Assessment of channel changes due to long-term bedload supply decrease, Roubion River, France

This paper presents the observations made in the watershed of the Upper-Roubion River (which drains a submediterranean mountainous environment) and in two of its main tributaries, the Soubrion and Bine Creeks. Our aim was to analyze contemporary morphological and sedimentary changes in response to a progressive decrease in bedload supply. This decrease is linked to an increase of forest cover in the watershed during the 20th century following rural depopulation and to a correlated decrease in peak flows. Several variables were systematically measured (width, slope, bed level change, sediment size) along 23 km of river; whereas dendrochronological data, floodplain cross-sections, and estimated recurrence intervals of sediment entrainment based on critical shear stress calculations were also produced for specific sites to complete our analysis. Evidence of three major phenomena was obtained on both the Roubion and its tributaries: (i) bed degradation, (ii) development of stable pavement in incised reaches, and (iii) channel narrowing associated with the development of alluvial forest on recently abandoned portions of the pre-existing active channel (forest development began in the 1950s). These three phenomena show positively covariant relationships that lead us to propose a descriptive model of mountain fluvial system adjustments to progressive decrease of bedload.