Cairo 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, a Civil War camp that was built in 1862 by Union General Ulysses S. Grant. Cairo has the lowest elevation of any location in Illinois and is the only Illinois city surrounded by levees. It is in the area known as Little Egypt.Several blocks in the town comprise the Cairo Historic District, listed on the National Register of Historic Places. The Old Customs House is also on the NRHP. The city 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 during the 2011 Mississippi River Floods, after the Ohio River rose higher than the 1937 flood levels, with the possibility of 15 feet of water inundating Cairo.