Colorado and access laws are very tough. In your example, I believe he CAN deny access. I know when I float one certain river in Colorado, my driftboat or raft cannot touch shore or the bottom(including an anchor) or I am trespassing. I think in CO the land owner owns the river bed also, not just the land on both sides of the river. I'm only in CO about 6 times per year though. I'm sure someone from CO can clear this up more.