Quantifying the physical alterations of river reaches using a regional river morphology reference model. A step towards river restoration

International audience; The aim of this study is to quantify the physical alterations of alluvial rivers and reach a better understanding of the links between these alterations and the ecological quality (derived from the European Fish Index, EFI) of river channels. This is an essential step towards river restorations, which are increasingly being considered in order to meet good ecological status as defined by the European Water Framework Directive. The methodology of this study involves comparing physically altered river reaches (mainly reshaped rivers) to a reference model built from data collected on 28 non-altered sites in the chalky parts of the Seine Basin (northern France). The physical quality of the reaches is determined from eight morphological variables such as the bankfull discharge, the channel width, the width/depth ratio and the depth of pools present. These variables are estimated at the reach scale (i.e. about 20 times the channel width) from topographical surveys and discharge measurements. The hydrological regime (at mean and bankfull flow) and river substrate (sediment composition, size and transport rate) are also considered as morphological drivers. Our study shows that a reference model for natural river reaches may be built from their morphological characteristics in regions with homogenous geology and hydrology. Physically altered reaches have statistically different characteristics from the reference model. From biological data (relative to fish populations) the structure of the fish communities in natural and altered reaches was characterised and fish index was calculated, showing a link between physical alteration and ecological quality.