I am not sure that pushing the issue is the best tactic now, but at some point this may need to adjudicated. The easiest way would be for the state to open the season with a warning and then step back. Once someone got arrested, it would be up the tribe to prove they own the river. Unfortunately, it sounds like what is more likely to happen is the tribe will detain you and hand you over to the state, which will then prosecute for fishing out of season, preventing the ability to for this to ever go to trial.
I remember sitting in a counsel meeting years ago when they were passing a controversial law. They asked their attorney what their options were. He looked up and stated that there were some serious problems with the law, but that they could enforce it up to the the time someone fought it. At that point they could fight up to the court date and then drop the charges, effectively penalizing the defendant but never setting a precedent. It seems that is the approach the state and tribe are taking. Passing rules and laws that effectively mask the real question, whether the tribe has any right to the river. As long as the state does not open it, they effectively own the river. Opening the river up with a statement that no access is allowed, would then allow the legality of ownership to come up.