For every 100 wild salmon that used to run the rivers, two remain. It is easy to drown in data. But the solution is not complicated at all. Each big dam kills on average 10 percent of the total run of young salmon making their way to the sea. Spill causes O-3 percent per dam, mostly from nitrogen bubbles it produces.
Spill gives the runs a fighting chance.
To some extent, spill has been employed, though with much groaning and mumbling from power interests. They expressed public concern about fish getting the bends from those nitrogen bubbles. But water has often been spilled during wet years, when the mountains run with abundant snowpack. Those happen to be the years that produced the best return of salmon. Power interests did not complain in high-water years because their turbines were full.