Molino Bajo and Molino del Cabrillas, Spain

Molino Bajo and Molino del Cabrillas weir removals: An awarded project to restore River Cabrillas

The Tagus basin

The Tagus basin is the drainage basin of River Tagus, which flows through the Iberian Peninsula to the west and runs into the Atlantic Ocean next to Lisbon (Figure 1). It is one of the great Spanish hydrographic demarcations and extends to several regions, including Aragon, Castilla-La Mancha, Madrid, Castilla y León, and Extremadura. With an area of 55,781 km2, it is the third largest basin in the Iberian Peninsula in both total surface area and water flow, after the Ebro and Duero basins. The Tagus basin is the most populated basin in Spain and the Iberian Peninsula, with nearly 7.9 million people live within its borders. The main tributaries of River Tagus, both in basin area, length and flow are the Tajuña, Alagón, Jarama, Alberche, Tiétar, Henares or Guadarrama rivers.

Figure 1. Tagus basin (photo by Port(u*o)s – Own work, based on an image of http://www.maps-for-free.com/ (Creative Commons Attribution Share Alike 3.0), CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2994049)

In recent years, a number of especially unique and valuable rivers or river sections have been characterized as River Nature Reserves and are thus protected by law. In the Tagus basin, 30 rivers/river sections have been declared so far as such in rivers (Alagón, Alberche, Almonte, Arbillas, Arrago, Barbaón, Barquillo y Cardal, Batuecas, Cuervo, Dulce, Escabas, Francia, Gévalo, Guadiela, Gualija, Hozseca, Jarama, Jaramilla, Lozoya, Madarquillos, Malvecino, Manzanares, Mesto, Muelas, Navahondilla, Pelagallinas, Sorbe, Tajo, Viejas, Riato and Puebla), 6 in streams (Cabrera, Canencia, la Dehesa, los Huecos, Ompolveda and Vallosera), 5 in the gorges of Las Lanchas, Las Torres, Infiernos, Iruelas and Mayor, and 1 in a temporary river (Rambla de la Sarguilla). 

The River Cabrillas

River Cabrillas is located at the most eastern part of the Tagus basin (Figure 2). It inhabits several fish species, most of which are migratory, and some are also protected by European and Spanish legislation. The river’s ichthyofauna includes Squalius pyrenaicus, Luciobarbus bocagei, Squalius alburnoides, Pseudochondrostoma polylepis, Achondrostoma arcasii and Salmo trutta. Genetically unique populations of the latter species are present in the river, and thus considered of special interest on a regional scale. Other unique species and/or protected by European and Spanish legislation, associated with the river’s aquatic ecosystems are: Alcedo atthis, Cinclus cinclus, Galemys pyrenaicus, Arvicola sapidus and Lutra lutra. 

Figure 2. Location of River Cabrillas (photo by Wikimedia maps – Map data © OpenStreetMap contributors) 

 River Cabrillas has fragmented by longitudinal barriers for decades (Figure 3). Two of these barriers, Molino Bajo (latitude: 40.678622, longitude: -1.968067) and Molino del Cabrillas (latitude: 40.647206, longitude: -1.908942), were located in the northern area of the province of Guadalajara, within the region of Castilla-La Mancha. This area is registered in the Special Conservation Zone ZEC ES4240016 “Alto Tajo” and in the Special Bird Protection Zone ZEPA ES0000092 “Alto Tajo”. It is also part of the Natural Park of “Alto Tajo”, as declared by the regional government Castilla La Mancha under Spanish law in 2000. 

Figure 3. Location of barriers along River Cabrillas 

Molino Bajo was a 3-m high and 10-m long weir (Figure 4), while Molino del Cabrillas was a 3.5-m tall and 60-m long weir (Figure 5). The construction date of the weirs is unknown, but they were both privately owned by local residents. Molino Bajo had a registered concession to supply water to a flour mill which was revoked by the river basin authority in 2019. Molino del Cabrillas never had a legally registered concession.  

Figure 4. Molino Bajo at River Cabrillas (photo by AEMS Rios con Vida) 

Figure 5. Molino del Cabrillas at River Cabrillas (photo by AEMS Rios con Vida) 

In Spain, continental, surface and underground waters, as well as channels and aquifers, are publicly owned, constituting the so-called Public Hydraulic Domain (DPH). The responsible entities for DPH are the general administration of the State in each basin and the territorial administrations within their jurisdiction. On the other hand, infrastructure in rivers can be privately owned, but any construction and use of the DPH assets can occur after the concession of the respective river basin authority. The responsibility for eliminating a riverine barrier lies – in principle – with the last owner. However, when ownership of a barrier is unknown or ambiguous, the State has to carry out the removal works and cover the associated costs. In the event that a barrier removal is scheduled by a non-profit and/or a non-governmental organization, the necessary permits need to be obtained from the river basin authority. 

Dam removal 

In July 2020 the prospects of removing any barriers at River Cabrillas were initiated, including successive field visits to barrier sites, which were accompanied by personnel of the River Tagus basin authority. The final selection of the barriers to be demolished was made almost a year later, in June 2021. The preparation phase of the removals commenced in January 2022 and lasted for almost a full year. It included field surveys, meetings with local authorities, technicians from the regional administration, and from the river basin authority, topographical works, and the acquisition of the necessary permits from the river basin authority. Prior to the removal works, analyses of the potential environmental impact of the removals were performed. It was concluded that the mobilization of fine sediments accumulated upstream of the weirs would be very limited. The demolition works of the two weirs were lasted for 8 days, from November 24 till December 2, 2022 (Figures 6-7; Videos 1-2).  

