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.”