Complex sentence: The plains are intersected by many wadis which are the only serious obstacles to the movement of mounted troops during the wet winters, when they are transformed them into "raging torrents" rushing down from the bare rocky Judaean mountains.
Split into simpler sentences: The plains are intersected by many wadis, which are the only serious obstacles to the movement of mounted troops. During the wet winters, they are transformed into "raging torrents," rushing down from the bare rocky Judaean mountains.