The Ohio River is, by volume, the largest tributary to the Mississippi River.
Louisville was founded at the only major natural navigational barrier on the river, the Falls of the Ohio. The Falls were a series of rapids where the river dropped 26 feet (7.9 m) in a stretch of about 2 miles (3.2 km). In this area, the river flowed over hard, fossil-rich beds of limestone. The first locks on the river were built in 1825 at Louisville to circumnavigate the falls. Today it is the site ofMcAlpine Locks and Dam.