Could some kind soul have a look here. I think I have the essentials. TM is discussing the usage of a word in the plural form when describing the beginning or end of a river, and he uses the Acheloios and Nile as examples.
Thomas Magister, Ecloga nominum et verborum Atticorum Letter Pi 302.11-14
Πηγαί λέγονται πληθυντικῶς ἐπὶ τῆς ἀρχῆς τῶν ποταμῶν, οὐ
πηγή, οἷον αἱ πηγαὶ τοῦ Ἀχελῴου καὶ αἱ πηγαὶ τοῦ
Νείλου· καὶ ἐκβολαί αἱ εἰσελεύσεις αὐτῶν εἰς τὴν θά-
λασσαν.
Running waters, they say in the plural, of the beginning of rivers, not running water, such as the running waters of the Acheloios and the running waters of the Nile. The same is likewise true concerning the mouth of the river flowing out to the sea.
That last clause I’m really uncertain about but I think I’ve captured its essence.