ירמיהו, פרק נ״א, פסוק ס״ג

A practical instruction marks the conclusion of the prophetic message, directing exactly what must be done with the physical writings after they are read aloud. As soon as the reading of the small scroll holding these prophecies is complete [ביאור שטיינזלץ], a specific physical action is required. A stone must be tied to the scroll to weigh it down, guaranteeing that it will sink completely into the depths of the water [מצודת דוד, ביאור שטיינזלץ].

The weighted scroll is then thrown into the Euphrates River. While the Euphrates is frequently recognized throughout the Bible as the border of the Land of Israel, its mention in this context highlights that the river's flow extends all the way into the region of Babylon [רד״ק].