What is "mavroneri"

Mavronéri (Greek: “Black Water”) is a river identified with the River Styx of Greek mythology according to Hesiod’s description in Theogony. It is located near Nonakris in the Aroania Mountains of Arcadia on the Peloponnesian peninsula. Some scientists have speculated that the waters of the Mavronéri were used to poison Alexander the Great in 323 BCE.