I actually fell in a river and lost a bunch of gear, but was with a larger group so others loaned me stuff. We were actually on our way out of the mountains, evacuating a person with a suspected concusion and walked 20+ miles out without stopping. I was carrying his gear on the top of my pack and toppled into a river as I was hopping rocks. I replaced my lost gear before going back in. We later found out he did not have a concusion, thank goodness.

So yes, people do fall into rivers! I did not loose my own pack, but the poor fellows pack was never found!