Dragon Soul - You have the distances right. You, Hornet and Dok are 10' from the river-sde end of the gulley. But the downhill slope of a gulley doesn't typically have a wall across it. It is an open wash that has eroded. Across the bottom of this one is a very dense wall of vegetation. There is a door like opening in it that leads out towards the river. So it is a wall of vegetation, not stone.
But it really doesn't change your post. Technically someone could come through the wall but it would be very slow going. Shooting through it is not really possible. Someone could try and shoot up and over, but it would be a hard shot.
Also - if you are moving 30' per turn you will reach the rivers edge this turn if nothing else interferes. Or, if two of you are jointly hauling Dok you can move 45' and could get 15' out into the river.