B. Räpple

What drives riparian vegetation encroachment in braided river channels at patch to reach scales? Insights from annual airborne surveys (Drôme River, SE France, 2005–2011)

Understanding riparian vegetation encroachment and its drivers is crucial for predicting alluvial river channel morphodynamics in different hydro-climatic contexts. We investigated fine-scale vegetation encroachment patterns and underlying physical controls along the Drome River, France, following a 50-year flood in 2003. Our study addressed the following questions: (a) What is the relative timing and the respective importance of vegetation patch recruitment versus patch expansion? (b) How well do regional/climatic factors explain the spatio-temporal patterns of recruitment and expansion versus local and reach-scale variation in geomorphic position and habitat? Along a 3-km reach, we analysed a chronosequence of high-resolution aerial images acquired yearly between 2005 and 2011. From each image series, we digitized patches of newly recruited vegetation and their lateral extent in following years to determine their expansion. This was complemented by elevation and plant structure data from airborne LiDAR data and field observations. The initial post-flood vegetated area doubled within 6 years, with a pronounced recruitment peak in 2006–2007. Patch expansion showed a positive linear relationship to growing season temperature. Vegetation encroachment was greatest in a slightly aggrading sub-reach compared to an incising sub-reach. Initially, it occurred principally along low flow channel margins and on bar surfaces, later in intermittent channels. Our results confirmed that extensive recruitment can succeed floods with recurrence intervals smaller than 1-in-5 to 10 years, when the flood history resets the geomorphic framework. Recruitment extent depends in part on local hydrological connectivity, whereas higher temperatures can boost plant growth where there is a perennial water supply.

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

River restoration efforts require interdisciplinary approaches involving fluvial geomorphology, hydraulic engineering, ecology, sedimentology, chemistry, social geography, and sociology. We investigated the functioning of artificial structures called “Casiers Girardon” (groyne fields) in the Rhône River. We assessed potential benefits and risks linked to removing the Rhône groyne fields in a restoration context, with particular focus on the potential for increased bank erosion. Hydraulic, morphological, chemical, ecological, and social issues resulting from dismantlement were studied for terrestrialized and aquatic Casiers Girardon. Only 10% of Casiers Girardon have maintained their aquatic features, whereas most of the Casiers are terrestrialized. Our results help to confirm the effectiveness of restoration actions; however, they also indicate uncertainties and additional knowledge needs, especially in regard to potential incompatibilities between Casier restoration and conservation. Then, an interdisciplinary conceptual model was developed to identify interventions to be considered in Casiers Girardon, according to their terrestrialization rate and physiochemical characteristics (connectivity, amount of gravel vs. fine sediment, contamination level). This model synthetizes scientific results and expert judgment and provides management recommendations based on ecological and sociological expectations about the restoration of Casiers Girardon. The model highlights high heterogeneity in functioning and ecological potential between terrestrialized and aquatic Casiers. Dismantling of terrestrialized Casiers has strong potential to provide multiple benefits, whereas aquatic Casiers could be maintained as valuable backwaters. The managing guidelines for the Casiers Girardon of the Rhône River should be adapted according to local conditions, as well as expected benefits and needs, and conducted in co-ordination with all actors involved in and affected by the restoration.