Removal of three consecutive barriers on the Tajuña River, Guadalajara, Spain

Introduction

The Tajuña is a third-order river in central Spain, the second longest in the Tagus basin and a tributary of the Jarama. The section containing the three dams is classified as a “Mediterranean calcareous mountain river” (R-T12) ecological type. It is a permanent body of water 86.56 km long, with a natural regime not regulated by large reservoirs.

Listed in two protected areas by the Natura 2000 Network, this stretch of the Tajuña, surrounded by forests of oak (Quercus pyrenaica), holm oak (Quercus ilex) and gall oak (Quercus faginea), has a riparian vegetation community composed mainly of poplar species, such as Populus nigra and willows (Salix sp.), and has different aquatic species of interest, highlighting demanding and vulnerable fish such as the brown trout (Salmo trutta), and several endemic cyprinids such as the common barbel (Luciobarbus bocagei), the russet (Achondrostoma arcasii), the bream (Pseudochondrostoma polylepis), and the chub (Squalius pyrenaicus).

Project context and opportunity

The three weirs to be removed have been abandoned for decades, but they continue to fragment the habitat and populations of several protected native and endemic species, altering hydrogeomorphology and disrupting the transport of water, sediment and nutrients in the river.

Following a previous pre-demolition grant from the programme, the new project proposal has the express support of the river basin authority (CHT) and the regional environmental authority, which collaborated in the selection and characterisation of the barriers to be removed and in the definition of the technical projects, allowing for the application for authorisation for the works at the end of January 2025. The technical projects were also presented to the local population in a second citizen participation meeting, where a reasonable social consensus was established around the restoration initiative. Besides funding from the Open Rivers Programme, the project also counts with the support of Suntory Beverage & Food Europe (SBFE). This opportunity is reinforced by the parallel project to restore the riverbank occupied by abandoned fish farms, funded by the N&E Peretti Foundation, which seeks to restore and enhance the river territory.

Project aims

The demolition of the three barriers would restore the river’s natural dynamics in a 9.1 km stretch of natural river upstream from La Tajera Reservoir, unregulated by large dams and in good ecological condition. The future removal of these three barriers will facilitate species transit and genetic exchange, improve habitat conditions and correct negative impacts on hydromorphology, promoting the creation of differentiated habitats key to biodiversity. The restoration of ecological connectivity will directly contribute to the conservation of the fish community, facilitating its resilience to the effects of global warming, which are already being strongly felt in the area.

The technical projects include preliminary work to condition accesses and construction areas, avoiding any potential impact or minimising it as much as possible; the physical demolition of the barriers; the extraction, transportation and controlled dumping of the artificial material; and the application of bioengineering to mitigate the environmental and landscape impact following the construction works. The approved proposal also includes monitoring the outcome of the demolitions, with pre and post-construction physical data collection and photogrammetric flights to verify potential hydromorphological changes above and below the obstacles.

The project’s conclusions will integrate results, lessons learned, best practices, and preventive and corrective measures to be applied in this type of project, which can contribute to their greater social acceptance.