Hydrogeomorphological alteration and ecological quality of three small peri-urban rivers in the Paris metropolitan area
Altération hydrogéomorphologique et qualité écologique de trois petites rivières périurbaines de la métropole parisienne
Résumé
Even though they represent 80% of Ile-de-France's hydrographic network, small peri-urban rivers are poorly studied. These low-energy and small-scale rivers, whose morphological adjustments happen in the long run, are distinguished by the plurality of (public and private) actors mobilised, by the fact that they escape state control and by the diversity of the geographical situations in which they flow. Peri-urban areas are mainly affected by both the intensification of agricultural practices and urban sprawl, the latter growing the risk of runoff by increasing the number of artificial hydraulic networks and impermeable surfaces. The rising number of urban and agricultural discharges leads to significant ecological degradations. This study focuses on three rivers in the Île-de-France region: the Biberonne, the Mérantaise and the Morbras. Two methods were used to determine the relationship between their hydrogeomorphological and ecological alteration: the first one is CARHYCE (Caractérisation Hydrogéomorphologique des Cours d'Eau), and the second one is I2M2 (Indice Invertébrés Multimétrique) which assesses the ecological quality of a river after its populations of benthic macroinvertebrates and their habitats. The width/depth ratio of the three rivers shows a tendency of the channel to incise itself with almost no bottom load (apparent bedrock). Depletion of the macroinvertebrate populations indicates poor water quality, and appears to respond to a gradient of local physical conditions. However, it is still necessary to describe more precisely and explain the relationship between hydrogeomorphological and biological alteration. Nevertheless, the results show the weight of urban transformations on contemporary alterations.