Encyclopædia Britannica, Ninth Edition/Alpheus

ALPHEUS, (Ἀλφειός), the chief river of Peloponnesus, now called Rufia or Rouphi. Its sources are in the mountains of Arcadia, to the east of Megalopolis. Being fed by a great number of small streams, it becomes navigable, and traversing Elis, empties itself into the Ionian sea. At several points in its course it runs in a subterranean channel. This fact probably gave rise to the well-known myth which represents Alpheus, the river-god, as passing under the sea to the nymph Arethusa, who had been changed into a fountain in the island of Ortygia. Milton in his Arcades thus alludes to the story—

⁠⁠⁠“That renowned flood, so often sung,

Divine Alpheus, who by secret sluice

Stole under seas to meet his Arethuse.”