Nebraska ( nə-BRASS-kə) is a landlocked state in the Midwestern region of the United States. It borders 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. Nebraska is the 16th-largest state by land area, with just over 77,220 square miles (200,000 km2). With a population of over 1.9 million, it is the 38th-most populous state and the eighth-least densely populated. Its capital is Lincoln, and its most populous 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 one of only two states that divide electoral college votes by district, and is not winner-take-all. 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. Eastern Nebraska has a humid continental climate while western Nebraska is primarily semi-arid. The state has wide variations between winter and summer temperatures; the variations decrease in southern Nebraska. Violent thunderstorms and tornadoes occur primarily during spring and summer, and sometimes in autumn. The Chinook wind tends to warm the state significantly in the winter and early spring. 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 discovery and exploration. The state is crossed by many historic trails, including that of the Lewis and Clark Expedition. The completion of the Transcontinental Railroad through Nebraska and passage of the Homestead Acts led to rapid growth in the population of American settlers in the 1870s and 1880s and the development of a large agriculture sector for which the state is known to this day.