Sediment transport constraints for restoration of the Ebro Delta

Water 2025, 17, x https://doi.org/10.3390/xxxxx Type of the Paper Article 1 Sediment transport constraints for restoration of the Ebro Delta 2 Francisco Martin-Carrasco 1, David Santillán 1, David López-Gómez 2, Ana Iglesias 3, and Luis Garrote 1,* 3 1 Department of Civil Engineering: Hydraulics, Energy and Environment, Universidad Politécnica de Ma4 drid, Madrid; [email protected] (FM); [email protected] (DS); l.garro[email protected]s (LG) 5 2 Hydraulics Laboratory, Centre for Hydrographic Studies of CEDEX, Madrid; [email protected] 6 3 Department of Agricultural Economics, Statistics and Business Management, Universidad Politécnica de 7 Madrid, Madrid; [email protected] 8 * Correspondence: [email protected] 9 Abstract: The natural flow of sediment in the Ebro River has been altered by a variety of 10 factors that have impacted the geomorphic and ecological balance of the delta. Ongoing 11 restoration efforts in the delta would benefit if the flow of sediments in the river could be 12 increased. Understanding the dynamics of sediment flow in the Ebro River is an important 13 component in the design of effective management strategies for the Ebro Delta. This study 14 estimates the sediment transport potential of the Ebro River under current and future con15 ditions through numerical simulation. Historical data from the late 19th century indicate 16 that the river once transported up to 28.1 million tons of sediment per year. However, due 17 to water abstractions and flow regulations, the current sediment transport capacity is lim18 ited to 9 million tons annually, a reduction of 67%. Future projections suggest further de19 creases in flow and sediment transport potential, with reductions of up to 30% by 2060 20 and 50% by 2100, depending on climate conditions and water management practices. The 21 findings underscore the need for integrated management strategies to mitigate the im22 pacts of reduced sediment flow, emphasizing the importance of restoring sediment 23 transport as a crucial component of the delta restoration efforts. 24 Keywords: sediment transport; Ebro River; climate projections; ecosystem restoration 25 26 1. Introduction 27 The rivers in the Mediterranean region have undergone significant transformations 28 due to human activities, particularly through the construction of dams, which profoundly 29 impact their hydrology and ecology. One of the most affected rivers is the Ebro River in 30 Spain, which serves as a compelling case study of how large-scale damming disrupts nat31 ural sediment transport and flow regimes [1,2]. The Ebro, one of the longest rivers in the 32 Iberian Peninsula, has been heavily dammed, with over one hundred reservoirs altering 33 its natural dynamics. These dams regulate the flow of water to support irrigation, hydro34 electric power and urban water supplies, but they also cause severe disruptions in the 35 river sediment load. 36 Under natural conditions, the Ebro transported large amounts of sediment to its 37 delta, nourishing wetlands and maintaining coastal stability [3]. However, with the reten38 tion of sediment behind dams, the downstream portions of the river have become sedi39 ment starved, leading to bed armoring, erosion of riverbanks, and a significant retreat of 40 the Ebro Delta, which is now highly vulnerable to rising sea levels [4-6]. Furthermore, the 41 alteration of flow regimes has negatively affected aquatic ecosystems by reducing 42 Academic Editor: Firstname Lastname Received: date Revised: date Accepted: date Published: date Citation: To be added by editorial staff during production. Copyright: © 2025 by the authors. Submitted for possible open access publication under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/license s/by/4.0/). Water 2025, 17, x FOR PEER REVIEW 2 of 32 seasonal flooding. The disruption of these natural processes has weakened the ability to 43 act as a natural buffer against storms and coastal erosion [7]. Similar patterns of river al44 teration can be observed in the Mediterranean, where the balance between human water 45 demands and environmental sustainability remains a contentious issue [8]. The case of the 46 Ebro River underscores the long-term consequences of intensive river regulation, empha47 sizing the need for integrated water management strategies that consider not only human 48 needs but also the ecological integrity of river systems. 