This situation screams for engineering innovation. We need a way to repair/replace bridges with less disruption. How about a civilian version of the portable Army bridge (Wolverine?). Something that could temporarily carry the load while the permanent span is repaired. It wouldn't work for the longest spans but we need creative solutions.
How about using old aircraft carriers? Use the flight decks as bridges. Two of them, one on each bank, with a temporary span between them to allow river traffic to pass. 800-1000 ft.each plus a 100+ ft span and you could bridge a significant gap.
the aircraft idea is cool. not sure if it would work, but the idea of driving across a carrier deck is an awesome feeling!