Impact of rainfall variability on sedimentary and hydropower dynamics in a dam reservoir of southern France This dataset presents the results of particle size, elemental geochemistry, radionuclide, and dry bulk density analyses carried out on the ORB09 sediment core (IGSN: 10.58052/IEFOU008X), collected on 5 October 2023 in the Monts d’Orb hydropower reservoir (southern France). These data support the publication "Impact of rainfall variability on sedimentary and hydropower dynamics in a dam reservoir of southern France" (Hazet et al., 2025, Hydrology and Earth System Sciences). Corresponding author: paul.hazet@lsce.ipsl.fr Particle size analysisMeasurements were performed using a Malvern Mastersizer 3000 laser granulometer, covering a particle size range from 10 nm to 3.5 mm. Grain size was measured continuously along the core at 0.5 cm resolution (n = 197). Parameters such as D10, D50, D90, and raw distribution data are provided in the file Particle_size_Orb09_Hazet-et-al.xlsx. Elemental geochemistry (XRF)The sediment core was analyzed using an Avaatech X-ray Fluorescence (XRF) core scanner at the Laboratoire des Sciences du Climat et de l’Environnement (Gif-sur-Yvette, France), with a 0.5 cm measurement interval. The resulting geochemical profiles are provided in XRF_core_scanner_ORB_Hazet-et-al.xlsx (sheets “10 kV” and “30 kV”).Arsenic (As) values, although part of the original XRF spectra, are not included in this public dataset due to data-sharing restrictions from BRL, the reservoir operator. This restriction does not affect the reproducibility of the analyses presented in the associated publication. Radionuclide analysesGamma spectrometry was performed using HPGe detectors (Canberra/Ortec) at LSCE. Short-lived radionuclides, including caesium-137 (^137Cs) and excess lead-210 (^210Pb_ex), were measured in 26 dry sediment samples (≈12 g) collected along the sequence. These results are available in Radionuclide_Hazet-et-al.xlsx. Dry bulk density (DBD)DBD was determined from 25 discrete samples collected along the core, representing contrasting sedimentary facies. Data are provided in Dry_Bulk_Density_Hazet-et-al.xlsx. Precipitation dataDaily precipitation data from the Roqueronde station, located in the Orb River catchment upstream of the Monts d’Orb dam, were obtained from the Météo-France database. These data are available in Precipitation_Roqueronde_Station_Hazet-et-al.xlsx. Relative density (CT-scan)CT scan data were acquired using a GE Discovery CT750 HD scanner at the DOSEO platform (French Atomic Energy and Alternative Energies Commission – CEA, Paris-Saclay, France). The relative density (expressed in grey value) was smoothed along the width of the core to reduce artefacts and quantify internal density variations. The data are provided in Relative_density_CT-SCAN_Hazet-et-al.xlsx.