Controls, budgets and variability of riverine sediment fluxes to the Gulf of Lions (NW Mediterranean Sea)

International audienceThe present study investigates the spatio-temporal variability of riverine sediment fluxes to the Gulf of Lions, one of the most extensive shelf regions in the Mediterranean Sea. Small coastal rivers compete here with the Rhone River, nowadays the largest Mediterranean river in terms of water discharge. Our scientific objectives were to investigate the major controls of riverine sediment yields (SY) in this area and to quantify the role of the small coastal rivers, largely ignored in previous studies, in the total sediment budgets. Another objective concerned the source identification of the Rhone sediments with regard to the major tributary contributions, and to test whether the sediment fluxes are in equilibrium in the basin. For the calculation of representative long-term fluxes, we used a Simplified Rating Curve Approach (SiRCA) which could be validated by high resolution monitoring and existing literature data. An overall of 13 drainage basins could be distinguished, covering about 86% of the study area (6 coastal rivers, the Rhone River, and 6 of its tributaries). Representative SY range from 19 to 151 t km−2 yr−1 in the investigated drainage basins. Despite their smaller basin areas and more torrential discharge regimes, SY of the coastal rivers were generally lower compared to SY of the Rhone River and its tributaries. Confrontation with the lithological, morphological and hydroclimatic basin characteristics indicate that lithology exerts the dominant control on SY in the study region. In particular, the existence of erodible sedimentary rocks in the headwater regions yields high SY. Peak values of 135 and 151 t km−2 yr−1 were observed for the Isere and Durance tributaries of the Rhone River, where badlands exist. The coastal rivers contribute on average only to slightly more than 5% of the long-term sediment inputs to the Gulf of Lions. During individual years however, their contribution can strongly increase (up to 27% in 2011). Their contribution is generally important during late spring and autumn, when flash-floods are frequent (up to 90% of the monthly sediment discharge). Summing the various tributary contributions of the Rhone River produces a sediment budget that is close to the sediment export at the river mouth. This indicates that at spatial scales, the average sediment fluxes are equilibrated in the basin. About 40% of the Rhone sediments originate from the Isere and Durance tributaries. However, fully closure of the budgets requires high SY in the lowermost basin parts. Omitting in our calculations only a few years with exceptional flooding considerably reduce these values. Trend analyses show at the same time that the sediment fluxes from the Saone and Isere tributaries decreased during the study period. At temporal scales, sediment transport from upstream to downstream was therefore not in equilibrium and the remobilisation of older sediments from the downstream basin parts may have been important