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• Very low bedload distances at the event scale. • Event bedload distances were lower than spacing between identical bedforms. • Bedload velocity varied with year, bedform type and injection method. • Tracer mixing was incomplete 10 years after injection at the bed surface. This work analysed the factors controling the spatiotemporal variability in bedload mobility in a low-energy gravel-bed meandering river, with a particular focus on interactions between bedload transport distance and bedforms (bar-riffle-pool units). We first addressed the influence of pre-existing morphology, especially bars, on event transport distance, and examined to what extent the maintenance of bedforms requires hydrological events that can move bedload over distances at least equivalent to the length of two successive identical geomorphic units. Specifically, we tracked the displacement of passive integrated transponder (PIT)-tag particles 10 years after their injection, and applied bedload formulae to reconstruct the event transport distance for a 32-year study period. The results showed that event bedload distances for bed surface median size sediments (D 50) could not exceed an active channel width, roughly equivalent to 0.3 times the median distance between riffles and between pools. This has two main implications; first, trapping by bars is rare, or perhaps even absent, at the scale of competent events for most particles located at a distance greater than an active channel width from a bar; second, because no event is able to transport bedload over distances at least equal to the length of one riffle-pool unit, maintenance of bedforms must arise from successive minor changes. Furthermore, our data reveal important variability in bedload velocity at the decadal scale according to the year of injection, type of geomorphic unit of injection, and method of injection. This highlights the long-term effects in low-energy rivers of location, timing, and method of injection on velocity estimate. Lastly, we inferred the long-term bedload velocity from PIT-tag data and applied it to bedload formulae. The resulting velocity of 0.95–1.3 km century-1 is considered an overestimate because the slowdown of tracers seeded at the bed surface due to their progressive mixing in the active layer and their trapping in bars is still ongoing. [ABSTRACT FROM AUTHOR]

Dams are ones of the main sources of anthropogenic disturbance to the ecology and geomorphology of rivers. The aim of the present study is to understand the mechanisms underlying their influence on downstream bedload transport in three gravel-bed rivers in the Morvan massif, France. The hydrological disturbance caused by four dams is examined at a short (2.4–2.7 years) and longer (21–28 year) time scale. At the short time scale, bedload displacement was monitored (RFID) at 8 study sites around the dams. Morpho-sedimentary characterization of the bed substrate was performed at the study sites and combined with analyses of the long profile evolution and the current cross profile. The flood regime has been to a varying extent durably reduced by the dams depending on their size and purpose: the mean annual maximum flood was reduced by 9 to 40% and the number of flood events by 27 to 73.5% over the 21–28 years period. Sediment availability and loose structures were found above the dams and below medium-sized dams (<10 m) with a null or moderate influence on the flood regime (configuration I). Sediment deficit, consolidated structures, bed coarsening and vegetation encroachment were observed within 20 km downstream of large dams (>15 m) influencing strongly or moderately the flood regime (configuration II). These morpho-sedimentary features significantly affect the current bedload dynamics, creating conditions more or less favorable for the mobility of the present and incoming bedload. The cumulative mean bedload distances of RFID tracers in configuration II are significantly lower (6.8–45 m) than in configuration I (78–315 m). The current flow management of the dams has only a moderate effect on the bedload distances recorded, as shown by the mean virtual bedload velocities, which confirm the different dynamics (0.42–0.91 m/d, and 0.62–6.44 m/d, for II and I respectively). Our results demonstrate how modifications of dams on flow and bedload discharge altered the downstream morphology, but also that this inherited morphology may now be the main controlling factor of bedload transport. These findings invite further discussion about the most appropriate ways to restore rivers downstream of dams when dealing with multi-decadal inherited morphological features. • Dams of different sizes and purposes reduce the flood regime to a varying extent. • Bed coarsening, stable sediment structures, gravel deficit occur below large dams. • Sediment availability, loose clusters occur above dams and below medium-sized dams. • Flow energy only partly explains the low bedload mobility below large dams. • Substrate conditions shaped under dams'influence highly control bedload mobility. [ABSTRACT FROM AUTHOR]