The damage began most dramatically in the 1960s and 1970s, when large reservoirs entered operation and the Ebro’s natural flows were increasingly diverted for agriculture, hydropower and other uses. Today, 187 dams in the basin regulate the river, and the lower Ebro’s total solid discharge has fallen by more than 90%; the sand fraction has declined by almost 100%. River-borne sand reaching the delta is estimated at only about 30,000 cubic metres per year, with the southern half receiving roughly 1,500 cubic metres. This starvation has encouraged wave-driven reshaping of the coast, including severe erosion near the river mouth, where local retreat rates have reached 40 metres per year. The emerged delta covers about 320 square kilometres and has a coastline of roughly 50 kilometres, with approximately 50,000 inhabitants, rice fields, aquaculture areas and habitats that depend on the river’s freshwater and sediment discharges.

Climate change compounds the geological squeeze. Relative sea level is rising by more than 5 millimetres per year at the Ebro, while differential interferometric synthetic aperture radar and precision levelling indicate subsidence averaging roughly 1–3 millimetres per year in the central and northern delta. Estimates vary spatially and include substantial standard deviations in some comparisons, reflecting limitations in observation periods, methods and data sets. Groundwater extraction and reduced flood-borne sediment supply are associated with higher subsidence rates in some areas. The coast faces annual sand deficits estimated within a broad range of 50,000 to 500,000 cubic metres, depending on the sediment-balance definition, coastal sector and period analysed, with about two-thirds of the aggregate deficit attributed to the southern hemi-delta. Relative sea-level rise around 4 millimetres per year under current conditions would require about 200,000 cubic metres of additional sand annually; combined wave-driven and sea-level-related losses were estimated at roughly 700,000 cubic metres per year.

The study finds that a conventional return to historical sediment loads is neither easy nor plausible under future conditions. The Ebro’s potential sediment-transport capacity, estimated with a two-dimensional hydrodynamic and sediment model, averages about 11.1 million tonnes per year under observed flows but only about 6.9 million tonnes under regulated conditions. Natural-flow simulations give a much higher figure of roughly 22.4 million tonnes, revealing how irrigation and reservoir operations have suppressed not only water volume but also the high-flow events that move most sand. Climate projections under the severe SSP5-8.5 pathway indicate that future natural-flow transport capacity could fall to approximately 10–17 million tonnes per year. Under a similarly reduced altered-flow regime, the corresponding range would be 3.5–5 million tonnes per year; if current abstractions of about 6 cubic kilometres per year continued, available flow would fall to 6–7.5 cubic kilometres per year. The wide differences among climate and hydrological models indicate substantial uncertainty, but the projections show that restoration would increasingly depend on recovering trapped sediment and guiding it efficiently toward the coast.

Reservoirs are therefore both part of the cause of the crisis and a potential store of part of the solution. Surveys indicate that Mequinenza has accumulated roughly 75–100 million cubic metres of sediment since 1964, with an estimated trapping efficiency near 100%, whereas Ribarroja has accumulated about 13.1 million cubic metres and traps roughly 40% of incoming sediment. Flushing could evacuate substantial material from the smaller downstream reservoirs, but Mequinenza’s length, gentle gradient and limited bottom-outlet capacity make hydraulic flushing difficult. Mechanical bypass is technically feasible and has been estimated at roughly 700,000 euros per year for 150,000 tonnes annually to about 14 million euros for 1.5 million tonnes, depending strongly on local conditions. Most reservoir sediment is fine-grained, however, and the useful sand fraction averages only 18.5% at Mequinenza and 7% at Ribarroja. Pilot work is therefore examining mechanical and hydraulic removal, flushing at Ribarroja and the delivery of deposits from the Siurana tributary, with isotopic tracers and electromagnetic transmitters used to monitor downstream sediment transport.

That limitation makes coastal recycling especially important. The authors estimate that bypassing sediment at the technically feasible range would supply only about 7,500 to 75,000 cubic metres of sand per year, equivalent to roughly 1–10% of the estimated 700,000-cubic-metre coastal deficit. Controlled floods could increase downstream transport, with monitored releases showing sediment concentrations nearly an order of magnitude higher after reservoir levels were lowered. Yet sand begins moving effectively only when flows exceed about 575 cubic metres per second, far above the lower Ebro’s legally protected environmental-flow range of 80–100 cubic metres per second. Floods capable of transporting sand therefore compete directly with irrigation, water supply, environmental flows and the reservoirs’ hydroelectric role, while mobilised sediment may temporarily increase turbidity or release contaminants stored in reservoir sludge. The first releases would also help rebuild an armoured lower river bed, meaning that some transported material could remain in the river before reaching the delta.

