Nebraska ( (listen)) is a state in the Midwestern region of the United States. It is bordered by South Dakota to the north; Iowa to the east and Missouri to the southeast, both across the Missouri River; Kansas to the south; Colorado to the southwest; and Wyoming to the west. It is the only triply landlocked U.S. state.Indigenous peoples, including Omaha, Missouria, Ponca, Pawnee, Otoe, and various branches of the Lakota (Sioux) tribes, lived in the region for thousands of years before European exploration. The state is crossed by many historic trails, including that of the Lewis and Clark Expedition. Nebraska's area is just over 77,220 square miles (200,000 km2) with a population of over 1.9 million. Its capital is Lincoln, and its largest city is Omaha, which is on the Missouri River. Nebraska was admitted into the United States in 1867, two years after the end of the American Civil War. The Nebraska Legislature is unlike any other American legislature in that it is unicameral, and its members are elected without any official reference to political party affiliation. Nebraska is composed of two major land regions: the Dissected Till Plains and the Great Plains. The Dissected Till Plains region consists of gently rolling hills and contains the state's largest cities, Omaha and Lincoln. The Great Plains region, occupying most of western Nebraska, is characterized by treeless prairie. Nebraska has two major climatic zones. The eastern two-thirds of the state has a humid continental climate (Köppen climate classification Dfa); a unique warmer subtype considered "warm-temperate" exists near the southern plains, which is analogous to that in Kansas and Oklahoma, which have a predominantly humid subtropical climate. The Panhandle and adjacent areas bordering Colorado have a primarily semi-arid climate (Köppen BSk). The state has wide variations between winter and summer temperatures, variations that decrease moving south within the state. Violent thunderstorms and tornadoes occur primarily during spring and summer and sometimes in autumn. Chinook wind tends to warm the state significantly in the winter and early spring.