How constructive is channel reconstruction?

Posted on January 5, 2013

Channel reconstruction designed to improve habitat conditions for particular species can have unforseen consequences.  Attempts to improve salmon spawning habitat typically involve the removal of fine substrate particles to reduce smothering of eggs and juveniles, the removal of boulders to increase the available area for nest construction, and the addition of gravels to keep the spawning beds functional.  Because these changes reduce the range of particle sizes and increase substrate mobility they could prove to be  counterproductive if they have negative impacts on the invertebrates that provide food for salmon and other consumers.  A study on the Merced River, California, monitored a restored reach and the adjacent unrestored upstream reach for over a year and carried out an experiment to investigate the effects of in situ manipulation of river bed substrates.  Compared to the pebbles in the reference reach, those in the restored reach were more homogenous in size and much more mobile under flood conditions.  Invertebrate biomass in the restored reach was only 62% of that in the unrestored reach.  Filter-feeding caddisflies (Hydropsyche) were the dominant species in the unrestored reach, but grazing mayflies (Baetis) were dominant in the restored reach.  In the field experiment, which examined the relative effects of substrate homogeneity and mobility, small baskets containing particles of various sizes were buried flush with the river substrate and macroinvertebrates colonising the baskets were sampled about four and a half months later.  The influence of substrate heterogeneity was investigated by comparing (a) baskets containing pebbles having the median rock size for the restored reach with (b) baskets containing pebbles with the normal range of particle sizes for the same reach.  The effect of substrate mobility was examined by mixing and turning rocks in some of the baskets on two occasions to simulate the action of floods.  The results showed that homogenous, mobile substrates encouraged invertebrate communities to shift towards those found in the restored reach: Baetis abundances fell in response to increased substrate homogeneity, but Hydropsyche abundances dropped disproportionately more, due to increased substrate mobility.  These results illustrate how channel restoration can have unintended consequences for non-target species and life stages.

Reference:  Albertson, L.K. et al.  2011.    Impacts of channel reconstruction on invertebrate assemblages in a restored river.  Restoration Ecology 19 (5), 627–638.