Laughing Bear is right about naturally exposed roots, but in this case the erosion of the bank on which the trees grow is causing some rather severe exposure. One willow tree (a non-native) has roots exposed about halfway around the base of the tree, and it has gotten worse in the last year. Human activity probably speeds up the erosion of the bank, and the trash buried beneath it keeps surfacing. I'm not sure what we can, or should, do about this. Rebuilding the bank would take extensive work and require some sort of retaining wall, which I seriously doubt the USFS would be in favor of. Simply pouring dirt over the roots will not help, and the dirt would quickly be eroded away again.
Hopefully this tree will not be falling down soon.
It is possible that some recent users have added some fresh trash to the old buried trash under the bank.