Found in a waterway in Butte County, CA. First thought it was a beaver kit, but it has no tail. What aquatic mammal has no tail?
Looks like a mountain beaver (Aplodontia rufa) to me. Location, appearance, and minimal tail all match.
A bit more detail:
Where I'm from on Washington's Olympic Peninsula, they're not uncommon, but Butte County is near the southern end of their distribution (eol map page) and they may be less common there, and limited to cooler higher elevations (Beier 1989), and to wet habitats (their inability to concentrate urine (!) means they need to drink enormous quantities of water).
Arjo, Wendy M. (2007). "Mountain Beaver: A Primitive Fossorial Rodent". USDA National Wildlife Research Center - Staff Publications. Paper 675.
Beir, Paul (1989). Journal of Wildlife Management 53(3) 649-654