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, most Atlantic salmon die, and the salmon life cycle starts over again.
The salmon constructs a nest or "redd" in the gravel bed of a stream. The female creates a powerful downdraught of water with her tail near the gravel to excavate a depression. After she and a male fish have eggs and milt (sperm), respectively, upstream of the depression, the female again uses her tail, this time to shift gravel to cover the eggs and milt which have lodged in the depression. The young hatch in spring time whilst still under the gravel. They have yolk sacs which they use for food whilst buried in the redds. They emerge from the gravel about four to six weeks after hatching (depending on water temperature).
Unlike the various Pacific salmon species which die after spawning, the Atlantic salmon is iteroparous, which means the fish may recondition themselves and return to the sea to repeat the migration and spawning pattern several times, although most spawn only once or twice.