The streams are starving from lack of salmon, and that lack of salmon is hurting the ability of the next generation of salmon to survive. By going out into the ocean, feeding and storing nutrients, then returning to their stream of origin to spawn and die enriches the habitat, giving the young salmon a better chance for survival. Salmon really are a keystone species. For example, the reestablishment of the grizzly bear may be impeded by not having the primary food source it evolved with. The abundance of all these things is interrelated.
Pacific salmon are the food base for much of what lives in the watershed.
When the salmon disappear, their importance becomes evident. Cederholm points to the example of McDonald Creek in Glacier National Park. In 1981, more than 600 bald eagles gathered to feed on kokanee salmon carcasses. By 1989, only 25 eagles were found at McDonald Creek. But behind it, it's the whole ecosystem.