Face downstream, so the bunker is on Lara's left. Climb onto the wooden ledge on the right side of the stream.
Follow the ledge around to the right.
Jump the gap and scramble up the wooden wall on the right.
On top of this rock formation are more arrows and a GPS Cache (1/5).
Jump down off this ledge. You wind up just uphill from the bunker. Head downhill until you are once again standing in the stream bed between the overturned truck and the wooden ledge.