Looks like a cool spot. Get some people together and lease the land. Anytime an owner can make a few bucks on his land, he maybe willing to go for it. Where is this (which state are you in). Looks similar to here in Central California. Unfortunately, people get handed big sums of money when they sue you and win. Gates and no trespassing signs are much cheaper.

Sometimes land owners just put up signs in case someone tries to sue them. Otherwise, it really doesn't bother them that there are people on their land. You really don't know though unless you politely ask the guy. Which can be difficult to do for many reasons I understand. My bother ownes a property with a really big fishing pond. If he catches people trespassing he will run them off, but if someone asks him upfront he will usually let them go on and fish. It is all, how you ask sometimes. Even if the answer is no, you really haven't lost anything at this point. Good luck with it.