49 Alterations in sediment dynamics due to river regulation and climate change have 50 been documented in other large river systems. In the Mississippi River Delta, human in51 terventions and reduced sediment supply have led to extensive land loss and increased 52 vulnerability to sea level rise and hurricanes [9]. The Mekong River Delta has also experi53 enced a decrease in sediment loads, exacerbated by dam construction upstream and pro54 jected to worsen under future climate scenarios, posing risks to food security and liveli55 hoods [10]. Similarly, in the Changjiang Estuary, Xie et al. [10] documented how reduced 56 sediment input altered estuarine transport patterns and depositional dynamics. In Eu57 rope, the Rhône River Delta exhibits signs of sediment starvation and morphological 58 changes due to both hydropower development and climatic changes [12]. The Nile Delta, 59 once nourished by seasonal floods, now suffers subsidence and erosion as a consequence 60 of the Aswan High Dam and reduced discharge [13]. These cases highlight the global rel61 evance of sediment management and emphasize the need for integrated approaches that 62 consider future climatic variability and flow alteration. Under climate change scenarios, 63 many studies anticipate a reduction in river discharge and sediment transport potential. 64 For example, in the Ganges–Brahmaputra–Meghna system, changes in monsoonal pat65 terns and upstream dams could lead to major disruptions in sediment delivery to the 66 delta, with potential socioecological consequences [14]. These studies highlight the 67 broader implications of disrupted sediment continuity in large river systems, reinforcing 68 the need to understand how sediment availability and flow regimes affect downstream 69 channel morphology and ecological integrity. They provide relevant context for assessing 70 the sediment dynamics of the Ebro River and offer comparative insights into restoration 71 and management strategies under changing climatic and anthropogenic pressures. 72 The existing literature on sediment transport highlights the critical role of sediment 73 dynamics in maintaining the ecological and geomorphological integrity of river systems. 74 Hauer et al. [15] emphasize that natural rivers operate in a delicate balance between sedi75 ment erosion, transport, and deposition, a process essential to maintain riverbanks, flood76 plains, and deltas. Research has consistently shown that the construction of dams along 77 the Ebro has severely disrupted the sediment balance by trapping significant amounts of 78 sediment in reservoirs, reducing the availability of sediment downstream. Studies by 79 Guillén and Palanques [16], Batalla and Vericat [17], Tena and Batalla [18] or Polo et al. 80 [19] demonstrate that sediment load has drastically decreased since the mid-20th century 81 due to damming, leading to intensified coastal erosion, delta retreat, and habitat degrada82 tion. As a result of the construction of the upstream dams and the regulation of flow, the 83 current sediment supply to the Ebro Delta has declined dramatically, with current esti84 mates indicating suspended sediment loads of only 30,000 to 50,000 t/yr [18]. This repre85 sents a reduction of more than 99% compared to pre-dam conditions. Furthermore, sedi86 ment retention in reservoirs alters the morphology of the riverbed [20], affecting aquatic 87 habitats and the distribution of nutrients [21]. 88 Restoring sediment transport is crucial for preserving both the physical structure and 89 ecological health of the Ebro River, making it an essential consideration in future water 90 management policies. Some scholars advocate for sediment bypass systems or controlled 91 sediment releases as potential solutions to restore sediment continuity in the Ebro River 92 [22]. Bathymetric surveys of the Mequinenza (1961-2012) and Ribarroja (1967-2007) 93 Water 2025, 17, x FOR PEER REVIEW 3 of 32 reservoirs show significant sediment retention. The loss in capacity was 5.8 106 m³/yr ini94 tially, later estimated at 1.5 to 2 106 m3/yr over 48 years. Ribarroja accumulated 0.34–0.94 95 106 m3/yr. Mequinenza traps nearly 100% of sediment, while Ribarroja retains approxi96 mately 40% [23]. Historical sediment transport upstream of the reservoirs of the Lower 97 Ebro River decreased from 8 Mt/yr in the 1960s-1970s to 5 