Passage 9.38.7
Θηβαῖοι δὲ τὸν ποταμὸν τὸν Κηφισόν φασιν ὑπὸ Ἡρακλέους ἐς τὸ πεδίον ἀποστραφῆναι τὸ Ὀρχομένιον· τέως δὲ αὐτὸν ὑπὸ τὸ ὄρος ἐς θάλασσαν ἐξιέναι, πρὶν ἢ τὸν Ἡρακλέα τὸ χάσμα ἐμφράξαι τὸ διὰ τοῦ ὄρους. ἐπίσταται μὲν οὖν καὶ Ὅμηρος λίμνην ἄλλως τὴν Κηφισίδα οὖσαν καὶ οὐχ ὑπὸ Ἡρακλέους πεποιημένην, καὶ ἐπὶ τῷδε εἴρηκε λίμνῃ κεκλιμένος Κηφισίδι· Hom. Il. 5.709
The Thebans say that the river Cephisus was diverted by Heracles into the plain of Orchomenus. Previously the river passed under the mountain and flowed into the sea, until Heracles blocked the cleft through the mountain. Homer, however, knows that Lake Cephisus existed naturally, not as a creation of Heracles, and for this reason he says, "dwelling by Lake Cephisus." (Homer, Iliad 5.709)