II.

Physical Geography

Oklahoma covers 181,035 sq km (69,898 sq mi), including 3,188 sq km (1,231 sq mi) of inland water, and it ranks 20th in size among the 50 states. Along its southern border, Oklahoma measures 507 km (315 mi). The Panhandle, which is 269 km (167 mi) long, brings Oklahoma’s northern border to 747 km (464 mi). From north to south the length ranges from 267 to 357 km (166 to 222 mi), except in the Panhandle, which measures only 55 km (34 mi). The mean elevation is 400 m (1,300 ft).

Oklahoma has mountainous lands as well as vast areas of level plains. Soils vary from rich black grassland soils to sterile blow sand, and vegetation ranges from sagebrush to grassland to dense forest. The climate varies from semiarid to humid.

A.

Natural Regions

Three of the natural regions, or physiographic provinces of the United States extend within Oklahoma’s territory. These are the Coastal Plain, the Interior Highlands, and the Interior Plains. Of these, the Interior Plains make up the greater part of the state, the Coastal Plain and Interior Highlands flanking these plains on the south and east. Elevations in Oklahoma range from under 90 m (about 300 ft) in the southeast corner to 1,500 m (5,000 ft) in the northwest edge of the Panhandle.

The Gulf Coastal Plain forms a narrow strip along the southeastern Texas-Oklahoma border. The Red River Plains, as it is known to some because it parallels that stream, are low, relatively flat, and sometimes swampy.

The topography changes dramatically in the Interior Highlands north of the Coastal Plains where peaks in the Ouachita Mountains reach as high as 800 m (2,600 ft). The Ouachita Mountains, a series of steeply folded ridges and valleys, resemble parts of the Appalachians farther to the east. Ouachita peaks such as Winding Stair, Kiamichi, Blackfoot, and Rich tower 460 m (1,500 ft) above their valleys.

North of the Ouachitas, set apart from them by the valley of the Arkansas River, is the Ozark Plateau, also part of the Interior Highlands. Broad, flat-topped hills are separated one from another by narrow, V-shaped river valleys. Elevations here range from 180 m (600 ft) to 365 m (1,200 ft). Cookson Hill and Boston Mountains are names sometimes attached to this region.

By far the greater part of the state is the Interior Plains province. From east to west the elevation in this region, reaching 600 m (2,000 ft), divides it into the Osage Plains on the east and the High Plains to the west. The Osage Plains are themselves divided into subregions: the central Red Bed Plains and the Prairie Plains (or Arkansas River valley); the hilly sections of the Sandstone and Gypsum hills; the folded limestones, shales, and other strata of the Arbuckle Mountains and the more rugged, chiefly granite Wichitas.

The High Plains, a part of the Great Plains, occupy northwestern Oklahoma and the Panhandle. Elevations on the plains in the western Panhandle exceed those of the mountains farther east. The highest point in the state is located here at Black Mesa (1,516 m/4,973 ft), the remnants of an ancient lava flow.

B.

Rivers and Lakes

There are about 500 named creeks and rivers in Oklahoma; many are short, intermittent streams. Most rivers flow across the state from northwest to southeast. Two major tributaries of the Mississippi River drain the state—the Arkansas and Red rivers. The principal tributaries of the Arkansas River are the Cimarron and Canadian rivers from the west, the Verdigris, Grand, and Illinois rivers from the north and northeast, and the Poteau River from the south. As with most rivers that rise on the western plains and flow east, rivers such as the Cimarron and Canadian are characterized by broad, shallow, and sandy channels. In dry season there will be little surface flow, although much subsurface water will flow through the rivers’ sandy beds. The chief tributaries of the Red River are the North Fork, Washita, Blue, Boggy, and Kiamichi rivers. Waterways originating in the more humid eastern regions are characterized by their steep banks, swift current, and depth.

All of the larger lakes in the state are artificial and more than three-fourths of them are in the eastern portion of the state, where the rainfall is greater. These reservoirs were created for flood control, navigation, water supply, power generation, and recreation. The United States Army Corps of Engineers, the Bureau of Reclamation, and the Grand River Dam Authority created the major reservoirs of the state, including Altus, Arbuckle, Canton, Eufaula, Fort Supply, Keystone, Oologah, Texoma, Thunderbird, and Wister. Some of these reservoirs are part of the McClellan-Kerr Arkansas River Navigation System that connect Tulsa’s port of Catoosa to barge traffic on the Mississippi system.

C.

Climate

Oklahoma’s geographic location and topography have a critical effect on the state’s climate. Like many plains states, Oklahoma is known for its changeable and varied weather patterns. During the winter it is common for the south and southeast regions to bask in springlike temperatures when as much as 300 mm (12 in) of snow falls in the Panhandle.

About four-fifths of Oklahoma outside of the Panhandle is categorized as humid subtropical, with very hot, long summers and moderate short winters. The western portion and the Panhandle are classified as a steppe, where precipitation, typically 250 to 500 mm (10 to 20 in), is the controlling characteristic.

