If it's a valley, there's wires waiting to pluck you out of the sky. You can conduct a thorough map recce, but not all wires are marked on maps (although these look pretty significant), you can do a physical recce if time permits, but if it's the first time down the valley, you need to assume the worst around every bend and adjust your speed and height accordingly. Not sure how far these in cockpit wire detector systems are from being viable, but until then, it's the Mark 1 eyeball. This is a very familiar story.