Passage 8.14.3
διὰ μέσου δὲ ὤρυξεν Ἡρακλῆς τοῦ Φενεατῶν πεδίου ἔλυτρον , ῥεῦμα εἶναι τῷ ποταμῷ τῷ Ὀλβίῳ, ὅν τινα Ἀροάνιον Ἀρκάδων καλοῦσιν ἕτεροι καὶ οὐκ Ὄλβιον· μῆκος μὲν τοῦ ὀρύγματος στάδιοι πεντήκοντά εἰσι, βάθος δέ, ὅσον μὴ πεπτωκός ἐστιν αὐτοῦ, καὶ ἐς τριάκοντα καθήκει πόδας. οὐ μὴν ταύτῃ γε ἔτι κάτεισιν ὁ ποταμός, ἀλλὰ ἐς τὸ ῥεῦμα ἀπεχώρησεν αὖθις τὸ ἀρχαῖον, καταλιπὼν ἔλυτρον τοῦ Ἡρακλέους τὸ ἔργον.
Heracles dug a channel through the middle of the plain of Pheneus, to provide an outlet for the river Olbios—whom some of the Arcadians call Aroanios rather than Olbios. The length of this excavation is fifty stadia, and its depth, where it has not collapsed, reaches up to thirty feet. However, the river no longer flows through this channel, but has reverted again to its original course, leaving behind the channel as evidence of the labor of Heracles.