Passage 8.22.8
λέγεται δὲ καὶ ἐφʼ ἡμῶν γενέσθαι θαῦμα τοιόνδε. ἐν Στυμφάλῳ τῆς Ἀρτέμιδος τῆς Στυμφαλίας τὴν ἑορτὴν κα τά τε ἄλλα ἦγον οὐ σπουδῇ καὶ τὰ ἐς αὐτὴν καθεστηκότα ὑπερέβαινον τὰ πολλά. ἐσπεσοῦσα οὖν ὕλη κατὰ τοῦ βαράθρου τὸ στόμα, ᾗ κάτεισιν ὁ ποταμός ὅς ἐστιν ὁ Στύμφαλος , ἀνεῖργε μὴ καταδύεσθαι τὸ ὕδωρ, λίμνην τε ὅσον ἐπὶ τετρακοσίους σταδίους τὸ πεδίον σφίσι γενέσθαι λέγουσι.
And they say that a marvel of the following sort also occurred in our time. At Stymphalos, they celebrated the festival of Artemis Stymphalia and generally conducted it without due reverence, neglecting to a great extent the established customs. Thus, wood having fallen into the mouth of the chasm through which the river called Stymphalos descends, prevented the water from flowing down, and it is said that a lake formed covering the plain to an extent of four hundred stades.