Cairo ( /kɛəroʊ/ kair-oh) is the southernmost city in the U.S. state of Illinois. It 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, an American Civil War fort that was commanded by General Ulysses S. Grant. Cairo is the only city in Illinois completely 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 drop 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 Army Corps of Engineers breached levees in the Mississippi flood zone below Cairo in Missouri in order to save the city.