Borbála Hortobágyi , Stéphane Petit, Baptiste Marteau, Gabriel Melun, Herve Piegay

Large active channels usually store more wood than channels with a narrow flow because of the availability of large unvegetated bars for wood deposition and inner functioning that usually supplies more wood through channel shifting. However, the dynamics of the wood supply (wood input, output, or stability) can vary substantially over time and the drivers are largely unknown. To explore them, we studied the temporal variability of large wood pieces and logjams along a 12-km reach of the lower Allier River using six series of aerial images of variable resolution acquired between 2009 and 2020, during which maximum river discharge fluctuated around the biannual (Q2) flood magnitude. We show that the wood budget was controlled by specific hydrological conditions. Wood output was best explained by water levels exceeding bankfull discharge (Q1.5). The duration of the highest magnitude flood (over bankfull discharge) was the best predictor of wood inputs, with shorter floods resulting in higher input rates. Finally, most of the wood remained stable when the river discharge did not exceed 60% of the bankfull discharge over a long period of time. Hydrological conditions driving jam build-up and removal were similar to those controlling individual wood piece dynamics. A succession of floods of similar (relatively low ~ Q2) magnitude and decreasing flood duration since 2016 have probably reinforced the filtering effect of wood obstacles, leading to positive feedback, which has been strengthened by riparian vegetation colonisation of the active channel.