Aplodontia rufa nigra
I checked the location for Point Arena mountain beaver (don’t remember who recommended it, but thanks!) at the end of Alder Creek Beach Rd. that branches of Hwy 1 just N of Manchester, CA. It worked! The place is now a bit overgrown, but I still managed to see this gorgeous black-and-white subspecies after less than 2 hours of waiting – try the area about 100 m before the terminal parking lot (the road is closed for traffic, so you have to walk for about a mile, but it’s a beautiful place and the views of sunset from the road’s end are superb). As an added bonus for birders, a brown shrike is now hanging out just below that parking circle at the road’s end.
But if you’ve never seen any mountain beavers, Lolo Pass in Oregon is still a bit better because you can watch dozens of burrows at the same time, rather than just a few, and you can sit in your car while waiting.