Confluence of Ohio and Mississippi Rivers at Cairo, IL
The Ohio River becomes a tributary of the Mississippi River directly south of Cairo, Illinois, a small city on the spit of land where the rivers converge (at center of this astronaut photograph). Brown, sediment-laden water flowing generally northeast to south from the Ohio River is distinct from the green and relatively sediment-poor water of the Mississippi River (flowing northwest to south). The color of the rivers in this image is reversed from the usual condition of a green Ohio and a brown Mississippi. This suggests that the very high rainfall in December 2005 over the Appalachians and the northeastern United States has led to greater-than-normal amounts of sediment in the rivers and streams of the Ohio River watershed. The distinct boundary between the two river's waters indicates that little to no mixing occurs even 5-6 kilometers (3-4 miles) downstream.
Cairo became a prosperous port following the Civil War due to increased riverboat and railroad commerce. Small features visible in the image on the Ohio are river barges, which indicate the continued importance of Cairo as a transport hub. Flooding of the Ohio and Mississippi Rivers presents a continual danger to the city; this danger is lessened by the Birds Point-New Madrid Floodway that begins directly to the south of the river confluence. During major flood events, the floodway lessens flood stages upstream (such as at Cairo) and adjacent to the floodway. Part of the extensive levee system associated with flood control of the Mississippi River is visible in the image. Barlow Bottoms (image right), located in adjacent Kentucky, is a wetland bird-watching location that is replenished by periodic floods and releases of Ohio River water.