Avoid the Styx.
The Styx is a river in Greek mythology that formed the boundary between Earth and the Underworld (often called Hades). The rivers Styx, Phlegethon, Acheron, and Cocytus all converge at the centre of the underworld on a great marsh, which is also sometimes called the Styx. The important rivers of the underworld are Lethe, Eridanos, and Alpheus.
The gods were bound by the Styx and swore oaths on it. The reason for this is that during the Titan war, Styx, the goddess of the river Styx, sided with Zeus. After the war, Zeus promised every oath be sworn upon her. Zeus swore to give Semele whatever she wanted and was then obliged to follow through when he realised to his horror that her request would lead to her death. Helios similarly promised his son Phaëton whatever he desired, also resulting in the boy’s death. According to some versions, Styx had miraculous powers and could make someone invulnerable. According to one tradition, Achilles was dipped in it in his childhood, acquiring invulnerability, with the exception of his heel, by which his mother held him.
The rule is if you find a river with any of these names I’d take a hike or paddle elsewhere. The good news is that these Stygian rivers are hard to find and most rivers are of course benign, although some, naturally, can be dangerous.