Influences of major flood sediment inputs on sedimentary and geochemical signals archived in a reservoir core (Upper Loire Basin, France)

• Conservative trace element and grain-size variations inform about flood dynamics.
• Sediment-laden underflows during major floods magnify annual aggradation rate.
• Contamination levels of sediments are enhanced during flood events.
• Solid anthropogenic inputs are diluted by detrital material inputs during flood events.

The Villerest flood-control reservoir was built in the Upper Loire River (France) during the early 1980s, downstream from the most important industrial and coal-mining area of the basin. This reservoir has constituted an important trap for sediments and associated pollutants since its operation in 1984. A 154-cm-long core was recovered in 2010 in the deepest area of the reservoir and shows the influence of sedimentary infill processes on aggradation rates and selected chemical patterns. During dam operations, the lacustrine aggradation rate is not linear because of three turbiditic-like layers resulting from sediment-laden underflows during major flood events in 1996, 2003 and 2008. These three events contribute to 43% of the 151 kg·m− 2 of accumulated sediments over the 1984–2010 period. Trace element solid sources are activated during these floods as selected metals present the highest enrichment factors, levels never reached during interflood periods. In addition, sedimentological and detrital geochemical signals during these events influence anthropogenic trace element signals by a variable dilution effect (maximum during the increasing discharge stage). Hence, coupling sedimentological and geochemical approaches allows us to understand the sedimentary infill dynamics and, more specifically, to take into account the influence of floods on the temporal trajectories of pollutants.