Improving river hydromorphological assessment through better integration of riparian vegetation: Scientific evidence and guidelines

River hydromorphology has long been subjected to huge anthropogenic pressures with severe negative impacts on related ecosystems’ functioning and water quality. Therefore, improving river hydromorphological conditions represents a priority task in sustainable river management and requires proper assessment tools. It is well known that riparian vegetation plays a crucial role in sustaining river hydromorphological conditions. However, it has been nearly neglected in most hydromorphological assessment protocols, including the European Water Framework Directive (WFD). This paper reviews and synthesizes the relevance of riparian vegetation for river hydromorphology, focusing on its contribution to streamflow and sediment regime conditions. We also examine how riparian vegetation is considered in the WFD and how it is included in national hydromorphological protocols currently in use. Our findings point to a temporal mismatch between the date when the WFD came into force and the emergence of scientific and technologic advances in riparian vegetation dynamism and bio-geomorphic modeling. To overcome this misalignment, we present promising approaches for the characterization and assessment of riparian vegetation, which include the identification of vegetation units and indicators at multiple scales to support management and restoration measures. We discuss the complexity of riparian vegetation assessment, particularly with respect to the establishment of river-type-based reference conditions and the monitoring and management targets, and propose some attributes that can serve as novel indicators of the naturalness vs. artificiality of riparian vegetation. We argue that the hydromorphological context of the WFD should be revisited and offer guidance to integrate riparian vegetation in river hydromorphological monitoring and assessment.The work leading to this paper was developed within the COST Action (CA16208) – CONVERGES (Knowledge Conversion for Enhancing Management of European Riparian Ecosystems and Services), supported by COST (European Cooperation in Science and Technology; www.cost. eu). Vanesa Martínez-Fernández was funded by a “Juan de la Cierva” research contract (Spanish MINECO grant no. FJC2018-035451-I at National Museum of Natural Sciences, CSIC, Spain. Francisca C.Aguiar is supported by national funds via FCT under the contracts Norma Transitoria - DL57/2016/CP1382/CT0028. The Portuguese Foundation for Science and Technology (FCT) supported Patricia María Rodríguez- González through FCT investigator programme IF/00059/2015 and the Forest Research Centre through grants UID/AGR/00239/2019 and UIDB/00239/2020.Peer reviewe