Years ago I was the Government Documents section at the University Library doing research for a Geology paper. Got bored and just started poking around in the stacks.
I found a document that listed all of the natural thermal waters in the state of CO. Temps, mineral content and flows, maps. Done about 1932.
One of my friends that I was going to school with was hired to do a survey in AZ if the hot springs.
I'm sure you can find one for CA.
Yes, I did put it back in the stacks.
I was a librarian before I went to college after all.
I didn't mean to jack your thread. it was about your adventure after all and I just wanted to say with a bit of investigation you can find out of the way places that won't have lots of other people. Then, again, you may not meet some of the more eccentric ones either...
It makes me want to go find some more hot springs to soak in.
Monkey.