A retired firefighter said every Portland, OR bridge would cut off each part of the city. This report doesn't cover this explicitly, but it's pretty clear that is a major complications to overcome anecdotally.
Regular commute and business traffic would disappear, of course.
None of those things are true for the Columbia River separating Oregon and Washington. There aren’t many bridges and they’re very long. The river is non-trivial to cross. That would be a big mess.
(many bridges would have to go down for this to be the only way, of course)
I figure equipment to unload boats might be damaged, but the boats themselves should be able to come and go as before, no?
The result is that most buildings in Portland will survive. Only unreinforced masonry and liquefaction areas will have serious damage. The quake is survivable, the problem is infrastructure wasn't built with quake in mind.
The Oakland-San Francisco Bay Bridge was damaged in the quake, though other crossings: the Golden Gate, Dunbarton, San Mateo, Richmond-San Rafael, Carquinez Strait, and Martinez bridges, all survived intact.
Suspension bridges are also far more resilient than fixed (truss, viaduct) spans, though several of the structures that did survive are fixed designs.
Which is why word "rediscovered" was working hard in my earlier comment, rather than merely loitering as some may have suspected. Whilst there was some ferry service prior to the quake, existing route service was expanded greatly, and new routes added:
"Water Emergency Transportation Authority | Draft Final Transition Plan"
The Alameda/Oakland Ferry Service (AOFS) was started after the Loma Prieta earthquake on October 17, 1989, in direct response to the collapse of a section of the San Francisco-Oakland Bay Bridge and the nearly month-long closure that followed. the evening of the earthquake, private excursion vessel operators moved people across the Bay. By the following Monday, emergency funding had been secured and ferries were being operated between the San Francisco ferry Building, Oakland’s Jack London Square, and a temporary terminal at the foot of Main Street in Alameda. After the Bay Bridge was reopened, ferry service operated by Red and White Fleet and sponsored and funded by the city of Alameda, Port of Oakland, MTC and CalTrans was continued....
Vallejo’s current-day ferry service started in 1986 when Red & White fleet started limited commuter ferry service to and from San Francisco and midday service to marine world, which had just opened.... After the Loma Prieta earthquake in 1989, ferry ridership increased dramatically and remained high for several years. In 1994, the city of vallejo purchased their first high-speed ferry boat...
<https://weta.sanfranciscobayferry.com/sites/weta/files/weta-...>
A more general history at Wikipedia notes that Oakland, Alemeda, Berkeley, Richmond, and Treasure Island service all post-date the Loma Prieta quake: <https://en.wikipedia.org/wiki/Ferries_of_San_Francisco_Bay>
Lots of smaller bridges are going to come down. I don’t know why there isn’t more focus on them. They should be easier to retrofit. And at least have one in each area that will stay up to keep city connected. For example, I live near hospital but it is on other side of freeway and have no idea if will be reachable.
Every other bridge has been touched since the 80s/90s, but I don't have a lot of faith that, say, the Steel or Hawthorne will stay up.