January is usually the coldest month with an average of about 3°C (38°F) and extremes from -33°C (-27°F), the lowest ever recorded, to 33°C (92°F). Summer are long and hot with temperatures in the upper 30°s C (lower 100°s F) common from May until September across the state. The growing season varies from less than 180 days in the western Panhandle to more than 240 days in the southeastern Coastal Plain.

Oklahoma occupies a transitional precipitation zone, with a humid east and a semi-arid west. Rainfall averages from 1,270 mm (50 in) in the Ouachita Mountains to just 380 mm (15 in) in the far western Panhandle. Spring is generally the wettest, but in the west this advantage is offset by the high evaporation rate.

Two defining weather phenomena in Oklahoma are drought and tornadoes. Periodic droughts occur particularly in semiarid areas of western Oklahoma, the most famous of which occurred during the Dust Bowl era of the 1930s. The state averages dozens of tornadoes annually, especially during the months of April and May. These destructive storms are embedded in thunderstorms and move from southwest to northeast across the state.

D.

Soils

Within its borders, Oklahoma has a number of different soils of varying fertility. The ultisols (red and yellow podzols) of the forested Ouachitas and Ozarks have been leached of much of their nutrients. Alfisols and mollisols (chernozems and chestnut soils) of the grassy prairies are known for their natural fertility although agricultural overuse and limited precipitation restrict their natural richness. Alluvial soils are found along the river valleys while loess, a wind-deposited soil, can be found on the uplands between the rivers.

E.

Plant Life

Vegetation responds to variation in the water, temperature, elevation, slope, soil, drainage, and competition among native and introduced species. Three broad categories exist in the state—forest, woodland and savannas, and grasslands. The largest forested area can be found in eastern Oklahoma. Deciduous forest of oak, hickory and other species, mixed forests of pines and hardwoods, or pure stands of southern pine, are located here.

Woodlands and savanna cover the mid-section of the state with trees becoming less frequent moving westward. Tall grasses dominate the drier areas in this region. The largest woodland area was the “cross timbers” in the east central region, so called because the branches of the blackjack and post oak grew so close that their branches became intertwined, creating a barrier to passage.

Still farther west, the ground cover is dominated by short grasses, sagebrush, and eastern redcedar. The northwestern Panhandle has a piñon-juniper woodland like that of the Rocky Mountains. Here, too, though less dramatically than elsewhere in the state, the natural vegetation has been altered by cultivation and grazing.

Flowering trees found in Oklahoma include dogwood and redbud. Among the flowers found throughout the state are the sunflower, goldenrod, wild indigo, verbena, violet, primrose, anemone, and phlox.

F.

Animal Life

Oklahoma’s animal population includes jackrabbits, cottontails, coyotes, prairie dogs, mink, squirrels, raccoons, and skunks. Some of the larger animals found in the state are pronghorns, white-tailed and mule deer, elk, red and gray fox, bobcat and beaver. Birds commonly found are the cardinal, English sparrow, swallow, robin, meadowlark, mockingbird, quail, wild turkey, prairie chicken, mourning dove, and pheasant.

G.

Conservation

Only a few small areas of land, usually in stream bottoms, were cultivated until the opening of the Unassigned Lands and western reservations after 1889. After that, however, many areas were overgrazed and the semiarid lands were plowed. The vegetation that held the soil in place and kept the water on the land was thus destroyed. Subsequently much farmland was ruined by sheet, gully, and wind erosion. In the dry years violent dust storms developed and blew the rich topsoils away. In the wet years great gullies were carved out of the furrows where the land was steep.

Since the Dust Bowl tragedy of the 1930s, farmers and ranchers have worked with the Natural Resources Conservation Service and other government agencies to conserve the soil. They have terraced the hillsides, used contour plowing, and built dams to make ponds and reservoirs. The lands of the drier areas have been planted in grasses to prevent soil from blowing away. The first Soil Conservation District in the United States was developed in eastern Oklahoma, and today each county in the state has one or more such districts.

Water conservation is just as important as soil conservation to Oklahoma. When there is danger of flooding, water is stored by means of dams and reservoirs. The water is released into the rivers when their level is low. During dry spells, water is taken from some western lakes for irrigation. Flood prevention methods have kept river-bottom land under cultivation that otherwise would have been abandoned. The Sandstone Creek Project in Roger Mills County was the first upstream flood prevention project in the nation. So successful has this project been that even in drought years water has continued to flow in the streams. Even groundwater supplies have become a cause for concern. Groundwater supplies more than 70 percent of the irrigation water used in the state. Aquifers which filled over the millennia are being strained with growing agricultural usage. The Ogallala Aquifer in the Panhandle has thus far generated the most concern over the mining of this resource faster than it can be replaced.

In 2006 the state had 10 hazardous waste sites on a national priority list for cleanup due to their severity or proximity to people. Progress was being made in efforts to reduce pollution; in the period 1995–2000 the amount of toxic chemicals discharged into the environment was reduced by 26 percent.