Assessing the Restoration Success of River Widenings: A Landscape Approach

Abstract

During the last 200 years, many rivers in industrialized countries have been modified by canalization. In the last two decades, the philosophy of river management has changed considerably, and restoration of ecological integrity has become an important management goal. One appealing restoration approach is to create "river widenings" that permit braiding within a limited area. This study presents a new and efficient framework for rapidly assessing such widening projects and offers a novel method to comparing restored sites with near-natural stretches (stencil technique). The proposed framework evaluates spatial patterns of riparian habitat types using landscape metrics as indicators. Three case studies from river restoration (river widening) in Switzerland are presented for demonstration purposes. The method compares restored sites with prerestoration conditions and near-natural conditions, which are assumed to represent the worst and best case states of a river system. To take into account the limited spatial extent of the restored sites, the so-called "stencil technique" was developed, where the landscape metrics of the near-natural reference sites are calculated for both the entire study area and smaller sections (clips). The clips are created by using a stencil that has the exact shape and size of the restored area (random window-sampling technique). Subsequently, the calculated metrics for the restored sites are compared to the range of values calculated for the near-natural data subset. Our studies show that the proposed method is easy to apply and provides a valid way to assess the restoration success of river widenings. We found that river widenings offer real opportunities for establishing riparian habitats. However, they promote mainly pioneer successional stages and the habitat mosaic of the restored section is more complex than at the near-natural reference sites.

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).…”

Impact des extractions de graviers dans le lit mineur sur la géométrie des zones aquatiques périfluviales du Doubs (France)

“…Le cours d'eau retrouve alors ses capacités d'auto régénération des habitats et de maintien d'espèces pionnières spécifiques des milieux riverains. Les élar-gissements résultant des extractions ont des effets assez semblables à ceux d'opérations de redynamisation de cours d'eau par érosion latérale (Rohde et al, 2004 ;Piégay et al, 2005). Ce constat vient donc conforter les conclusions de certains auteurs montrant les effets bénéfiques des extractions de granulats, notamment pour les populations avicoles (Fustec et Frochot 1995) et piscicoles.…”

Gravel bar inundation frequency: an important parameter for understanding riparian corridor dynamics

“…Riparian corridor restoration may involve the removal of river training structures to allow fluvial processes to be re-established in riparian corridors, physical restoration of channel morphologies through construction measures, removal of levees, bioengineering, grooming and enhanced planting of terrestrial species, and protection and preservation of wild areas (see for example Schropp and Bakker 1998;Roni et al 2002;Rohde et al 2004).…”