It’s gray because, yes, the forest preserve does own “property” with a public launch just down stream of the old guys property but the old guy does own both sides of the property for a stretch. So therefore I would think that he can exercise the same rights as the folks in Boone County and run barbed wire across the Kish if he so chooses to do so. So I do believe that the old guy could make an argument for your guessing of “good to go.”
At the end of the day the result is going to be the same as in the video I posted above whereas the public officer will just ask you or assist you in leaving the area to neutralize the situation until answers can be found, which will never happen because nobody really knows.
On a side note, when this problem and a couple others were brought to the forest preserves attention their reply was that they don’t have anything to do with the water that flows thru their land property, they only care about the land property.