The rivers of Babylon refers to the waterways of the whole Babylonian country. These included the Euphrates and Tigris, as well as canals and related streams such as the Chalonas and Ulai.
The rivers of Babylon are remembered in Psalm 137 as the places beside which the exiles sat and remembered Zion.
The expression includes the extensive river and canal system of Babylonia rather than a single river alone.