Sihor - The Nile. To lean upon Egypt was a violation of the principles of theocracy.
The two rivers are the two empires, and to drink their waters is to adopt their principles and religion. Compare also Isaiah 8:6-7.
Related Commentaries of Jeremiah 2:18
Jeremiah 2:18
18 And now what hast thou to do in the way of Egypt, to drink the waters of Sihor? or what hast thou to do in the way of Assyria, to drink the waters of the river?