Abstract
Abstract. The district of the France Rhône basin is characterised by several braided reaches, preserved from the widespread disappearing occurred in the 20th century. Even if the overall pattern is evolving through a general river narrowing, some reaches have shown to be still active and have widened. The preliminary results suggest that differences in width pattern could be related to several factors, such as high magnitude and low frequency floods, and geographical position in the catchment which influences bedload delivery conditions and vegetation recruitment related to climate. From an initial set of 53 braided reaches, we selected 12 sites, distributed into four main hydro-geographical regions. Reaches were selected to be representative of the overall study area. We analysed the braiding width pattern and the vegetation pattern dynamic among five observation periods dating from the 1950s to the 2000s. We hypothesised that a comparative analysis of a detailed temporal trajectory (i.e. five dates) of a set of rivers within several hydro-geographical contexts would allow us to better distinguish the relative role of floods (in terms of magnitude and duration) and other controlling factors acting at the regional scale. We showed that active channel width is controlled mainly by Q10 flood and secondarily by bedload availability whereas island pattern is in large part independent of flood series characters. Moreover a clear regional differentiation, constant over time, in terms of riverscape response is observed, mainly opposing south-western and south-eastern reaches. This opposition depends on several concurring factors, i.e. the flood characters, the river activity, the human influence and the climate. Finally, these findings allowed us to highlight those sectors in which the braided pattern could disappear, and those sectors in which the braided pattern is still active, because critical processes responsible of channel dynamic are still present.
Highlights
Gravel-bed braided rivers are dynamic systems in which mainly flow, vegetation and sediment regime interact to form the rich mosaic of physical habitat characteristics of such fluvial landscapes
The preliminary results suggest that differences in width pattern could be related to several factors, such as high magnitude and low frequency floods, and geographical position in the catchment which influences bedload delivery conditions and vegetation recruitment related to climate
Reaches are selected considering a strict condition: having photos of good resolution and quality and according to sequence of periods with and without floods. These reaches are representative of the large subset (53 reaches) in Belletti et al (2013b) by including 2 to 4 reaches for each of the four main hydro-geographical regions, belonging to five sub-basins of the Rhône catchment (Fig. 1): four reaches are located in the North Alpine hydrogeographical region (N; upper Rhône basin); three reaches are located in the south-west of the study area, in the Rhône river corridor (W; lower Rhône basin); three reaches are located in the south-east of the study area (E; piedmont zone of the Durance basin); the Bans (Durance basin) and the Tinée (Var basin), are situated in the high mountain zone (HM)
Summary
Gravel-bed braided rivers are dynamic systems in which mainly flow, vegetation and sediment regime interact to form the rich mosaic of physical habitat characteristics of such fluvial landscapes. Major causes of decreases in European braided rivers can be attributed, for example, to gravel mining in Italy Surian et al, 2009; Comiti et al, 2011), to a combination of river channelization and gravel mining in the Polish Carpathian rivers The role of flow and sediment regimes on braided river evolution has recently been the object of several studies. The role of floods on long and mid-term braided river patterns has already been demonstrated; these studies generally focus on a single river The role of floods on long and mid-term braided river patterns has already been demonstrated; these studies generally focus on a single river (e.g. Zanoni et al, 2008; Comiti et al, 2011; Toone et al, 2012), on a small river subset (e.g. Surian et al, 2009) or on observations at smaller temporal scales (e.g. Hicks et al, 2008; Bertoldi et al, 2009; Welber et al, 2012).
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