Geomorphological methods to characterise wetlands at the scale of the Seine watershed

Based on easily available morphological data within the Seine river watershed (76750 km2), two approaches were used for wetland delineation and characterisation. Their common assumption is that geomorphology largely governs the spatial distribution of wetlands, because it determines topography and the nature of deposits, thus water pathways and residence times. The first approach relies on the topographic index introduced by Beven and Kirkby [Beven KJ, Kirkby MJ. A physically based, variable contributing area model of basin hydrology. Hydrol Sci Bull 1979; 24: 43–69.], that has been widely used to characterise saturated areas in small catchments. We mapped this index for the Seine watershed using a 100 m resolution DEM typical of DEMs easily available at this scale. The second approach relies on a geomorphological classification of river corridors which was specifically developed for the Seine basin. It is based on genetic concepts, and defines 13 types of river corridors as a function of the geometry of the river bed with respect to bedrock (incised, aggraded, encased, stable), the nature of alluvial fills, and the small scale morphology in the corridors. We used geological, hydrogeological and topographical maps of the Seine basin to delineate the river corridors and characterise the type of all the comprising streams with 2 km resolution. Two cartographic sources that were not exploited by the above methods were used to assess their performances. The wetlands depicted on 1:25000 topographic maps cover 2% of the Seine basin but are limited. The waterlogged soils from two 1:50000 pedologic maps are more reliable, but these maps only cover 5% of the watershed. In the river corridors, most wetlands fall in the encased and aggraded subsystems of the geomorphological classification, where the mean of the topographic index is significantly higher than in the other subsystems. High values of the topographic index are good general indicators of wetlands, even when calculated from a 100-m DEM. The agreement between the two studied methods confirms that geomorphology is the major driving factor for wetland distribution, even in a sedimentary basin with a strong influence of aquifers on hydrology. These complementary methods provide a powerful tool to complement the gaps of classical wetland databases at the scale of large watersheds.