The early "D" scholia on the Iliad explain this as a reference to an unnamed river flowing past it; the geographical writer Stephanus of Byzantium explains it as a reference to the brief journey from the nearby town Thespiae to Thebes.
According to the "D" scholia, an unnamed river is referenced in this passage.  The geographical writer Stephanus of Byzantium suggests that the passage refers to a brief trip from Thespiae to Thebes.