Mt/yr due to land use and cli98 mate change. The total sediment accumulation is estimated at 320 Mt in Mequinenza and 99 64 Mt in Ribarroja. Given the configuration of the reservoirs, the most promising options 100 for sediment mobilization in the Mequinenza-Ribarroja-Flix system are hydrodynamic re101 moval by flushing or sluicing and mechanical removal by excavation or dredging. Flush102 ing is technically possible in the Ribarroja and Flix reservoirs, but raises environmental, 103 economic, and social concerns, including water quality issues, impacts on hydropower 104 production, and disruptions to water users. Mechanical removal, especially hydraulic 105 dredging, is always feasible but costly. Given these challenges, sediment management re106 quires careful planning to balance feasibility and potential negative effects. The Ebro River 107 Basin Management Plan includes funding for pilot tests for sediment mobilization in col108 laboration with all relevant stakeholders [24]. These tests will be carried out with appro109 priate environmental, safety, and economic safeguards. The proposed pilot studies will 110 take place in the Mequinenza regatta area, a rowing course located at the tail end of the 111 Ribarroja reservoir. The project will assess different sediment mobilization techniques, fo112 cusing on mechanical methods and hydraulic dredging to determine their feasibility and 113 effectiveness in managing accumulated sediments. 114 The interplay between flow regime and sediment transport capacity is a critical factor 115 in determining the success of management strategies aimed at restoring sediment flow in 116 the Lower Ebro River, where sediment bypass operations are being explored. Under nat117 ural conditions, sediment transport is governed by river discharge, with higher flows hav118 ing the energy needed to mobilize and carry sediment downstream. However, in a regu119 lated river such as the Ebro, dam-induced flow alterations have significantly reduced river 120 discharge, which in turn lowers the flow energy and sediment transport capacity. Even 121 when successful sediment bypass systems or controlled sediment releases are imple122 mented, their effectiveness depends on whether the discharge levels are sufficient to 123 maintain suspended sediment in motion and prevent deposition before reaching the Ebro 124 Delta. Studies such as those by Tena et al. [25] highlight that sediment transport efficiency 125 is directly related to flow magnitude and variability, which means that without adequate 126 discharges, sediment restoration efforts may fail. Additionally, low-energy flows encour127 age the accumulation of fine sediments, promoting vegetation encroachment and further 128 stabilizing riverbeds, making it even more difficult to restore dynamic sediment transport 129 processes. Therefore, management strategies for sediment bypass in the Lower Ebro must 130 consider not only sediment supply, but also ensure that flow releases are powerful enough 131 to maintain sediment in suspension and transport it to the delta. Without addressing this 132 key interaction between flow and transport capacity, restoration efforts risk being ineffec133 tive, reinforcing the need for integrated strategies that align sediment management with 134 hydrological restoration. 135 In recent years, a variety of sediment management strategies have emerged to miti136 gate the impacts of sediment retention in regulated rivers. Among these, sediment aug137 mentation, which involves adding sediment downstream of barriers, has been applied in 138 rivers such as the Rhine to partially restore geomorphic and ecological functions [26, 27]. 139 In mountainous rivers, artificial sediment pulses have been released to simulate natural 140 transport events and to support riverbed dynamics [28]. These interventions are often 141 supported by detailed modeling, field monitoring, and laboratory experimentation to as142 sess their effectiveness [29]. At the delta scale, eco-engineering and nature-based solutions 143 are increasingly integrated into restoration projects to enhance sediment retention, restore 144 Water 2025, 17, x FOR PEER REVIEW 4 of 32 natural hydromorphological processes, and improve ecosystem resilience to sea-level rise 145 and climate impacts [7,30]. These approaches often involve multi-stakeholder processes 146 and adaptive management frameworks, highlighting the need for integrated strategies 147 that combine engineering feasibility with ecological functionality. Incorporating such ap148 proaches in the context of the Ebro River is crucial, particularly given the magnitude of 149 sediment deficit and the vulnerability of the delta. This study contributes to that effort by 150 assessing the current and future potential for sediment transport as a prerequisite for eval151 uating the feasibility of sediment management interventions. 