To conserve scarce sand, the team has tested nature-based coastal designs that increase roughness rather than simply building higher barriers. At the Trabucador bar, alternating embryo dunes create partial barriers that can be overtopped, trapping sand on the back beach and rebuilding resilience after a storm. At Marquesa beach, transverse embryo dunes and small lagoons develop a ridge-and-runnel topography intended to dissipate wave and surge energy while retaining sediment. In simulations calibrated against satellite and drone observations, embryo dunes reduced breaching volume from an unprotected range of 17,000–120,000 cubic metres to about 5,000–30,000 cubic metres. Protection was close to complete for low- and medium-energy storms, while breached volume fell by roughly 30% during high-energy events. Proactive deployment before a storm can reduce breaching by a factor of five to ten, and the partial-barrier design preserves overwash rather than attempting to make the beach permanently impermeable. Construction at the pilot sites was scheduled outside the breeding season, and monitoring observations at Marquesa remain too short to establish long-term performance under cumulative storm action.

The pilots also exploit the coast’s natural conveyor belt. Prevailing northwesterly winds move sand southeast toward the river mouth, while refracted and diffracted wave systems drive nearshore transport northwest. By stabilising selected dunes with vegetation and sediment traps, the design uses these opposed fluxes to redistribute sand rather than export it entirely from the active shoreline. Estimated wind transport of 25,000–200,000 cubic metres per year overlaps with alongshore wave transport of about 50,000–60,000 cubic metres per year, making a partially closed recycling system plausible. The approach could reduce the net redistribution required at the ends of the belt to approximately 25,000–150,000 cubic metres per year, far less than conventional large-scale nourishment, but it depends on sufficiently regular wind patterns and will require continuing monitoring and maintenance. Another possible source is submerged prodeltas left by former river mouths, but extracting this relict sediment creates a trade-off between using a finite deposit to protect present coastal areas and potentially reducing the natural protection supplied by the submerged landform.

The analysis frames these interventions as parts of an adaptive system rather than ready-made solutions. Near-term storm warnings can guide rapid action on exposed beaches, including dune reinforcement, sandbags or beach scraping, while decadal projections can serve as a climate alarm system for remaining beach width, sand availability and risk. It also identifies difficult trade-offs: mobilised reservoir sediment may improve downstream habitats but disrupt water users; artificial nourishment may protect coastal assets while affecting beach use or disturbing the geomorphic role of submerged prodeltas; and water released for sediment transport may reduce water available for farming. Synergies emerge when one intervention serves several goals, as controlled floods improve river habitat and support deltaic resilience, while rough coastal forms increase habitat diversity, carbon storage and the lifespan of supplied sand. Overall, the Ebro is presented as a test case for restoring dynamic river-to-coast connectivity, but its future will depend on explicit stakeholder agreement over baseline conditions, priorities, funding, environmental safeguards and the timing of actions as sea level and coastal risk continue to change.

Subject of Research: Restoration trade-offs and synergies in Spain’s sediment-starved Ebro Delta and coastal system

Article Title: Trade-offs and synergies in river-coastal restoration for the Ebro case (Spanish Mediterranean)

Article References: Sánchez-Arcilla, A., Garrote, L., Gracia, V., Cáceres, I., Sánchez-Artús, X., Caiola, N., Espanya, A., Espino, M., García, M. Á., Grassa, J. M., Ibáñez, C., López, D., Mestres, M., de la Peña, J. M., Bladé, E., Pernice, U., Puértolas, L., Santillan, D., Zemah-Shamir, S., & Iglesias, A. (2025). Trade-offs and synergies in river-coastal restoration for the Ebro case (Spanish Mediterranean). Nature Conservation, 59, 101-137. https://doi.org/10.3897/natureconservation.59.142061

Image Credits: AI Generated

DOI: 10.3897/natureconservation.59.142061

Keywords: Ebro Delta, river restoration, coastal restoration, sediment transport, nature-based solutions, sea-level rise, coastal erosion, reservoir sediment, controlled floods, Mediterranean coast, Trade-offs, synergies