Jess, you were right!

You were right, Jess! It was a car. 

If you look you can see that the whole frame of the car is there, floorboard and all...or what's left of it. I did a little internet research and all I could find were some shady urban-legend-like sources that said it was a car from a moonshiner who took a wrong turn while drunk and drove off the falls. Creepy, but a valuable lesson. Apparently there is another truck from the 50s that can also be seen around the falls. I'll have to look for that one next time we go.