When asked if he would like water in his whisky W.C Fields famously remarked that he didn’t drink water because fish procreate in it (his actual words were somewhat racier). Migratory salmon do so in their millions with a great deal of energy, specifically in the gravel beds of high-energy streams. Before spawning, females lash the stream bed with their tails to create a pit or redd in the gravel, in which they lay their eggs to be fertilised by males. Then she fills-in the redd with more gravel excavated from upstream. Salmon spawning grounds are thus easily recognised as pale patches of freshly overturned gravel on a stream bed that also contain lower amounts of fine sediment and are thereby loosened. As well as discouraging bibulous old men from diluting their liquor, it occurred to Alexander Fremier of Washington State University and other American colleagues that here was a noteworthy example of an active part of the biosphere physically intervening in the rock cycle. Not that it comes even close to what humans have become capable of since the Industrial Revolution, but it might be an object lesson in the fragility of what are otherwise the robust processes of erosion. Moreover, since salmon emerged at some time in the past, their actions might help demonstrate that evolutionary events – speciation, adaptive radiations, mass extinctions etc – play a role in transforming geological processes.
Fremier and colleagues (Fremier, A.K. et al. 2017. Sex that moves mountains: The influence of spawning fish on river profiles over geologic timescales. Geomorphology online publication; doi.org/10.1016/j.geomorph.2017.09.033) modeled the consequences of salmon spawning habits for the critical stress needed to set grains in motion, theoretically and in a flume tank. Based on a significant reduction of the critical stress, models for the evolution on various river profiles in the vicinity of salmon spawning grounds suggest that river beds may cut deeper at rates up to 30% faster than they would in the absence of salmon. Were salmon to be reduced or extirpated through dam construction or overfishing then sedimentation in channels would increase. In some areas of extensive farming of salmon in offshore pens, escape and colonization of rivers would eventually change sedimentation and erosion patterns. The findings vary from species to species, but salmon may have had a significant effect on generally rugged landscapes following their appearance in local ecosystems.
The terrestrial-marine-terrestrial migratory habits of salmon, including the return of adults to their birth rivers to spawn, are uncommon if not unique. Their forbears must have evolved to this behaviour at some time in the geological past, separately in the case of North Atlantic and North Pacific species. The authors suggest that adaptive radiation of salmon may have been favoured by orogenic events in western North America around 100 Ma ago that created the system of fast flowing rivers that salmon favour. In turn, salmon may have significantly influenced Western Cordillera landscapes of Alaska, Canada and the conterminous Unites States. A nice example of the inseparability of cause and effect on the scale of the Earth System.