As a simple earthquake event, you're not alone, civilization hasn't collapsed and disaster relief will consist of authorities collecting resources to ensure survival of all and reopening access somehow. You might get helicoptered out of a stretch of road that's isolated by bridge collapse or landslide at either end. Or high-lined over the river or something. But there would be rescue. Eventually. Your long-term survival isn't completely dependent on your own resources though getting back to work on time won't be a factor for your rescuers.
You mostly need the ability to wait it out and deal with your family and others who might be stuck with you but who were dressed and supplied for the beach they were heading for on their vacation.

This situation appears to me to be one where you'll have to take up temporary residence wherever you are until the damage is repaired. That river looks to be a natural barrier with a capital BARRIER!

Maybe you should look at how many places could be used to buy a charter aircraft ride from where you might get stuck to where you need to go? I'd find all the small airports along the route and then make some inquiries as to whether a charter service or someone you know who has an airplane could be retained just in case. (if they are allowed in/out of the area)

Bear in mind that while there may be a canyon airstrip, mountain/canyon flying requires special skills and only a limited assortment of aircraft are compatible with those places.

I'm thinking that backtracking to Lewiston and catching a flight out is vastly preferable to trying to walk 200 miles. Even waiting a month for the flight would be a better choice than walking 200 miles. (Cost is not being considered in my discussion)