152 The sediment flow restoration strategy consists of two key components: mobilizing 153 accumulated reservoir sediments and transporting them to the Ebro Delta. The first com154 ponent is primarily an economic challenge, addressed through pilot tests to identify the 155 most effective sediment mobilization techniques. The second component relies on the nat156 ural transport capacity, though uncertainties remain due to the altered hydrological re157 gime caused by water abstractions and regulation. The objective of the work presented 158 here is to assess whether the current and future flow regime of the river will be sufficient 159 to transport the sediments that can be released from the reservoirs. Since current sediment 160 transport is very low, the adopted methodology is based on hydrodynamic simulation. 161 Hydrodynamic and sediment transport models have become essential tools in rivers, 162 providing critical information on sediment dynamics and forming more effective river 163 management strategies. These models simulate complex interactions between water flow, 164 sediment transport, and river morphology, allowing scientists to predict how sediment 165 moves through the system under various conditions. 166 The purpose of this study is to estimate the sediment transport capacity of the Lower 167 Ebro River using a sediment transport rating curve based on hydrodynamic simulation. 168 The specific objectives of this study are to: (1) develop a sediment transport rating curve 169 for the Lower Ebro River based on available data and hydrodynamic simulation; (2) quan170 tify sediment transport capacity under current, natural and projected future hydrological 171 conditions; and (3) assess whether future flow regimes, under climate change and water 172 management scenarios, will be sufficient to support restoration of sediment delivery to 173 the Ebro Delta. The Materials and Methods section details the modeling approach, includ174 ing the study area, the development of a sediment transport rating curve through numer175 ical simulation, and the calibration of the model using observed data. This is followed by 176 the Results and Discussion, which are structured into subsections that analyze sediment 177 transport capacity under current, natural, and projected future flow regimes using climate 178 scenarios. The results compare natural versus altered conditions, assess seasonal and 179 long-term trends, and discuss the interplay between flow regulation and sediment mobil180 ity. The Conclusions section summarizes key findings, including projected reduction in 181 sediment transport capacity and its implications for delta restoration. An Appendix pro182 vides the mathematical foundation of the hydrodynamic and sediment transport models 183 used in the study. 184 2. Materials and Methods 185 This section outlines the methodological approach of the study. The methodology 186 consists of two main steps: (1) constructing the rating curve by simulating sediment 187 transport under different discharges and (2) applying the rating curve to estimate sedi188 ment transport under various flow scenarios. 189 2.1 Study area 190 The Lower Ebro River extends from the Mequinenza Reservoir to the Ebro Delta, 191 encompassing a series of reservoirs and natural river stretches that shape its hydrology 192 and sediment dynamics (Figure 1). The Mequinenza Reservoir, the largest in the system 193 Water 2025, 17, x FOR PEER REVIEW 5 of 32 with a storage capacity of 1.5 km3, is located at the confluence of the Segre and Ebro rivers. 194 It spans more than 110 km, with a high sediment trapping efficiency due to its storage 195 capacity and elongated shape. Downstream, Ribarroja Reservoir follows, smaller in size 196 (0.21 km3), but still significantly affecting sediment transport. It stretches for 30 km and 197 serves as a secondary barrier to sediment movement. Further downstream, Flix Reservoir 198 is a smaller impoundment (0.011 km3) located just before the natural river reaches, used 199 primarily for hydroelectric purposes. It has a history of industrial pollution that requires 200 careful sediment management interventions. Below Flix Reservoir, the Lower Ebro flows 201 freely for approximately 100 km toward the Mediterranean Sea. 202 203 Figure 1. Location of the study areas. 