The Ohio River is the largest tributary, by volume, of the Mississippi River. At the confluence, the Ohio is even bigger than the Mississippi (Ohio at Cairo: 281,500 cu ft/s (7,960 m³/s); Mississippi at Thebes: 208,200 cu ft/s (5,897 m³/s)) and, thus, is hydrologically the main stream of the whole river system, including the Allegheny River further upstream. It is approximately 981 miles (1,579 km) long and is located in the Eastern United States.

The river had great significance in the history of the Native Americans, as numerous civilizations formed along its valley. In the five centuries prior to European contact, the Mississippian culture built numerous regional chiefdoms and major earthwork mounds in the Ohio Valley, such as Angel Mounds near Evansville, Indiana, as well as in the Mississippi Valley and the Southeast. For thousands of years, Native Americans, like the European explorers and settlers who followed them, used the river as a major transportation and trading route. Its waters connected communities. The Osage, Omaha, Ponca and Kaw lived in the Ohio Valley, but under pressure from the Iroquois to the northeast, migrated west of the Mississippi River to Missouri, Arkansas, and Oklahoma in the 1600s.

Ohio (ⁱ/oʊˈhaɪ.oʊ/) is a state in the Midwestern United States. Ohio is the 34th most extensive, the 7th most populous, and the 10th most densely populated of the 50 United States. The state's capital and largest city is Columbus.

The name "Ohio" originated from Iroquois word ohi-yo’, meaning "great river". The state, originally partitioned from the Northwest Territory, was admitted to the Union as the 17th state (and the first under the Northwest Ordinance) on March 1, 1803. Although there are conflicting narratives regarding the origin of the nickname, Ohio is historically known as the "Buckeye State" (relating to the Ohio buckeye tree) and Ohioans are also known as "Buckeyes".

The government of Ohio is composed of the executive branch, led by the Governor; the legislative branch, which comprises the Ohio General Assembly; and the judicial branch, which is led by the Supreme Court. Currently, Ohio occupies 18 seats in the United States House of Representatives. Ohio is known for its status as both a swing state and a bellwether in national elections.

Cairo ( /ˈkɛəroʊ/ KAIR-oh) is the southernmost city in the U.S. state of Illinois, and is the county seat of Alexander County.

Cairo is located at the confluence of the Mississippi and Ohio Rivers. The rivers converge at Fort Defiance State Park, a Civil War fort that was commanded by General Ulysses S. Grant. Cairo has the lowest elevation of any location within Illinois and is the only city in the state surrounded by levees.

Several blocks in the town comprise the Cairo Historic District, listed on the National Register of Historic Places (NRHP). The Old Customs House is also on the NRHP. It is part of the Cape Girardeau–Jackson, MO-IL Metropolitan Statistical Area. The population at the 2010 census was 2,831, a significant decline from its peak population of 15,203 in 1920.

The entire city was evacuated in early May 2011, after the Ohio River rose above the 1937 flood levels, out of fear of a 15-foot wall of water inundating the city. The United States Army Corps of Engineers breached levees in the Mississippi flood zone below Cairo in Missouri in order to save the city.