Instead of building a fish ladder only long enough to let migrating fishes go around just a single dam, how about one that bypasses not just that one dam or all the dams on the river but the entire river itself?
Starting at the mouth of the river, this fish ladder loops and curlicues a new hydrological continuum, passing through forests and hills and even cities, doing arabesques around mountains, sometimes on stilts and sometimes tethered. Like an aqueduct staggering through the plains of Latium.
Sometimes it flows besides its parent river, which has been nearly desiccated by global warming, Aral Sea-style irrigations or over damming by energy gluttons from faraway Los Angeles and Phoenix. Awaiting the fishes at the end is a reservoir replicating their ancient spawning grounds.
Along the banks of this artificial river valley, this infrastructure of pure ethology, might be a series of public viewing areas from which to view the endangered spectacle of wild animal migration. And/or catch dinner.