204 Figure 2 illustrates the development of reservoir storage in the Ebro basin, based on 205 data from the ICOLD World Register of Dams [31]. According to this dataset, the basin 206 contains 142 reservoirs with a storage capacity exceeding 106 m³, totaling 8.438 km³. Res207 ervoir construction peaked in the 1950s and 1960s, accounting for 54% of the total capacity. 208 Water 2025, 17, x FOR PEER REVIEW 6 of 32 209 Figure 2. Development of reservoir storage in the Ebro basin. 210 2.2 Development of the sediment transport rating curve 211 The first step is to establish a quantitative relationship between river discharge and 212 sediment transport using a hydrodynamic and sediment transport model. River simula213 tions were performed with the Iber code [32,33]. Iber is a two-dimensional (2-D) freely 214 available code that solves the 2-D hydrodynamic equations coupled to the sediment 215 transport equations using a finite volume approach applied to the shallow water equa216 tions (SWE). The model can handle steady and unsteady flows in 2D and includes multi217 ple modules for different physical processes. Iber has been used successfully for modeling 218 sediment transport in environments such as that analyzed in this study. Santillán et al. 219 [34] benchmarked the hydrodynamic and erosion modules of Iber against laboratory ex220 periments with satisfactory results. Their study included simulations of erosion processes 221 at open-channel junctions based on experimental data available in the literature. The nu222 merical results closely matched the observed erosion patterns. In addition, the model was 223 applied to a real-world case study involving erosion at a river junction. Furthermore, the 224 hydrodynamic module of Iber was validated using laboratory experiments and classical 225 one-dimensional open-channel contraction theory [35]. Iber has been used to simulate the 226 hydrodynamic behavior of the Lower Ebro and other Mediterranean rivers. Dehghan-227 Souraki et al. [36] applied it to the Ribarroja reservoir. López-Gómez et al. [37] used Iber 228 to simulate flushing flows in the Lower Ebro River. López Gómez et al. [38] also applied 229 Iber to analyze sediment management options for Marmolejo Reservoir in the Guadalqui230 vir River. Due to computational requirements, the recently developed parallel computing 231 version of the model was used [39]. The hydrodynamic equations and the erosion model 232 are described in Annex 1. The model setup is described in the next section in terms of 233 model configuration data: topography, hydrodynamic parameters, and boundary condi234 tions. The calibration of the hydrodynamic numerical model with controlled flood obser235 vations is described in the following section. The methodology for constructing the rating 236 curve is presented in the last section. 237 Hydrodynamic model setup 238 The domain of numerical simulations to model the transport of sediment to the Ebro 239 Delta includes the reach of the river between the reservoir system and the delta. The mod240 eling objectives are: (1) to study alternatives to restore, at least in part, the sediment flow, 241 Water 2025, 17, x FOR PEER REVIEW 7 of 32 (2) to estimate the quantity of sediments that can be mobilized, (3) to determine how long 242 the sediments would take to reach the delta site, and (4) to know how the sediment would 243 be delivered to the delta system. 