A salmon run is an annual fish migration event where many salmonid species, which are typically hatched in fresh water and live most of their adult life downstream in the ocean, swim back against the stream to the upper reaches of rivers to spawn on the gravel beds of small creeks.

In Northwest America, salmons are keystone species, which means the ecological impact they have on other wildlife is greater than would be expected in relation to their biomass.

Most salmon species migrate during the autumn (September through November)[failed verification],[2] which coincides with the pre-winter activities of many hibernating animals.

The annual salmon run can be a major feeding event for predators such as grizzly bears and bald eagles, as well as an important window period for sport fishermen.

The post-spawning death of salmon also has important ecological consequences, because the significant nutrients in their carcasses, rich in nitrogen, sulfur, carbon and phosphorus, are transferred from the ocean and released to inland aquatic ecosystems, terrestrial animals (such as bears) and the wetlands and riparian woodlands adjacent to the rivers.

[6] In the early 19th century, Chinook salmon were successfully established in the Southern Hemisphere, far from their native range, in New Zealand rivers.

[16][17] As they approach the time when they are ready to migrate out to the sea the parr lose their camouflage bars and undergo a process of physiological changes which allows them to survive the shift from freshwater to saltwater.

Smolt spend time in the brackish waters of the river estuary while their body chemistry adjusts their osmoregulation to cope with the higher salt levels they will encounter in the ocean.

They then spend up to four more years as adult ocean salmon while their full swimming ability and reproductive capacity develop.

[16][17][18] Then, in one of the animal kingdom's more extreme migrations, the salmon return from the saltwater ocean back to a freshwater river to spawn afresh.

[19] After several years wandering huge distances in the ocean, most surviving salmon return to the same natal rivers where they were spawned.

[23] In 1978, Hasler and his students found that the way salmon locate their home rivers with such precision was because they could recognise its characteristic smell.

[29] In 1973, it was shown that Atlantic salmon have conditioned cardiac responses to electric fields with strengths similar to those found in oceans.

All their energy goes into the physical rigours of the journey and the dramatic morphological transformations they must still complete before they are ready for the spawning events ahead.

The run up the river can be exhausting, sometimes requiring the salmon to battle hundreds of miles upstream against strong currents and rapids.

[5] Chinook and sockeye salmon from central Idaho must travel 900 miles (1,400 km) and climb nearly 7,000 feet (2,100 m) before they are ready to spawn.

[39] The height that can be achieved by a salmon depends on the position of the standing wave or hydraulic jump at the base of the fall, as well as how deep the water is.

[40] Data suggest that navigation locks have a potential to be operated as vertical slot fishways to provide increased access for a range of biota, including poor swimmers.

[41][clarification needed] Skilled predators, such as bears, bald eagles and fishermen can await the salmon during the run.

To lay her roe, the female salmon builds a spawning nest, called a redd, in a riffle with gravel as its streambed.

She builds the redd by using her tail (caudal fin) to create a low-pressure zone, lifting gravel to be swept downstream, and excavating a shallow depression.

[60] This is because upstream freshwater bodies (especially creeks) typically do not have sufficient food available for the adult salmon diet, and they have used large amounts of energy swimming upriver, thus exhausting their own internal nutrient reserves.

[65] It is an evolutionary strategy that concentrates all available resources into maximizing reproduction, at the expense of individual organism's life, which is common among insects but rare among vertebrates.

[62] All six species of Pacific salmons live for many years in the ocean before swimming to the freshwater stream of its birth, spawning, and then dying.

[66] The bodies of salmon represent a transfer of nutrients from the ocean, rich in nitrogen, sulfur, carbon and phosphorus, to the forest ecosystem.

It has been estimated that bears leave up to half the salmon they harvest on the forest floor,[67][68] in densities that can reach 4,000 kilograms per hectare,[69] providing as much as 24% of the total nitrogen available to the riparian woodlands.

[3] Salmon continue to surprise us, showing us new ways in which their oceanic migrations eventually permeate entire terrestrial ecosystems.

A 2010 study suggests the density and diversity of many estuarine breeding birds in the summer "were strongly predicted by salmon biomass in the autumn".

[73] Anadromous salmon provide nutrients to these "diverse assemblages ... ecologically comparable to the migrating herds of wildebeest in the Serengeti".

Among the key driving factors are (1) harvest of salmon by commercial, recreational, and subsistence fishing, (2) alterations in stream and river channels, including construction of dikes and other riparian corridor modifications, (3) electricity generation, flood control, and irrigation supplied by dams, (4) alteration by humans of freshwater, estuarine, and marine environments used by salmon, coupled with aquatic changes due to climate and ocean circulatory regimes, (5) water withdrawals from rivers and reservoirs for agricultural, municipal, or commercial purposes, (6) changes in climate caused at least in part by human activities, (7) competition from non-native fishes, (8) salmon predation by marine mammals, birds, and other fish species, (9) diseases and parasites, including those from outside the native region, and (10) reduced nutrient replenishment from decomposing salmon.