Assessing the Impact of Riprap Bank Stabilization on Fish Habitat

Riprap is a form of riverbank stabilization that is commonly used to protect road and bridge infrastructures from fluvial erosion.  However, little is known about how riprap can alter local hydro-geomorphology and how this can affect fish habitat. The objective of this study is therefore to assess potential impacts of riprap on fish habitat quantity and quality through a pairwise comparison of stabilized and non-stabilized stream reaches at 10 field sites in the Lowlands of Montérégie-Est (Quebec).  For each stream reach high-resolution measurements of velocity and depth were taken along transects. The proportions of in-stream cover and habitat units (pools, riffles, runs and glides) were also documented, longitudinally at the reach scale. Results demonstrated a significant (p<0.05) decrease in the abundance of woody debris, undercut banks and overhanging vegetation at stabilized reaches. Riprapped reaches also showed a significant decrease in the number of pools as well as the overall proportion of slower flowing water, compared to non-stabilized reaches. These results show the pertinence of altering riprap design in order to increase proportions of slower flowing water, which can be important refuge for fish during flooding events.