244 The topography of the simulated reach is shown in Figure 3. Fluid flow and sediment 245 transport are simulated from the Flix reservoir, located in Tarragona province at 41.1 me246 ters above sea level, to the delta site, located in the mouth of the river at 0 meters above 247 mean sea level. The Flix dam is the last sediment barrier on the Ebro River and is located 248 about 100 km from the river mouth. The domain topography was obtained from a high-249 resolution Digital Terrain Model (DTM) with a 0.5 m resolution of the Ebro River that 250 includes river bathymetry. A Triangulated Irregular Network (TIN) model of the river 251 was built using this DTM, with a tolerance of 0.1 meter, maximum side of 2000 meters, 252 and a minimum side of 12 m. The model includes about 100 km of the topography of the 253 Ebro River, and is composed of approximately 5 million triangles. An unstructured com254 putational mesh was generated from the TIN, composed of about 2.5 million triangles; 255 each triangle of the TIN model is one finite volume in the numerical model, i.e. the TIN 256 model coincides with the computational mesh. The picture to the left in Figure 3 shows 257 the coverage of the TIN model of the Ebro topography in blue. Two detailed snapshots of 258 the computational mesh are shown on the right. 259 260 Figure 3. Domain of the numerical simulations in the Ebro River. 261 The hydrodynamics boundary conditions for the numerical model are: (1) the im262 posed flow rate in Flix in subcritical regime, and (2) the outlet boundary condition at the 263 level imposed by the sea. 264 Model calibration 265 Water 2025, 17, x FOR PEER REVIEW 8 of 32 The riverbed roughness, given by the Manning coefficient, was calibrated with the 266 observed flood event on 5 May 2022. This event also allowed testing of the performance 267 of the hydrodynamic model [37]. The Ebro Automatic Hydrological Information System 268 (SAIH) provided 30-minute flow rate observations at two points on the river: Ascó and 269 Tortosa (see Figure 1). A set of numerical simulations with several values of the Manning 270 coefficient was run. The best agreement between the simulated and observed flow rate in 271 Tortosa was obtained with a Manning coefficient equal to 0.03. The results are shown in 272 Figure 4. The model can capture the propagation of the flood wave along the river in terms 273 of time and flood peak downstream of the inlet. 274 275 Figure 4. Observed inflow in Ascó and observed and simulated flow rate in Tortosa for the con276 trolled flood of May 2022. 277 Rating curve for sediment transport 278 A rating curve for sediment transport was built in the Lower Ebro River with the aid 279 of the numerical model described above. The sediment transport part was calibrated using 280 data compiled by Ibàñez et al. [40]. They provided estimates of the annual transport of 281 suspended sediments for several flow ranges in the Ebro. Data were obtained from Gor282 ria’s experiments, conducted in 1877, well before the dam construction period. They iden283 tified two representative regimes: normal flow, between 300 m3/s and 1500 m3/s, with an 284 estimated mean flow of 710 m3/s and a duration of 256 days and an average sediment 285 concentration of 0.9 g/L, and annual flood, for discharge larger than 1500 m3/s, with a 286 mean flow of 1764 m3/s, a mean duration of 11 days and an estimated sediment concen287 tration of 7.2 g/L. Sediment transport was estimated to be 14.1 Mt/yr for normal flow and 288 12.1 Mt/yr for annual flood. They added a contribution to the mean sediment transport of 289 1.7 Mt/yr from exceptional floods. The remaining 0.2 Mt/yr corresponds to flows of less 290 than 300 m3/s. These values imply a total annual volume of 19.29 km3/yr and sediment 291 transport of 28.1 Mt/yr. 292 Numerical simulations were performed for the two identified flow regimes, estimat293 ing sediment transport parameters to fit the observed data until a good agreement was 294 obtained. In numerical experiments, the suspended sediment concentration correspond295 ing to the flow regimes was specified at the upstream boundary condition. The numerical 296 simulations are aimed at analyzing whether the Ebro River can transport those sediment 297 concentrations down to the Ebro Delta. The analysis focused on the size of the suspended 298 Water 2025, 17, x FOR PEER REVIEW 9 of 32 particles, characterized in the model with the median diameter, 𝐷𝐷50. The parameter 𝐷𝐷50 299 was estimated with a calibration process for the two representative regimes. A set of sim300 ulations was run varying the value of 𝐷𝐷50, checking for the evolution of suspended sedi301 ment concentration along the river. The adopted value for 𝐷𝐷50 was the maximum diam302 eter that does not produce substantial net sedimentation in the river and is able to carry 303 the suspended load to the delta. The following characteristics were adopted for the sedi304 ment: relative density 2.65, friction angle 30 degrees, suspended sediment dispersion co305 efficient 0.001 m2/s, and Schmidt number is 1.1. The best performance was obtained for a 306 𝐷𝐷50 value equal to 0.0005 mm, clay. 307 A sample of the results obtained in the simulations performed during the calibration 308 process is presented in Figure 5. The figure shows two contour plots of the suspended 309 sediment concentration for two 𝐷𝐷50 values: 0.005 mm –silt–, and 0.0005 mm –clay– after 310 1,5 days of simulation. The flow is 710 m3/s in both cases. The flow cannot convey the 311 suspended silt load, and net sedimentation occurs during the transport process. The silt 312 concentration decreases to 0.5 g/L approximately in the delta, as shown in Figure 5(a). 