Well I am filling it with quikcrete today, I really have no choice, no way to get equipment back there so we are hand trailering everything in. At least there is plenty of water for the quikcrete.
That's what I would do in your situation. If you just fill it with clay or soil it would seem to me you could be looking at the same problem again.
I wonder if some fencing set just above and below the water line in any vunerable bank would prevent digging in the future after you fill the hole? Maybe weight it down with stones, or possibly you wouldn't have to if it laid on the bottom?
