Salmon change river appearance

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Spawning salmon are capable of considerably altering the appearance of streambeds, according to research published in the journal Geophysical Research Letters.

Scientists used sediment traps to track the movement of pre-placed magnetised particles and detailed channel maps, to study the physical effects of salmon on four mountain streams in British Columbia.

Spawning salmon construct a nest called a redd by excavation with their tails; in the course of excavation, some of the material is moved downstream, gradually altering the appearance of the streambed.

The authors found that up to 60% of the annual amount of sediment migration in a given stream is caused by salmon redding, leading to deepening of channels in the headwaters and the gradual filling in of pools and channels downstream.

The authors conclude egional physiographic change is thought to have spurred the evolution of the Pacific salmon, but our results suggest that conversely the rise of mass spawning salmon may have also influenced channel morphology and sediment transport, and potentially reach slopes and hence to some degree perhaps even the physiographic evolution of the region.

Moreover, efforts to recover salmon stocks both in Europe and North America through hydrogeomorphic (habitat) restoration would be further complicated if, as suggested here, historic river morphology and dynamics were greatly influenced by larger numbers of mass-spawning fish.

For therein lies a potential conundrum for restoration ecologists: what must one recover first, the fish or the stream?

For more information, see the paper: Hassan, MA, AS Gottesfeld, DR Montgomery, JF Tunnicliffe, GKC Clarke, G Wynn, H Jones-Cox, R Poirier, E MacIsaac, H Herunter and SJ MacDonald (2008) Salmon-driven bed load transport and bed morphology in mountain streams. Geophysical Research Letters 35, L04405, doi: 10.1029/2007GL032997.