Found some interesting info on the subject at http://www.adventuresports.com/river/nors/us-law-who-owns.htm

If the physical characteristics of the river are such that it meets the federal test of title navigability, it is public land up to the ordinary high water line.

1. The waterway must be capable of or susceptible to use as a highway for the transportation of people or goods;
2. The waterway must be usable for transportation conducted in customary modes of trade and travel on water;
3. Waters must be navigable in their natural and ordinary condition; and
4. Navigability is determined as of the date of statehood.


I know nothing about the area in question, but if that stream was navigable as of June 1, 1792, then those signs are on public land. Another good point that web site makes though (and I'm sure you already know this) is that even though the law is on your side, ya gotta pick your battles. Good Luck!