Physical Geography

South Dakota consists of three major regions—the Prairie, the Great Plains, and the Black Hills.

The Prairie, which is part of a large physiographic province of the United States known as the Central Lowlands, extends westward almost to the Missouri River. All of it was covered by glaciers during the last Ice Age and owes its present appearance mainly to glacial deposits and to subsequent erosion. Flat to gently rolling land prevails, except in a somewhat hilly belt of glacial debris, known as the Prairie Coteau, or the Prairie Hills. The James River Valley, just west of the coteau, is exceptionally flat terrain, especially the northern part, which occupies the bed of an old glacial lake. Equally flat is the low-lying valley containing Lake Traverse and Big Stone Lake in the northeastern corner of the state. The surface of Big Stone Lake, at 966 feet (294 m) above sea level, is the lowest elevation in the state.

The Great Plains extend westward from the Missouri Coteau, or the Missouri Hills, a belt of moraines and other debris left by glaciers. The coteau marks the farthest advance of the ice into the state. Elevations here are about the same as those in the Prairie Coteau. Beyond the Missouri River lies the unglaciated section of South Dakota's Great Plains. In many areas the land stretches westward as gradually rising, flat to undulating terrain. Elsewhere, it has been eroded into deep valleys and intricately carved canyons and badlands with narrow ravines and high-rising spires. Most spectacular are the badlands of the southwest, a portion of which is preserved as a national park. There are also scattered mesas and buttes throughout the plains, which further vary the landscape.

The Black Hills, in the southwest, consist of rough, heavily forested mountains geologically similar to the Rockies. They rise as much as 4,000 feet (1,200 m) above the general level of the plains, reaching 7,242 feet (2,207 m) above sea level at Harney Peak, highest point in the state.

The Missouri is the chief river of South Dakota. Together with many tributaries, it drains virtually the entire state. The Missouri flows in a somewhat southeasterly course through the center of the state, then easterly along the Nebraska–South Dakota border. From the west, it receives the waters of five major tributaries: the Grand, Moreau, Cheyenne, Bad, and White rivers. Eastern South Dakota is drained primarily by the Big Sioux and James rivers, both of which are southerly flowing tributaries of the Missouri. Though much shorter, the Vermillion, in the southeast, is also a significant river.

Only the northeastern corner of the state lies outside the Missouri drainage basin. There the water of Lake Traverse drains northward to Hudson Bay, while that of Big Stone Lake drains eastward to the Minnesota River and eventually to the Gulf of Mexico. The drainage divide between the two lakes is very poorly defined.

All of South Dakota's large lakes—Oahe, Sharpe, Francis Case, and Lewis and Clark—are reservoirs created by dams built on the Missouri River during the 1950's and 60's. Of the four, Oahe is by far the largest. Besides controlling floods, the dams provide hydroelectric power, water for irrigation, and numerous recreational facilities. Small glacially created lakes dot parts of eastern South Dakota, particularly the Prairie Coteau. There are also artesian sources of water in the east and hot springs in the Black Hills.

Because of its central location in North America, far from any tempering influence of the ocean, South Dakota has a continental climate marked by great extremes in the weather. Winters are long and cold, with frequent readings below 0° F. (-18° C.). January is the coldest month, averaging between 10° and 20° F. (-12° and -7° C.) throughout most of the state. Summer daytime temperatures often rise to 100° F. (38° C.) or more. The hottest month, July, averages 72° to 75° F. (22° to 24° C.), with the Black Hills somewhat cooler.

Except for the Black Hills and the extreme southeastern corner of the state, which receive about 25 inches (635 mm) of moisture each year, South Dakota gets meager amounts of precipitation. It decreases from about 20 inches (510 mm) annually in the east to about 13 inches (330 mm) in the northwest, where semiarid conditions prevail. The total amount is highly variable from year to year, and serious droughts periodically occur. Normally, most of the precipitation comes as rain from April to September, which corresponds roughly to the crop growing season. Snows are usually light, but tend to accumulate to considerable depths during the long, cold winter. Blizzards occasionally occur.

The natural vegetation of South Dakota consists primarily of grasses—tall grass on the prairies and short grass, often widely dispersed, on the Great Plains. The boundary between the two is roughly the Missouri River. Much of the natural vegetation, however, particularly in the east, has been destroyed since the introduction of farming in the late 1800's.

Forests are confined primarily to the Black Hills, where ponderosa pine, white spruce, and other conifers predominate. Elsewhere, there are scattered areas of cedar in the west and stands of cottonwood, oak, and other hardwoods along the rivers and streams and around the margins of lakes.