| Summary: | According to a widespread opinion, weirs transform running waters into stagnant waters, but researches about upstream reaches of this type of hydraulic structure are rare. Based on the assumption that they are reservoirs, the objective of this study is to verify, through the observation of their hydrodynamic, sedimentary and thermal characteristics, if they exhibit lentic features or if they retain lotic traits. The flow has been studied, as well as its degree of sedimentary competence. This last aspect has been developed through the analysis of the bed's sediments. The thermal properties of the water have been used to understand if the water column splits into distinct water bodies, along a vertical temperature-density gradient. Results show that the presence of a weir does not mean that running waters is transformed into stagnant waters. Indeed, although the energy of the waters is weakened, the current is constantly moving in the direction of the slope and it can carry a sediment load. Furthermore, even in "extreme" cases, a significant vertical thermal gradient was not observed, such as that which could be observed in still waters.
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