Strabo Geography 8.8.4

[4] I have already mentioned the marvellous circumstances pertaining to the Alpeius and the Eurotas, ¹ and also to the Erasinus, which now flows underground from the Stymphalian Lake, ² and issues forth into the Argive country, although in earlier times it had no outlet, since the “berethra,” ³ which the Arcadians call “zerethra,” were stopped up and did not admit of the waters being carried off so that the city of the Stymphalians ⁴ is now fifty stadia ⁵ distant from the lake, although then it was situated on the lake. But the contrary was the case with the Ladon, since its stream was once checked because of the blocking up of its sources; for the “berethra” near Pheneus, through which it flowed, fell in as the result of an earthquake and checked the stream as far down into the depths of the earth as the veins which supplied its source. Thus some writers tell it. But Eratosthenes says that near Pheneus the river Anias, ⁶ as it is called, makes a lake of the region in front of the city and flows down into sink-holes, which are called “zerethra”; and when these are stopped up the water sometimes overflows into the plains, and when they are again opened up it rushes out of the plains all at once and empties into the Ladon and the Alpheius, so that even at Olympia the land around the temple was once inundated, while the lake was reduced; and the Erasinus, which flows past Stympllalus, sinks and flows beneath the mountain ⁷ and reappears in the Argive land; and it was on this account, also, that Iphicrates, when he was besieging Stymphalus and accomplishing nothing, tried to block up the sink with a large quantity of sponges with which he had supplied himself, but desisted when Zeus sent an omen from the sky. And near Pheneus is also the water of the Styx, as it is called—a small stream of deadly water which is held to be sacred. So much may be said concerning Arcadia.

[4] We have spoken of the extraordinary circumstances relative to the Alpheius, Eurotas, and the Erasinus, which issues out of the lake Stymphalis, and now flows into the Argive country.

Formerly, the Erasinus had no efflux, for the Berethra, which the Arcadians call Zerethra, ¹ had no outlet, so that the city of the Stymphalii, which at that time was situated upon the lake, is now at the distance of 50 stadia.

The contrary was the case with the Ladon, which was at one time prevented running in a continuous stream by the obstruction of its sources. For the Berethra near Pheneum, through which it now passes, fell in in consequence of an earthquake, which stopped the waters of the river, and affected far down the veins which supplied its source. This is the account of some writers.

Eratosthenes says, that about the Pheneus, the river called Anias forms a lake, and then sinks under-ground into certain openings, which they call Zerethra. When these are obstructed, the water sometimes overflows into the plains, and when they are again open the water escapes in a body from the plains, and is discharged into the Ladon ² and the Alpheius, ³ so that it happened once at Olympia, that the land about the temple was inundated, but the lake was partly emptied. The Erasinus ⁴ also, he says, which flows by Stymphalus, sinks into the ground under the mountain (Chaon?), and reappears in the Argive territory. It was this that induced Iphicrates, when besieging Stymphalus, and making no progress, to attempt to obstruct the descent of the river into the ground by means of a large quantity of sponges, but desisted in consequence of some portentous signs in the heavens.

Near the Pheneus there is also the water of the Styx, as it is called, a dripping spring of poisonous water, which was esteemed to be sacred.

So much then respecting Arcadia.