Fish Evolved to Climb Waterfalls with Its Mouth. One of the most iconic images of the Pacific Northwest is the salmon run, when salmon return from the ocean, swimming upstream to their place of birth in order to spawn. This requires tremendous physical effort, such as jumping up waterfalls. The salmon run is the time when salmon, which have migrated from the ocean, swim to the upper reaches of rivers where they spawn on gravel beds. After spawning, all Pacific salmon and most Atlantic salmon die, and the salmon life cycle starts over again. ... When they have matured, they return to the rivers to spawn.