Board Thread:General Discussion/@comment-6302057-20160307065615/@comment-1916997-20160328044323

NickM98 wrote: Regardless of any speculation, as of 5x15 we can now canonically name four of the rivers:
 * River of Lost Souls - green (named in 5x14)
 * Styx - blue (shown in 5x14)
 * Cocytus - grey (shown in 5x14)
 * Lethe  - yellow (shown in 5x15)

Only the lava river (likely to be Phlegethon) remains. River of Lost Souls is more a title than a name, the name of that river is most likely Acheron.