The estimation of sand transport in gravel bed rivers is a complex task since bedload or suspended load may prevail depending on hydrodynamic conditions and because sediment transport capacity may be affected by an insufficient sediment supply. A challenge is first to sample suspended load due to strong vertical and lateral gradients and such direct sampling, though necessary, has a prohibitive cost and cannot provide high frequency estimates. A sediment rating curve may be applicable if the system is not supply limited. Dramais (2020) measured suspended load in the River Rhône at Lyon-Perrache during the January 2018 flood. He proposed a sediment rating curve model based on Bayesian inferring to evaluate continuously the suspended load. Eventually, he obtained a total sand flux of 175 000 tons. The object of this study is to understand where the stocks of fine sediment were to supply this system.