Chico does not have any law on the books pertaining to public nudity. However it if found offensive and sexual (the definition to this is one that has seen much debate in the higher courts and is outside the jurisdiction of this wiki for now) then State and/or Federal laws can be ran afoul.
Therefore skinny dipping in Upper Bidwell Park is legal though the city does on occasion field complaints from those sensitive to it. Once in a while the City Council ponders the issue but so far they have not actively pursued ordinances against the activity. Often times freedom of religion or spiritual reasons are cited as reasons to not outlaw public nudity.