This entry seems to need more info and clarity - the 'lost' bridge looks to be intact and being used for the southbound lanes of I-5, next to a parallel newer bridge for northbound lanes, and a third bridge in place for the current Dunsmuir Ave.
Streetview:
https://www.google.com/maps/@41.2209375,-122.2748366,3a,75y,...
Doug K- I think what you're looking at is a modern modern concrete arch built in 1955 Sacramento River Bridge BH 11578
https://bridgehunter.com/ca/siskiyou/20002/