My understanding is that there are five rivers in the Underworld: Acheron - the river of woe; Styx - the river of hate; Lethe - the river of forgetfulness; Cocytus - the river of lamentation (which flows into Acheron); Phlegethon - the river of fire. Acheron and Styx are not the same, but it is a common misconception. Acheron is more like a lake but was often called a river, whilst the River Styx wrapped around the Underworld nine times. In the original Greek and Roman sources, aswell as in Dante, it was Acheron that Charon crossed.
Education without values, as useful as it is, seems rather to make man a more clever devil. – CS Lewis