313 However, the flow can trasnport the suspended clay load with the given concentration, 314 0.9 g/L, down to the delta without net sedimentation, as shown in Figure 5(b) 315 316 Figure 5. Suspended sediment concentration for the normal flow regime of the Ebro River after 1,5 317 days. Water flow is 710 m3/s and the sediment concentration at the inlet is 0.9 g/L. Plot (a) shows 318 sediment concentration for 𝐷𝐷50 equal to 0.005 mm. Plot (b) shows sediment concentration for 𝐷𝐷50 319 equal to 0.0005 mm, ten times smaller. 320 The resulting rating curve is presented in Figure 6. The curve was fitted to data com321 piled by Gorría in 1877 and to the results of the numerical simulations. The sediment 322 transport rating curve was fitted using a third-degree polynomial function of the 323 Water 2025, 17, x FOR PEER REVIEW 16 of 32 Figure 9 presents the monthly average values of flow and potential sediment 538 transport under natural and altered conditions. The figure shows that, in addition to water 539 abstractions, flow regulation is also relevant to determine the potential transport of sedi540 ments under altered conditions. The seasonality of the flows shows that most water ab541 stractions occur during the spring, summer, and fall months, leaving the average winter 542 flows almost unchanged. However, altered winter flows are concentrated in wet years, 543 when reservoirs release excess water. This leads to an increase in potential sediment 544 transport under altered conditions during the winter months, which can be used to effec545 tively manage sediment bypasses in reservoirs. 546 547 Figure 9. Mean seasonality of flow and potential sediment transport in the Lower Ebro River under 548 natural and altered conditions during the period 1980-81 to 2019-20. Left: annual flow. Right: annual 549 potential sediment transport. 550 In conclusion, the potential for sediment transport is significantly reduced under cur551 rent conditions of water management in the Ebro River basin, but there is still capacity to 552 transport almost 9 Mt/yr, which is a figure much larger than the sediment transport cur553 rently observed and exceeds what can be realistically expected to be bypassed from the 554 reservoirs. However, it should be noted that in 20 of the 40 years analyzed there would be 555 no sediment transport because the discharges measured in Tortosa do not reach the 556 transport activation threshold. 557 3.2 Potential sediment transport under climate projections 558 This section studies how the transport capacity of the Ebro River is expected to evolve 559 in the future using climate projections. First, the results under natural conditions are pre560 sented. The results corresponding to altered conditions are later analyzed, under the hy561 pothesis of maximum water use in the basin. 562 Sediment transport under natural conditions 563 The results produced by the SIMPA model and the ISIMIP hydrological models for 564 the mean annual flow in all scenarios analyzed are presented in Table 1. Each row of Table 565 1 corresponds to a specific combination of a hydrological model forced with an observed 566 data set or a global climate model, while each column corresponds to a specific scenario. 567 The values reported in the table are averages of mean annual flows taken over 40-year 568 periods, expressed in km3/yr. The time period for SIMPA and the obsclim scenario spans 569 from 1979-80 to 2018-19. This scenario corresponds to impact models forced with the ob570 served climate in the actual time sequence, based on reanalysis data. This option was se571 lected to validate the runoff obtained with the two global climate models against the re572 sults of the SIMPA model. The historical denomination corresponds to impact models 573 forced with results from climate models, not necessarily in the observed time sequence. 