The quantification and characterisation of hydrological and sedimentary transfers are now major challenges, not only in terms of knowledge of hydrosystems way of functioning, but also for the management of the environment and its natural resources. Indeed, the consequences associated with these transfers can be catastrophic for land ecosystems and aquatic biodiversity as well as for people, when they are accompanied by transfers of pollutants, flooding or pollution. These are so many issues critical to preserving and restoring water resources and soils that are now one of the government's environmental priorities (Law on Water; Water Framework Directive, WFD). While there are questions on watercourse and watersheds managements, we conducted a study focussed on typical feature for low-energy water system (<15 W/m²) in a large quarter of NW France, much less documented than in the southern part of the country, subject to transfers and specifically high levels of erosion rates. Indeed, if studies have attempted to assess the degree of soil erosion at the plot scale, to analyze the dynamics of runoff on slopes or to quantify the volume of sediment exported from watersheds, it remains difficult to analyze sediment dynamics into the catchment. Therefore, to improve the knowledge of these systems and to go beyond the "black box" study of the way of the catchment works, a network monitoring suspended sediment transfers was set up in the Seulles watershed (430-km²), representative of hydrosystems observed in Lower-Normandy. It has monitored SS fluxes for several years in different geological, geomorphological contexts and landscapes and identified the role of interlocking scales on the spatial and temporal dynamics of sediment transfers. Moreover, the identification of sediments in the basin and the prioritization of the contributions of each source provide nested sediment budgets. These studies lead the Seulles basin to become the first regional observatory of sediment transfers.
