the river was... hungry?

For a while, growing up, Mom was in charge of the orienteering hike for girl's camp each summer. For us kids, that meant a family camping trip a week before girl's camp during which she would plot out the course for the orienteering hike.

The road to the campground followed and crossed a river as they both twisted up the mountain.  Each crossing was different. Some bridges crossed the lazy flow of a wide spot in the river. Further up, the bridge ferried us over small canyons which the river pushed itself through the rock sides forcefully. I remember at least one where the water fell from a cliff behind the bridge on its way eternally down the mountainside.

It was at this bridge we saw something strange: people. The people weren't strange. They were the usual backpack and hiking folks we often saw at this bridge. What caught our attention was that they were on the wrong side. While most people were looking up at the waterfall, these folks were staring down as the river disappeared into the forest.

And then it hit me! Excitedly, I told everyone, "It was THEIR backpack!" That was all it took. Our collective genes kicked in and all the Nelson kids spent the rest of the trip coming up with stories about how they lost their backpack.

What do you think was going on?