In The Bible, where is the Yarmuk River mentioned in the Old Testament?
The Yarmuk wadi was a key geographical area in the times of the Old Testament, though it is never actually mentioned verbatim as far as I can establish. However, other geographical locations that lie close to the Yarmuk wadi definitely are mentioned. Just to take one example, consider 2 Samuel 9:1-8. In this passage, King David finds out that Mephibosheth, the lame son of his former best friend, Jonathan, Saul's son, is alive and well and residing at Lo Debar. This geographical location is east of the river Jordan and five miles south of the Yarmuk wadi.
Therefore, although the actual Yarmuk wadi itself is never mentioned as such in the Old Testament, it was definitely a key geographical area that was around during Biblical times and which Biblical characters would have known about and used as a landmark.