In a way, your body is a system. If any part has problems, other parts, or your entire body, may suffer. The system salmon are part of is huge. With many species, it stretches from the mountain headwaters of streams all the way to the ocean. With some species, it stretches thousands of miles into the ocean.
Ducks move in to feed on a dead salmon in the Willamette River.
Wild salmon are hatched and live part of their lives in a watershed. A watershed is the area drained by a distinct stream or river and is separated from other watersheds by topographic boundaries such as ridgetops. From the ecosystem perspective, the uplands and floodplain and riparian zone all affect the life in the stream. The surroundings farther down the stream affect it, too. Generally speaking, a good salmon stream has relatively cool and clean water, although the fish can adapt, within limits.