So the vertical cut is the enforced "line"?? That would be workable in most cases. For 6 weeks it is the edge of the "Valley" meaning all those flooded hay meadows would be fair game, a millionaires' nightmare... the fishing public's wet dream with all those landlocked "Ox bow" ponds full of huge fish.
Nope, flooded hay meadows are NOT part of the ordinary high water mark because they are not free of fine substrates. Plus, they are beyond what is considered bank full events (ordinary high water mark).
Any event where water goes over bank full is considered a flood event and those lands would NOT be accessible to recreation use via a stream access law. You're confusing flood events with ordinary high water mark.
The oxbow issue, again pretty easy to define that the same way. If during a bank full event, the river is flowing into the oxbow, then you're within the ordinary high water mark. If the river does not flow into the oxbow during a bank full event, then the public has no legal access to it.
Finally, the crowding issues you're explaining would be lessened if the public had more public access to the States waterways. The reason that the areas you're describing are "trashed up" is because you're concentrating use to small areas. Still no reason for it, but it would seem to me that those areas, because they're so limited in nature, would also be very easy to enforce, provide trash cans to, and maybe even someone like you could schedule a yearly clean up.
For the record, I experienced the same thing in Idaho at popular steelhead fishing areas. I made recommendations to the city of Orifino to provide a dumpster, which they did, at these areas. I also made it a point to pick up a plastic Wal-Mart bag of trash every time I left the area. Pretty quick, it caught on and I noticed several other people doing the same thing. I also wasn't one bit afraid to tell people to clean up after themselves. It works.
It's just lame to use the trash idea as a crutch to stop the public from using their public waterways. There are lots of ways to mitigate all these issues if you care to take the lead...not so much if you're a follower and just like to bitch.