Rio Grande

Rio Grande, fifth longest river of North America, and the 20th longest in the world, forming the border between the U.S. state of Texas and Mexico. Rising as a clear, snow-fed mountain stream more than 12,000 feet (3,700 metres) above sea level in the Rocky Mountains, the Rio Grande descends across steppes and deserts, watering rich agricultural regions as it flows on its way to the Gulf of Mexico. The total length of the river is about 1,900 miles (3,060 km).

The area within the entire watershed of the Rio Grande is some 336,000 square miles (870,000 square km). Because a large proportion of the river’s basin is arid or semiarid, however, only about half of the total area, or about 176,000 square miles (456,000 square km), actually contributes to the river’s flow.

Physical features

From its sources in the San Juan Mountains of southwestern Colorado, the Rio Grande flows to the southeast and south for 175 miles (280 km) in Colorado, southerly for about 470 miles (760 km) across New Mexico, and southeasterly for about 1,240 miles (2,000 km) between Texas and the Mexican states of Chihuahua, Coahuila, Nuevo León, and Tamaulipas to the Gulf of Mexico.

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Its early course follows a canyon through forests of spruce, fir, and aspen into the broad San Luis Valley in Colorado, after which it cuts the Rio Grande Gorge and White Rock Canyon of northern New Mexico and enters the open terrain of the Basin and Range and Mexican Plateau physiographic provinces. There, declining elevation, decreasing latitude, and increasing aridity and temperature produce a transition from a cold steppe climate with a vegetation of piñon pine, juniper, and sagebrush to a hot steppe and desert climate characterized by mesquite, creosote bush, cactus, yucca, and other desert plants. Shortly before entering the Gulf Coastal Plain, the Rio Grande cuts three canyons between 1,500 and 1,700 feet (460 and 520 metres) in depth across the faulted area occupied by the “big bend,” where the Texas side of the river comprises the Big Bend National Park. Along the remainder of its course the river wanders sluggishly across the Gulf Coastal Plain to end in a fertile delta where it joins the Gulf of Mexico.

The principal tributaries of the Rio Grande are the Pecos, Devils, Chama, and Puerco rivers in the United States and the Conchos, Salado, and San Juan in Mexico. The peak of flow may occur in any month from April to October. In the upper reaches of the Rio Grande it usually is in May or June because of melting snow and occasional thunderstorms, whereas the lower portion commonly experiences its highest water levels in June or September because of the occurrence of summer thunderstorms. It has been estimated that the Rio Grande has an average annual yield of some 2.6 million acre-feet (3.2 billion cubic metres), of which about a third reached the gulf before the building of the Falcon Dam, upstream from Rio Grande City, in 1953; the river’s average discharge rate is now about 3,000 cubic feet (85 cubic metres) per second.

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The economy

Irrigation has been practiced in the Rio Grande basin since prehistoric times, notably among the ancestors of the Pueblo Indians of New Mexico. Increases in population and in the use of water made necessary the water treaties (1905–07 and 1944–45) between the United States and Mexico, as well as the Rio Grande Compact (1939) among Colorado, New Mexico, and Texas, concerning shared use of the waters of the upper Rio Grande subbasin (above the site of former Fort Quitman, Texas), and the Pecos River Compact (1948) between New Mexico and Texas, concerning the Pecos above Girvin, Texas. Essentially all of the average annual production of more than three million acre-feet in the upper Rio Grande (including the 60,000 acre-feet allotted to Mexico by treaty) is consumed within this subbasin. Not only below Fort Quitman but also in many stretches of the river from the New Mexico–Colorado border to below Brownsville, Texas, there has been no surface flow at various times. In some places the depth of the river has varied from nearly 60 feet (18 metres) to a bare trickle or nothing. Below Fort Quitman the Rio Grande is renewed by the Conchos and other Mexican rivers. The Conchos, which rises high in the Sierra Madre Occidental of Chihuahua, is the Rio Grande’s most important tributary: it supplies more than one-sixth of the Rio Grande’s total and, with the other Mexican rivers, produces about two-thirds of the available water. A number of large springs in the area between Hot Springs in the Big Bend National Park and the town of Del Rio, Texas, including many in the bed of the river, are important and dependable producers of groundwater.

The major reservoirs in the basin are the Falcon Reservoir on the lower Rio Grande, Lake Toronto (impounded by La Boquilla Dam) on the Conchos, Elephant Butte on the Rio Grande in New Mexico, Marte Gómez (El Azúcar Dam) reservoir on the San Juan, and Venustiano Carranza (Don Martín Dam) on the Salado. The international Amistad Dam, below the confluence of Devils River, was completed in 1969 under terms of a U.S.-Mexico treaty. Considerable amounts of hydroelectricity are produced within the basin.

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Excluding areas irrigated above the reservoirs, the river provides water for some 2,100,000 acres (850,000 hectares) of cropland; slightly more than half of the irrigated land is in Mexico. The leading crops raised by irrigation vary from potatoes and alfalfa in Colorado; to cotton, pecans, and grapes in southern New Mexico and in the El Paso–Juárez region of Texas and Mexico; and citrus fruits, cotton, and vegetables in the valley of the lower Rio Grande delta region in Texas and Tamaulipas.

After agriculture and animal husbandry, the leading industries of the Rio Grande area are mining (petroleum, natural gas, coal, uranium ore, silver, lead, gold, potash, and gypsum) and recreation (national and state parks and monuments, dude ranches, rafting, fishing and hunting, and summer and winter resorts). Urban communities include Monterrey, Juárez, Chihuahua, Saltillo, Matamoros, Guadalupe, Nuevo Laredo, Reynosa, and San Nicolás de los Garzas in Mexico; Albuquerque and Las Cruces in New Mexico; and El Paso, Laredo, and Brownsville in Texas.