The Blair Witch Project/WMG

The kids were being harassed by the Blair Witch Cult
Pretty much every single thing can be attributed to something not supernatural. The little twig figures, the stones, (mostly) everything was done by the remnants of the Blair Witch-worshipping cult commune that was disbanded in the 60s, mentioned by Josh in one of the movie's deleted scenes.
 * Change that "pretty much every single thing" to a solid "every single thing". The same thought occurred to me: why else would they have deleted such a brief and otherwise harmless scene, unless they thought that it gave away too much?
 * Um, then how did the Blair Witch Cult magically cause them to come across the same river twice?
 * They are really bad at outdoor survival and navigation. After all, they threw their map in a river.

The kids are being stalked by one of the Gentry from Changeling: The Lost
They inadvertently walk through a rather nondescript portal to the hedge early on in the film. The rest of the movie is the Keeper who owns the realm stalking them, planning to add them to it's collection. Reality warping is perfectly within a True Fae's powers, and it explains why they come across the same river twice. Josh is the only one who survives the movie ironically, becoming a changeling when he disappears, and then luring his friends to their deaths at the hands of the True Fae.
 * I had the exact same thought. Of course, I figured they all became changelings, though the film ended before it was shown.
 * Well knowing how they were and what they were after or being chased by...they probably got ripped into becoming Darklings; the dark-woodlands of perpetual darkness and spiky-trees sure do LOOK like the Hedge, Autumn Court Changlings for the WIN!!

The kids are being chased by Slendy

 * ...holy CRAP, that makes sense.
 * This is now my personal canon.

The Blair Witch is the Demon.
Clearly, one of these kids has very naughty ancestors. Both the witch and the demon have similar motives and like to play with their prey. It eventually decides to stop dicking around, possess Josh, and then lure his friends to their deaths for an encore.