Physical Geography

Texas occupies parts of four physiographic regions of the United States. From east to west they are the Gulf Coastal Plain, the Central Lowlands, the Great Plains, and the Basin and Range.

The Gulf Coastal Plain, extending from Mexico to Florida, covers eastern and southern Texas. It roughly parallels the Gulf of Mexico and consists of a series of lowlands separated by hilly areas. The area along the coast consists of several long, narrow islands and peninsulas lying parallel to the mainland. Inland, the surface is at first flat, but it becomes more hilly toward the interior. The fertile, rolling Black Prairie belt, farthest west in the Gulf Coastal Plain, is one of the most densely populated and productive areas in the state.

The Central Lowlands, an interior region reaching as far north as Canada, is known in Texas as the North Central Plains. This area of rolling and hilly land lies west of the Black Prairie and north of the Colorado River. Its western boundary is the Cap Rock Escarpment, which rises 200 to 1,000 feet (60 to 300 m) above its base on the plains.

The Great Plains is a vast elevated region extending from Canada to Texas just east of the Rocky Mountains. In the Texas Panbandle, the state's northern extension between New Mexico and Oklahoma, the plains are about 4,000 feet (1,200 m) above sea level. This area, called the Llano Estacado, or Staked Plain, is flat and almost featureless except for its eastern margin, which is marked by a number of deep canyons. The most spectacular is Palo Duro Canyon, on a tributary of the Red River.

To the south is the Edwards Plateau. It lies at an average elevation of nearly 2,500 feet (760 m), and the surface is more rolling. Many valleys and canyons have been carved along the southern and eastern edges of this plateau. The hill country west of Austin is a popular tourist area and has numerous guest ranches.

The Basin and Range region, or Trans-Pecos, as it is known in Texas, is situated west of the Pecos River and the Stockton Plateau. It is a continuation of a series of mountain ranges and intervening basins extending southward from the Rocky Mountains of New Mexico. Highest among the ranges is the Guadalupe Mountains. Here is Guadalupe Peak, at 8,749 feet (2,667 m) the highest point in Texas. To the south are the Davis Mountains, with peaks up to 8,500 feet (2,590 m). The Chisos Mountains are in the Big Bend country, so named for the great northward turn made by the Rio Grande. Big Bend National Park was established here to preserve the area's natural beauty.

All streams and rivers in Texas drain to the Gulf of Mexico. The Red and Canadian rivers are part of the Mississippi River system, but all other rivers in the state flow directly into the Gulf. The Rio Grande, although the state's longest river, receives very little water from Texas as it flows along the 1,300-mile (2,100-km) international border. Its principal Texas tributary is the Pecos River. The longest river completely within the state is the Brazos, more than 800 miles (1,300 km) in length.

Many localities in Texas rely on wells for their water. Continuously increasing demands from agriculture and industry have threatened to exceed available supplies of well water. Streams and rivers are being used more and more, but because of the highly variable rainfall in Texas, they are often undependable sources.

To provide reliable water supplies, many state and federal conservation projects have been undertaken throughout the state, especially in the east and northeast. Among the largest of these is Lake Texoma, on the Red River, which provides electric power and recreation areas as well as water storage for Texas and Oklahoma. Toledo Bend Reservoir on the Sabine River is a joint Texas-Louisiana project. Nearby are the Sam Rayburn Dam and Reservoir. Falcon and Amistad reservoirs, international projects on the Rio Grande, are designed to provide irrigation water for both Texas and Mexico.

Texas has a wide variety of climates, ranging from subtropical to continental. Eastern Texas and the Gulf Coast have a humid subtropical climate, with mild to cool winters, hot summers, and abundant rainfall distributed fairly evenly throughout the year. Winters are mildest in the lower Rio Grande Valley, in the far south, where virtually no snow or freezes occur. January and July temperatures average 45° and 85° F. (7° and 29° C.) in Dallas; 52° and 83° F. (11° and 28° C.) in Houston; and 60° and 84° F. (16° and 29° C.) in Brownsville.

The climate in western Texas, north of the Balcones Escarpment, is a dry continental type. Winters are cold in the Panhandle, where frequent strong cold fronts move southward across the Great Plains. January and July averages for Amarillo are 36° and 79° F. (2° and 26° C.); for El Paso, 44° and 82° F. (7° and 28° C.); and for Del Rio, 51° and 87° F. (11° and 31° C.).

In general, the amount of precipitation decreases from east to west in Texas. Yearly totals in the east, near Beaumont, are close to 50 inches (1,270 mm). El Paso, in the far west, receives less than 8 inches (200 mm) a year. Total amounts throughout the state, however, are highly variable from year to year, and droughts are sometimes long and severe. Tornadoes occur at any time of the year in Texas, and hurricanes occasionally move in from the Gulf in summer or autumn, causing wind damage and flooding in eastern Texas.

Pine forests, mixed with a few such hardwoods as oak and hickory, lie between the Trinity and Sabine rivers; they provide most of the state's commercial timber production. West of the pine belt is a region of mixed hardwood forest, primarily blackjack and post oak, elm, and—along watercourses—pecan and walnut trees. A notable break in this woodland region is the Black Prairie belt, a grassland area with some timber along stream banks.

South of San Antonio is a region of brush, cactus, and mesquite trees. Similar vegetation is found in the Pecos River area. West of the Pecos, in the Basin and Range country, native plants are limited to low shrubs and grass, which grows in scattered patches. In the higher mountains, however, enough rain falls to support oaks and coniferous trees. Treeless grasslands occupy the Panhandle, while farther south on the Edwards Plateau increasing numbers of low trees and shrubs are found. Between the Great Plains and the Red River, in the Central Lowlands, is a prairie region with many patches of mesquite and scrub oak trees.