Passage 9.39.2
κεκόσμηται μὲν δὴ τὰ ἄλλα σφίσιν ἡ πόλις ὁμοίως τοῖς Ἑλλήνων μάλιστα εὐδαίμοσι, διείργει δὲ ἀπʼ αὐτῆς τὸ ἄλσος τοῦ Τροφωνίου ποταμὸς Ἕρκυνα. φασὶ δʼ ἐνταῦθα Ἕρκυναν ὁμοῦ Κόρῃ τῇ Δήμητρος παίζουσαν καὶ ἔχουσαν χῆνα ἀφεῖναι τοῦτον ἄκουσαν· ἐς δὲ ἄντρον κοῖλον ἐσπτάντος καὶ ὑπὸ λίθον ἀποκρύψαντος αὑτὸν ἐσελθοῦσα ἡ Κόρη λαμβάνει τὸν ὄρνιθα ὑπὸ τῷ λίθῳ κατακείμενον· ῥυῆναί τε δὴ τὸ ὕδωρ ὅθεν ἀνείλετο ἡ Κόρη τὸν λίθον καὶ ὀνομασθῆναι τὸν ποταμὸν ἐπὶ τούτῳ λέγουσιν Ἕρκυναν.
The city is adorned in other respects just like the most prosperous of Greek cities. A river, the Herkyna, separates from it the grove of Trophonius. They say that here Herkyna was once playing with Kore, the daughter of Demeter, and that as she held a goose, she let it go inadvertently. The goose flew away into a hollow cave and concealed itself under a rock. Kore entered the cave and retrieved the bird, which lay hidden beneath the stone. Then water, they say, began to flow forth from that very spot from which Kore had lifted the stone, and on account of this the river was named Herkyna.