I am guessing about what might be going on depending on how long you take to get to the dam there is an event that changes some of the locations at ~20 minutes into the run so that might be what’s going on. More likely is that you are getting there after the dam closes (thus allowing you to cross) and then opens again (locking the dam crossing if you haven’t made it across in any loop… I think, I haven’t actually been there for that long, like, ever). I think this happens at like, 15-18 minutes or something. Alternatively…you’re on the wrong version?
You're only able to cross the river while the dam is closed. It only stays closed for 2 minutes after the timer ends. So if you get there more than 2 minutes late, you won't be able to cross.
What does your map look like?