Public vs private access is a big issue and the issue is far from clear in many cases, and varies from state to state.
Here’s a link to CA access laws- note that you may have access where a bridge or crossing over a stream exists and in many cases there might be pull outs for parking where there is an easement. Generally it seems that you’d have access along the streambed, and up to the high water mark on “navigable waters”. This does not mean you can cross private property to access water though, and you may have a problem parking even though there might be public access nearby.
American Whitewater - access:ca
Along with the excellent advice about getting specific info on access from CA F&G and fly only stretches, it’s always a good idea to pick up a Delormers Street Atlas to be able to scout around and find you way around. You can also find a lot of info on specific rivers on the internet once you’ve narrowed it down, and I’m sure that there are probably some good FF guidebooks on CA that folks on the board or in a local fly shop could recommend or you can do the google thing.