It isn't easy to find........and you can easily drive right by the opening and never even know it.
As you are heading up to the sandbar, it is on your right hand side. If I had to guess, I would say about half way up to the sand bar from the gorge (or thereabouts). If you have never been before, you want to go slow.......and be looking out for the buoys (either one or two of them, I forget). They are located at the opening which is no more than about 20' wide or so. Once you enter the buoy area.......continue on back following the trail to the right hand side.