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When barriers are removed, what biotic and abiotic factors determine how fish populations will colonize newly available habitats? We used counts of adult pink salmon (Oncorhywnchus gorbuscha) from 1947 to 1987 in 66 streams of the Fraser River system, British Columbia, Canada, to determine when colonizing pink salmon populations became self- sustaining after a long-term migration blockage at Hell's Gate (river kilometre 209) was reduced. The abundance of salmon in available habitats were largely controlled by extrinsic factors such as an initially large source population, high intrinsic growth rates linked to favorable climate-driven conditions, a constant supply of dispersers, and large amounts of newly available habitat. Temporal variation in flows at Hell's Gate also affected recolonization success. Self-sustaining populations were developed within years of barrier removal and have continued to help expand the overall population of Fraser River pink salmon. However, pink salmon were considerably more abundant in the early 1900s than in the 1980s (~48 million vs. ~2.7 million), and the majority of spawning shifted from the historic areas above Hell's Gate prior to the rockslide to below Hell's Gate in the lower Fraser River after the long-term blockage was reduced, so the system has not returned to the former abundance and distribution patterns. Quels facteurs biotiques et abiotiques determinent comment des populations de poissons coloniseront des habitats nouvellement disponibles quand les barrieres en sont retirees? Les nombres de saumons roses (Oncorhynchus gorbuscha) adultes recenses de 1947 a 1987 dans 66 cours d'eau du reseau du fleuve Fraser (Colombie-Britannique, Canada) ont ete utilises pour determiner a quel moment les populations colonisatrices de saumons roses deviennent autosuffisantes apres la reduction d'un obstacle de longue duree a la migration a Hell's Gate (kilometre fluvial 209). L'abondance des saumons dans les habitats disponibles etait en bonne partie dictee par des facteurs extrinseques tels qu'une importante population source initiale, de forts taux de croissance intrinseque associes a des conditions favorables decoulant du climat, un apport constant d'individus se dispersant et l'abondance d'habitats nouvellement disponibles. Les variations temporelles des debits a Hell's Gate influaient egalement sur le succes de recolonisation. Des populations autosuffisantes s'etaient etablies apres quelques annees suivant le retrait des barrieres et ont continue de contribuer a l' augmentation de la population globale de saumons roses du fleuve Fraser. Cela dit, ces derniers etaient beaucoup plus abondants au debut du 20e siecle que dans les annees 1980 (~48 millions contre ~2,7 millions) et la majeure partie de l'activite de fraie s'est deplacee des frayeres historiques situees en amont de Hell's Gate avant le glissement de terrain vers des frayeres en aval de Hell's Gate, dans le bas Fraser, apres la reduction de l'obstacle de longue duree, de sorte que l'abondance et la repartition anterieures des saumons dans le systeme n'ont pas ete retablies. [Traduit par la Redaction], Introduction Barriers can limit the ability of fish populations and especially anadromous species to expand and occupy the full range of suitable habitats within river basins. Barriers can be permanent [...]

Debris flows are mass movements of sediment, wood and water down stream channels that profoundly impact streams and adjacent riparian areas and are a major erosion process in many steep mountainous terrains. Their impact on aquatic ecosystems, however, is poorly understood. In this report, we describe the ecological effects of a debris flow on a headwater stream in old-growth temperate forest that occurred in December 1999. This geomorphic disturbance scoured our study reach to bedrock and removed live trees 5 to 15 m into the adjacent riparian forest. Summer water temperature was higher in the affected reach after the debris flow than before; these changes have persisted through 2002. Algal biomass after the debris flow was similar to before; however, we speculate that algal growth was constrained by herbivory, as the abundance of two insect herbivores Chironomidae and Baetis spp. increased 380 and 9300%, respectively, within the first year after the debris flow. Overall, these data show that a rare disturbance event can mediate the population dynamics of stream herbivores in small, headwater streams such as those found in Olympic National Park likely via increased primary production and water temperature.