Metamorphoses Book 5: Arethusa
The second muse's song was the story of how Arethusa became a river. She had been a water nymph and huntress of Diana's cult. One day after hunting, she went swimming in a quiet stream. This stream was Alpheus, and he fell in love with her. She fled naked from his waters, and he gained human form and chased her. She ran until she could run no more and then cried out for Diana's help. The goddess shrouded Arethusa in a cloud, but Alpheus, her pursuer, waited, watching the spot where her footprints disappeared. Arethusa was so afraid that she began to feel a cold sweat pouring off her and she became a river. Alpheus resumed his water form and joined his river with hers. Diana created a riverbed that allowed Arethusa to flow past Ortygia, an island sacred to the huntress and her followers, and that is how Arethusa ended up beside Cyane's stream.