Figure 6. Demolition works of Molino Bajo at River Cabrillas (photo by AEMS Rios con Vida) 

Video 1. Demolition works of Molino Bajo at River Cabrillas (video by AEMS Rios con Vida) 

Figure 7. Demolition works of Molino del Cabrillas at River Cabrillas (photo by AEMS Rios con Vida) 

Video 2. Demolition works of Molino del Cabrillas at River Cabrillas (video by AEMS Rios con Vida)

Natural stone blocks present at the removal sites were distributed to locations upstream and downstream to decrease the newly formed slope and facilitate fish passage (Figure 8; Videos 3-4). Despite the original estimation and due to heavy rains and flood incidents that occurred a few days after the completion of the removal works, significant volume of sediment was mobilized at a faster pace than expected. The in-river works at the Molino del Cabrillas site, at the request of the local and regional administrations, included interventions to stabilize about 40 m of the left bank of the former reservoir.

Video 3. Distribution of natural stone blocks upstream and downstream of the removal site of Molino Bajo (video by AEMS Rios con Vida)

Figure 8. Distribution of natural stone blocks upstream and downstream of the removal site of Molino del Cabrillas (photo by AEMS Rios con Vida)

Video 4. Distribution of natural stone blocks upstream and downstream of the removal site of Molino del Cabrillas (video by AEMS Rios con Vida)

The project cost €86,432.9 in total and was mainly financed by the Open Rivers program. In May 2023 the project won the Dam Removal Award 2022,. After a pre-selection of the top five projects by an independent jury of international experts, a round of public votes took place until April 17. The jury’s and the public’s opinions were considered on a 50-50 ratio and the final count of votes elected the project as the winner. The award was hosted by Dam Removal Europe, the World Fish Migration Foundation, the European Investment Bank, The Nature Conservancy, and supported by the Dutch Postcode Lottery and Forest Peace Foundation. The award ceremony took place at the UK Dam Removal Conference on May 18 in Manchester.  

Part of the money prize was used to complete the in-river works at the Molino del Cabrillas site (Video 5). More specifically, monitoring visits following the initial removal works in 2022 revealed that a previously submerged plinth at the base of the former weir had been resurfaced thanks to flooding events in the area. That structure continued to impede free-flowing conditions and thus it was removed on September 5, 2023 (Figure 9-10; Video 6).  

Video 5. Molino Bajo and Molino del Cabrillas removal project (video by AEMS-Ríos con Vida) 

Figure 9. Additional removal works of the Molino del Cabrillas weir in September 2023 (photo by AEMS-Ríos con Vida) 

Video 6. Additional removal works of the Molino del Cabrillas weir in September 2023 (video by AEMS-Ríos con Vida) 

Figure 10. Removal site after the complete demolition of Molino Bajo at River Cabrillas in September 2023 (photo by AEMS-Ríos con Vida) 

Ecological and community benefits

The removals of Molino Bajo and Molino del Cabrillas (Figures 11-12; Video 7) were expected to contribute to the improvement of the River Cabrillas which is classified as “Natural”, “Permanent” and “Calcareous Mediterranean mountain river”. The removals will have a positive impact on the river ecological continuity by restoring the natural water flow, sedimentation, nutrients’ flow and fish passage of the last 25 km of the River Cabrillas up to its confluence with River Tagus. Fish populations are expected to be benefited and recreational activities to be boosted and expanded. The removals will therefore contribute significantly to the objective of the European Biodiversity Strategy of restoring the free-flowing conditions of at least 25,000 km of rivers in Europe.

Figure 11. Removal site after the demolition of Molino Bajo at River Cabrillas (photo by AEMS-Ríos con Vida) 

Video 7. Removal site after the demolition of Molino Bajo at River Cabrillas (video by AEMS-Ríos con Vida)

Figure 12. Removal site after the demolition of Molino del Cabrillas at River Cabrillas (photo by AEMS-Ríos con Vida)

The changes in the water flow pattern upstream of the removal sites and in the mobilization of the previously trapped sediment were already obvious during the demolition works. The hydro-morphological changes at the removal sites will be documented through fixed cameras and drone footage. In addition, the effects of the restored river connectivity will be assessed through analysis of environmental DNA samples. In these analyses the cyprinid fish species are considered the main indicators of a restored fish passage. 

The removal of the Molino Bajo was well received by the local community, as the barrier was obsolete and it had no sentimental value. On the other hand, the Molino del Cabrillas site was used by the locals for recreational activities, like bathing in the reservoir. Thus, the news of the removal of the weir was not welcomed initially. However, after the completion of the project, the perception of the local community towards the project has been shifted. 

Before & After photo pairs

The removal site at River Cabrillas: (left) before and (right) after the demolition of the Molino Bajo (photos by AEMS-Ríos con Vida)

The removal site at River Cabrillas: (left) before and (right) after the demolition of the Molino del Cabrillas (photos by AEMS-Ríos con Vida)

With many thanks to César Rodríguez and AEMS-Ríos con Vida for providing the information and the visuals presented herein. 

Written by Foivos A. Mouchlianitis