Unveiling Fluvial Processes in Urban Rivers: The Case of the Congost River at Granollers, Catalonia, Spain

The Congost River, a tributary of the Besòs River, flows through the city of Granollers, a city in the metropolitan area of Barcelona with 62,950 inhabitants in 2023. The river was heavily fixed and channelized during the 1970s and 1980s of the last century to prevent the city urban expansion from flooding. However, with the aim of recovering a “fluvial aspect of the river” and encouraging a “social use of the fluvial space,” a partial removal of the structures that kept the river fixed has been gradually undertaken since 2007 by the city council of Granollers. As a result, the fluvial morphology of the river has improved by recovering typical fluvial forms and recovering a “fluvial appearance.” To evaluate whether this transformation represented a “real improvement” in the fluvial characteristics and morphodynamics, a hydro-geomorphological index (IHG, Ollero et al., Limnetica 27:171–188, 2008) was applied in two different sections of the Congost River. The first section conserves the fixing structures and no naturalization works have been done. The second one, however, is located downstream and represents the area where structures have been removed. The evaluation was undertaken by applying a hydro-morphological index, in which the naturality of the flow regime, the quality of the riparian forest structure and the morphology of the river channel are considered. Results from the assessment indicate that the naturalized section obtained a better evaluation than the urbanized section. Despite this, the naturalized section shows symptoms of geomorphological processes that have been hidden while the section was fixed and urbanized and now, after some time, appear as a consequence of the “freedom” provided to the river. Riverbed incision, lateral erosion, river migration and bed lowering are the major processes taking on in this river reach. The index used has been useful in the assessment of geomorphological changes, but to unveil current processes undergoing, additional evaluations must be  applied to identify trends. The aim of this paper is to describe or unveil these processes that have been hidden for decades in fluvial urban areas, and that they should be considered by fluvial managers and authorities if a successful restoration project wants to be addressed.