I can't speak to Nebraska, pinrider. But here in Montana there are a couple of possible scenarios.
1. Eagles dropping them when flying over after they have taken them from the river.
2. Critters dragging them from the river over the roadway leaving the remains.
3. (most likely) Local fishermen tossing the parts-pieces onto the road after cleaning the fish.

You see, my commute is along the Missouri River for a good portion of it. Refer to the thread below for the whole skinny on the rural part of my commute. http://www.bikeforums.net/showthread.php?t=59011

I can deal with the fish parts. I'm just happy that I have only come upon a very dead rattle snake and not a live ticked off one.