Impacts of river bank stabilization using riprap on fish habitat in two contrasting environments

Abstract

Riverbank stabilization using rock riprap is commonly used for protecting road and bridge structures from fluvial erosion. However, little is known about how streams adjust to such perturbation or how this can affect fish habitat in different fluvial environments, particularly for non-salmonid species in small streams. The objective of this study is to assess impacts of riprap on fish habitat quantity and quality through a pairwise comparison of 27 stabilized and non-stabilized stream reaches in two physiographic regions, the Saint Lawrence Lowlands and the Appalachian highlands of Montérégie-Est (Quebec, Canada). Both quantitative (Hydro-morphological Index of Diversity, HMID) and qualitative (Qualitative Habitat Evaluation Index, QHEI) fish habitat assessment techniques are applied in order to compare results between methods. For each stream reach depth and velocity were measured to calculate HMID. In-stream cover (woody debris, overhanging vegetation, undercut banks, aquatic macrophytes) and habitat units (pools, riffles, runs, glides) were also documented and used to determine QHEI. Results show that overall bank stabilization using riprap at bridge and stream crossings alters fish habitat characteristics. Loss of in-stream covers and riparian vegetation lower QHEI scores at stabilized reaches, especially in more pristine Appalachian streams, but has less impact on already altered straightened Lowlands streams. In this latter context, some positive alterations of fish habitat were observed in riprapped reaches due to the coarsening of the substrate and an induced increase of slope. The two metrics (HMID and QHEI) revealed similar differences between stabilized and non-stabilized sites for Lowlands sites, but their level of agreement was much less in the Appalachian streams, suggesting caution when interpreting habitat quality results based on a single metric.

Partitioning of macroinvertebrate communities in a large New Zealand river highlights the role of multiple shore‐zone habitat types

“…Several other studies have highlighted that riprap can create novel habitats in low‐gradient rivers by introducing much larger substrate elements than occur on natural banks, which often consist of fine material with underwater structure from fallen trees (Brabender et al, ; Cavaillé et al, ; Eros, Toth, Sevcsik, & Schmera, ; Wensink & Tiegs, ). Riprap can also reduce wood recruitment and shade along river edges (Reid & Church, ; Massey, Biron, & Choné, ), although in this study, the combination of willow and riprap negated this effect. Shade along willow‐lined reaches, coupled with deciduous leaf fall, can be important bottom‐up drivers of macroinvertebrate community structure along vegetated shore‐zones, mediated by food‐web factors such as organic matter breakdown and algal growth, as reported in Australian streams by McInerney et al ().…”

“…Shade along willow‐lined reaches, coupled with deciduous leaf fall, can be important bottom‐up drivers of macroinvertebrate community structure along vegetated shore‐zones, mediated by food‐web factors such as organic matter breakdown and algal growth, as reported in Australian streams by McInerney et al (). Finally, riprap can be characterized by higher water velocity than other habitat types (e.g., Massey et al, ; Pander et al, ), but in this study, this was not evident at sampling points likely due to eddies created by large rocks upstream.…”

“…Several other studies have highlighted that riprap can create novel habitats in low-gradient rivers by introducing much larger substrate elements than occur on natural banks, which often consist of fine material with underwater structure from fallen trees (Brabender et al, 2016;Cavaillé et al, 2018;Eros, Toth, Sevcsik, & Schmera, 2008;Wensink & Tiegs, 2016). Riprap can also reduce wood recruitment and shade along river edges (Reid & Church, 2015;Massey, Biron, & Choné, 2017), although in this study, the combination of willow and riprap Finally, riprap can be characterized by higher water velocity than other habitat types (e.g., Massey et al, 2017;Pander et al, 2017), but in this study, this was not evident at sampling points likely due to eddies created by large rocks upstream.…”

A Q‐method survey of stream restoration practitioners in the Driftless Area, USA

“…Groups were divided evenly over the use of riprap; the tree removal and bank stabilization group and the active intervention group supported the use of riprap to reduce erosion, while the beyond‐the ‐ channel group and the post‐restoration monitoring and best available science group did not. The use of large rock riprap and dirting techniques to stabilize banks and reduce erosion has seen widespread use across the Driftless Area (Belby et al, 2019); however, this extensive use of riprap may actually cause additional erosion concerns, without addressing the underlying issue of the existence of post‐settlement alluvium, while also reducing hydrologic functioning and diminishing essential habitats (Massey, Biron, & Choné, 2017). In addition, the cumulative effect of widespread bank stabilization structures may limit riparian function and reduce important habitats for riparian species (Florsheim et al, 2008).…”

Creating wetland islands to enhance shoreline habitat for fish recruitment in a modified shallow lake

“…Shoreline hardening, with unnaturally steep land–water transitions, offers limited space for natural shallow‐area conditions, such as overhanging vegetation, input of terrestrial nutrients, and physical structures that function as feeding and refuge habitat for young fishes. Therefore, the diversity and density of young fishes are typically reduced in aquatic systems dominated by rip‐rap shores (Duncan & Kubečka, 1995; Kornis et al, 2018; Massey et al, 2017; Purcell et al, 2013).…”