Causes of 20th century channel narrowing in mountain and piedmont rivers of southeastern France

Abstract

Extensive channel narrowing in southeastern France provides an illustration of geomorphic response to land-use changes. The study region comprises a range of environments, from large piedmont and intramountain gravel-bed rivers, to small mountain streams. Field measurements and analysis of historical data demonstrate two distinct periods of channel change. From 1850 to 1950, channel narrowing is interpreted to be the result of a recovery process in response to widespread channel destabilization induced by major floods during the second half of the 19th century. At the time, the largely deforested basins were highly responsive to flooding, whereas the recovery process was accelerated by floodplain and basin-scale land use changes (afforestation) and torrential control works, which in turn reduced sediment delivery and enabled vegetation development in channels. From 1950 to 1970, channel narrowing accelerated in most of the studied rivers. This recent phenomenon is considered as a human-induced fluvial adjustment, directly related to forest development on river margins and human abandonment of intensive floodplain land uses. At the same time, long-profile degradation occurred as a result of long-term bedload supply decrease. In small mountain streams, channel narrowing is mainly explained by channel incision which seems to progress from upstream where sediment sources are progressively stabilized by afforestation.

Channel degradation and slope adjustment in steep streams controlled through bed sills

“…This bed degradation or channel incision has been reported as an extensive trend in alpine and Mediterranean environments, for example in Italy (Surian and Rinaldi, 2003) and southeast France (Liébault and Piégay, 2002). Mining, channelization and dams are the main causes in the former case, while a decrease in bedload supply due to land-use changes is the main cause in the latter.…”

Variabilidad espacial de los cambios de cauce en una rambla mediterránea en las seis últimas décadas (1946-2006)

“…In French rivers, channel narrowing in the first half of the twentieth century has been associated with decreased discharge and sediment supply at the end of the Little Ice Age. In contrast, channel reduction in the second half of the twentieth century is considered a human-induced fluvial adjustment (Bravard et al, 1997;Liébault and Piégay, 2002;Arnaud-Fassetta, 2003;Liébault et al, 2005). In Italy, Surian et al (2009) detected small width changes during the nineteenth century, but with no significant trend.…”

“…Causal interpretation has often been addressed through retrospective analyses of anthropogenic actions such as dam construction, reforestation, torrent control works, or river channelization (Bravard et al, 1997;Rinaldi, 2003;Surian and Rinaldi, 2003;Surian and Cisotto, 2007;García-Ruíz and Lana-Renault, 2011). Gravel extraction has also had a profound impact on rivers around the world (Kondolf et al, 2002;Liébault and Piégay, 2002;Surian and Rinaldi, 2003;Piégay et al, 2004;Rinaldi et al, 2005;Wishart et al, 2008;Surian et al, 2009).…”

“…Mean incision values have been estimated in the Italian perennial rivers (3-4 m) (Surian and Rinaldi, 2003;Surian and Cisotto, 2007;Rinaldi et al, 2009;Surian et al, 2009;Preciso et al, 2012;Gurnell et al, 2012), French rivers (1-5 m) (Bravard et al, 1997;Liébault and Piégay, 2002;Arnaud-Fassetta, 2003;Liébault et al, 2005), and Spanish rivers (0.6-5.5 m) (Boix-Fayos et al, 2007;Martín-Vide et al, 2010;SeguraBeltrán and Sanchis-Ibor, 2013). These values range in the same order of magnitude as those observed in ephemeral streams in the USA (Rinaldi et al, 2005;Cadol et al, 2011) and Israel (Rozin and Schick, 1996).…”

Transferts sédimentaires dans le Bas-Rhône depuis le milieu du 19e siècle : essai de quantification

“…1A), comme la plupart des systèmes fluviaux (Schumm, 1977 ;Milliman et Meade, 1983 ;Bravard, 1989 ;Knighton, 1998 ;Syvitski et al, 2005), a été soumis à des changements importants qui ont modifié l'afflux sédimentaire et ses conditions d'écoulement : réduction de la fréquence des fortes crues avant la fin du Petit Âge Glaciaire (Pichard, 1995(Pichard, , 1999, diminution de la charge solide liée aux changements de l'occupation du sol et aux ouvrages hydroélectriques (Peiry et al, 1994 ;Miramont et Guilbert, 1997 ;Liébault et al, 1999 ;Liébault et Piégay, 2002), transformation de la géométrie du chenal sous l'impact des aménage-ments pour la navigation et la protection contre les crues (Poinsard, 1992 ;Poinsard et Salvador, 1993). Ces forçages climato-anthropiques ont entraîné le réajustement morphologique des lits fluviaux et une modification des bilans sédi-mentaires dans le Bas-Rhône (Arnaud-Fassetta, 2003 ;Maillet et al, 2006a).…”