If the place is a huge lake or even an ocean, people could wait until the thick of winter to cross. By then, the low temperatures should have frozen the water surface hard enough for people to cross.
I'm gonna fail to quote the exact historic event here, but I remember seeing some hyphotesis that stated that people crossed continents like this (Europe to North America if I'm not mistaken) during a very harsh winter (this is just a theory though).
Depending on the place, low tides can reveal quite a lot of walking ground. Of course tides change rapidly, so people would both have to move fast and have specific places to wait out while the water is too high, but it's doable.
Maybe your story could have this rocky, seemingly endless, coastline where people would walk in low tides and hide in caves while the water's too high.
This is natural phenomenon that manages to cause quite the damage to what appears to be rock solid foundations. Maybe years and years of erosion could end up eating what was once a great natural barrier. Maybe it's not completely gone, but it's damaged enough to enable people to cross it.
If your scenario is canyon-like, a flood of biblic proportions could turn it into a huge lake. Yeah, I realize how ludicrous this is - It sounded awesome so I said it. You could also use the opposite of this (the drought), which was mentioned by Cyn in his answer.
I don't really see a forest as an obstacle, since you can push through a forest even if it's very dense (if you have the right gear, that is). But if your forest is dangerous (maybe inhabited by crazy perilous wild animals) this could be an answer. No forest, no problem.
I also thought about a vulcanic eruption, since lava turns solid in touch with water. I can't think of a specific situation where this would apply, but you have my permission to go nuts with this, if you want to.