Major transportation route in the present day is Interstate 80 for east/west traffic. You have the coastal area, then the Central Valley, which takes you to about Auburn, and then you start climbing the Sierra Nevada mountain range. Central Valley is lush in vegetation and farmlands, and runs north/south in direction.
The Sierra Nevada on their western slopes are heavily timbered, while on their eastern slopes are less so and rather dry. Significant amounts of the Sierra Nevada are above the timber line and it's not unusual to find pockets of snow well into the summer.
There are definitely places in the Sierra Nevada where the only means of transport are either foot or horse. But he'd better be very experienced in riding if he tries this area. It's not unknown in this area for riders to lose their horses down the slopes, and about the only thing you can do is shoot the horse and hike out.
There are very specific Indian tribes in this area, and there is a vast difference between the tribes and how they deal with Whites, more so then, but even now. In the present day, this is more noticeable east of Sacramento, and the further you get towards Nevada, the more pronounced this becomes.
There are a lot of books on both the Central Valley and the Sierra Nevada mountains with lots of pretty pictures. It's also a great place to go on vacation. There are also quite a few members of AW in this area. Take a look at
AW Members Listed by... You might want to try sending them a PM.
Best of luck,
Jim Clark-Dawe