River widening: an approach to restoring riparian habitats and plant species

Abstract

'River widenings' are commonly used in river restoration to allow channel movement within a spatially limited area. Restoration seeks to restore fluvial processes and to re-establish a more natural riparian community. This study investigates the performance of five river widenings in Switzerland, focusing on the re-establishment of riparian (semi-)terrestrial habitats and species, and highlights some factors that seem to influence their performance. The restoration projects are compared with pre-restoration conditions and near-natural conditions, which are assumed to represent the worst-and best-case conditions along a gradient of naturalness. Fuzzy ordination of vegetation data and calculation of landscape metrics based on habitat maps revealed marked differences between the degree of naturalness achieved by each individual restoration project. However, in general river widenings were found to increase the in-stream habitat heterogeneity and enhanced the establishment of pioneer habitats and riparian plants. Analyses of species pools based on a hierarchic list of indicator species and correspondence analysis showed that the ability of river widenings to host typical riparian species and to increase local plant diversity strongly depends on the distance to near-natural stretches. Species dispersal and establishment might be hampered by decisions taken outside the scope of the restoration project. Therefore we conclude that action on the catchment scale is needed to maximize the benefits of local management.

Spatio-temporal analysis of fish and their habitat: a case study on a highly degraded Swiss river system prior to extensive rehabilitation

“…Habersack (2000) describes the enhancement of aquatic habitat conditions and the increased stock of juvenile grayling in local widenings of the river Drau, Austria. A comparison of the riparian vegetation in 5 river widenings in Switzerland revealed an increased degree of naturalness at both habitat and species level in most of the cases studied (Rohde et al, 2004;Rohde et al, 2005).…”

“…One measure that would enhance both the shoreline and the low instream habitat diversity and quality in formerly braided rivers is local river widening (Rohde et al, 2005). This would involve a signifi cant widening of the river bed (Habersack et al, 2000), possibly by a factor >2 of the existing channel width, in order to re-create a braided river morphology.…”

“…Apart from river-engineering advantages -stabilization of the bottom by decreasing the transport capacity of the widened channel (Hunzinger, 1998) -local river widening offers many ecological improvements through morphodynamical processes such as braiding and gravel erosion and deposition, thus leading to increased structural and hydrological heterogeneity. Up to now, local river widening has mainly been carried out in Switzerland, Germany and Austria (Habersack and Nachtnebel, 1995;Rohde et al, 2005;Völkl et al, 2002).…”

Socio-environmental implications of process-based restoration strategies in large rivers: should we remove novel ecosystems along the Rhône (France)?

“…backwaters, and resulting gains in habitats and biological communities cited in the following section (Rohde et al 2005;Habersack and Piégay 2007).…”

Dismantling artificial levees and channel revetments promotes channel widening and regeneration of riparian vegetation over long river segments

“…In this social context, this type of restoration actions has been employed much less often than needed, and its effectiveness in restoring riparian vegetation has not been frequently assessed (Gonzalez et al, 2015). The few evaluations of this restoration method published to date have shown increases in riparian habitat heterogeneity and establishment of pioneer riparian plants when compared with unrestored control sites (e.g., Florsheim and Mount, 2002;Gonzalez et al, 2017a;Gothe et al, 2016;Hering et al, 2015;Jahnig et al, 2009;Poppe et al, 2016;Rohde et al, 2005; note that in some of these papers the restoration actions are semantically confounded with the restoration goal as this restoration technique is generally referred as to "channel widening"). Surprisingly though, and despite recommendations (Bernhardt et al, 2007;Gonzalez et al, 2015), we are unaware of any study taking into account river conditions prior to restoration (before-after-reference design).…”