releases and some reservoir seepage. Nonetheless, the Pecos can still become a dangerous river during heavy thunderstorms.
The earliest-known settlers along the river were the Pecos Pueblo Indians, who arrived about A.D. 800. Supposedly the first European to cross the river was Francisco Vázquez de Coronado, who reached the area in 1541. In 1583 Antonio de Espejo called the river the Río de las Vacas ("river of the cows") because of the number of buffalo in the vicinity. Gaspar Castaño de Sosa, who followed the Pecos northward, called it the Río Salado because of its salty taste, which caused it to be shunned by men and animals alike. According to Adolph F. Bandelier, the name Pecos first appears in Juan de Oñate's reports concerning the Indian pueblo of Cicuye, now known as the Pecos Pueblo, and is of unknown origin. To Mexicans the river was long known as the Río Puerco ("dirty river"). The earliest European settlement was founded in 1794 at San Miguel del Bado in the upper valley of the Pecos. With the Anglo-American occupation of Texas, the middle and upper Pecos valley became the chief western cattle trail to the north, as well as the site of several famous cattle ranches. A small church group settled at St. Gall, Texas, in 1845, and Fort Lancaster was built near the river in 1855. Except for settlement around the fort, the earliest Anglo settlement in Texas on the river was Pecos, founded in 1881 when the Texas and Pacific Railway crossed West Texas.
Water conservation practices on the Pecos River are overseen by the United States Bureau of Reclamation, along with the state engineer of New Mexico, the Pecos Valley Artesian Conservancy District, and the Carlsbad Irrigation District in the upper river valley. The Red Bluff Water Power Control District, along with its seven water improvement districts, oversees water use in the lower valley and works to provide equitable distribution of water supplies. For many years the amount of water available for irrigation was a matter of contention between New Mexico and Texas. Around 1948 the two states entered into an agreement known as the Pecos River Compact, which required New Mexico to maintain deliveries of water depending on the amount of water reaching the river in New Mexico by natural causes. Texas for years considered New Mexico to be deficient in living up to the terms of the contract and in 1974 filed suit. The United States Supreme Court ruled in June 1987 that New Mexico owed Texas 340,000 acre-feet of water for the period between 1950 and 1983, and ordered that New Mexico repay with deliveries of 34,000 acre-feet of water a year for ten years.