SDSouth Dakota Geography

East River, West River

South Dakotans rarely say eastern and western South Dakota. They say East River and West River, and the river in question is the Missouri, which enters from North Dakota, runs down the middle of the state and swings southeast to leave at Sioux City. The two halves behave like separate countries. East River was overrun by the ice sheets, so it has deep glacial till, more than twenty inches of rain in a normal year, and section lines full of corn and soybeans. West River was never glaciated. Its subsoil is Pierre Shale, a grey marine clay that turns to gumbo when wet, annual rainfall slides toward fourteen inches near the Wyoming line, and the working unit of land is the ranch rather than the quarter section. Sioux Falls sits at the far eastern edge on a set of falls cut into Sioux Quartzite. Rapid City sits roughly 350 miles west against the Black Hills. Pierre, the capital, sits on the seam.

Between 1946 and 1966 the Pick-Sloan plan put four dams across that seam: Oahe just above Pierre, Big Bend at Fort Thompson, Fort Randall at Pickstown and Gavins Point at Yankton. Lake Oahe alone backs water 231 miles upstream, past the North Dakota line and nearly to Bismarck. The reservoirs drowned the Missouri bottomland, the most fertile and most sheltered ground in the state, and a large share of what went under belonged to the Standing Rock, Cheyenne River, Lower Brule and Crow Creek reservations.

An island of granite on the plains

The Black Hills are not an outlier of the Rocky Mountains. They are a separate dome, pushed up around sixty million years ago and then stripped by erosion until the sedimentary cover peeled off the crest and exposed a core of Precambrian granite and schist close to 1.7 billion years old. Black Elk Peak reaches 7,242 feet, the highest ground in the United States east of the Rockies, and it carried the name Harney Peak until the federal board on geographic names changed it in 2016. Around the granite core runs the Red Valley, a ring of soft red Spearfish Formation that early riders called the Race Track, and outside that a hogback ridge of tilted Dakota Sandstone forms an almost unbroken rim.

The limestone plateau on the western flank of the dome is riddled with passage. Jewel Cave has more than 200 miles surveyed, and Wind Cave holds the finest boxwork anywhere, thin calcite fins left standing after the rock around them dissolved. Wind Cave was protected in 1903, the first cave in the world set aside as a national park.

Ground that is still washing away

Southeast of the Black Hills the White River has eaten into flat prairie and left the Badlands: banded spires and gullies carved from the Chadron and Brule formations, floodplain mud and volcanic ash laid down between roughly 34 and 37 million years ago. The rock is soft enough that the formation loses about an inch a year, fast enough that new fossils surface after heavy rain. Oreodonts, three toed horses, rhinoceros relatives and saber toothed nimravids all come out of those beds. Running through it is the Badlands Wall, a sixty mile escarpment that divides the higher prairie from the lower.

Two clocks and a watershed split in the northeast corner

The Central and Mountain time boundary crosses the state, following the Missouri through the northern counties before bending southwest, so Pierre keeps Central Time while Rapid City runs an hour behind it. The other dividing line worth knowing sits in the far northeast. Big Stone Lake, at 966 feet, is the lowest ground in South Dakota, and its outflow goes down the Minnesota River to the Mississippi and eventually the Gulf of Mexico. Lake Traverse begins a few miles up the same glacial trench and drains the opposite way, north through the Bois de Sioux and the Red River of the North to Hudson Bay. That trench was gouged by Glacial River Warren draining Lake Agassiz, and the divide between the two lakes amounts to a few feet of gravel. From the 966 foot low to Black Elk Peak is 6,276 feet of relief.

The eastern quarter of the state rides the Coteau des Prairies, a wedge of glacial drift about 200 miles long that stood high enough to split the advancing ice into separate lobes. That is why the pothole country either side of it is stippled with small lakes while the Coteau itself carries a different soil and a different crop mix.

Both temperature records in a single year

South Dakota set its hottest and coldest readings five months apart. The thermometer at McIntosh fell to 58 below zero on 17 February 1936, and on 5 July of that same year Gann Valley reached 120 degrees, a 178 degree spread inside one calendar year. A stranger reading came later. At Spearfish on 22 January 1943 a chinook wind dropped over the Black Hills and pushed the temperature from 4 below zero to 45 above in two minutes, the fastest rise ever recorded, and shop windows in town cracked from the shock.

Geography of the states around South Dakota

More about South Dakota