574 This option was selected as a reference to estimate changes in potential sediment transport 575 in the future. The time series of the historical scenario only cover up to year 2014, so the 576 time period selected for this scenario is 1974-75 to 2013-2014. To facilitate comparison, 577 Water 2025, 17, x FOR PEER REVIEW 17 of 32 results for the obsclim scenario in the time window 1974-75 to 2013-2014 are also presented. 578 Three future climate projections are included: ssp126, which corresponds to the SSP1-2.6 579 scenario, ssp370, which corresponds to the SSP3-7.0 scenario and ssp585, which corre580 sponds to the SSP5-8.5 scenario. Two time periods were selected for each climate projec581 tion: 2020-21 to 2059-60 and 2060-61 to 2099-2100.. Average values over the five climate 582 drivers for the H08 and CWatM models are also included in the table. 583 The results show a significant difference between the mean annual flow estimated 584 with the two global hydrological models. For the obsclim scenario, H08 provides a mean 585 annual flow for the Ebro River of 12.63 km3/yr, while the estimate of CWatM is 15.17 586 km3/yr. The estimate of the SIMPA model, 14.97 km3/yr, is almost coincident with that of 587 the CWatM model. The values for the average over the five climate drivers of the historical 588 scenario are 13.49 km3/yr for the H08 model and 15.62 km3/yr for the CWatM model. The 589 corresponding values of the obsclim scenario for the historical period, 1974-75 to 2013-2014, 590 are 13.19 km3/yr for the H08 model and 15.63 km3/yr for the CWatM model, which means 591 that there is an excellent agreement between the obsclim and the average of the results 592 obtained with the five climate drivers used in the historical scenario. The projections for 593 future scenarios show some sensitivity to the climate drivers used, but provide a con594 sistent picture of the expected evolution of mean annual flow in the Ebro basin. The mean 595 annual flow is expected to decrease in the Ebro basin for most scenarios. 596 Table 1. Mean annual flow obtained in the simulations of the scenarios for the Ebro basin, in km3/yr. 597 Scenario obsclim obsclim historic ssp126 ssp370 ssp585 ssp126 ssp370 ssp585 Period 79-18 74-13 74-13 20-59 20-59 20-59 60-99 60-99 60-99 SIMPA 14.97 15.19 H08 gswp3 12.63 13.19 gfdl 13.45 12.12 10.61 11.64 12.20 10.22 9.14 mrl 13.40 12.37 11.33 12.97 13.62 11.45 9.94 ipsl 13.61 10.91 10.42 9.83 10.57 8.10 7.27 mpi 13.03 10.95 10.33 10.61 11.39 8.80 8.55 ukesm1 13.98 11.63 9.63 9.24 10.62 7.64 7.97 mean 12.63 13.19 13.49 11.59 10.46 10.86 11.68 9.24 8.57 CWatM gswp3 15.17 15.63 gfdl 15.84 15.31 13.66 14.98 15.46 13.88 12.66 mrl 15.59 15.81 15.28 16.90 16.78 15.81 14.03 ipsl 15.84 13.95 13.98 12.81 13.84 11.82 10.60 mpi 15.44 14.02 12.90 13.71 14.49 11.73 11.87 ukesm1 15.40 14.92 13.82 12.13 14.30 11.89 11.38 mean 15.17 15.63 15.62 14.80 13.91 14.11 14.98 13.03 12.11 598 The comparison of the time sequence of annual flows in the Ebro River produced by 599 the SIMPA model and the average results of the two ISIMIP hydrological models is shown 600 in Figure 10. Both ISIMIP models show good agreement with SIMPA in the overlapping 601 period 1940-2019 for annual flows. The agreement of the CWatM model appears to be 602 better. Figure 11 shows the scatter plot of annual flows in the Ebro River. The plot on the 603 left shows the comparison between the H08 model and the SIMPA model and the plot on 604 the right shows the comparison between the CWatM model and the SIMPA model. For 605 the H08 model, the slope of the linear regression fit is 0.89, suggesting an underestimation 606 of the annual flow values for the H08 model compared to the SIMPA model. The coeffi607 cient of determination is very high, 0.986. For the CWatM model, the results are much 608 Water 2025, 17, x FOR PEER REVIEW 18 of 32 better. The slope of the linear regression fit is 1.04, suggesting only a small bias. The coef609 ficient of determination is excellent, 0.994. 610 611 Figure 10. Comparison of the time sequence of annual flows and potential sediment transport in the 612 Ebro River produced by SIMPA model and ISIMIP global hydrologic models. 613 614 Figure 11. Comparison of annual flows in the Ebro River produced by ISIMIP global hydrologic 615 models and SIMPA model. Left: H08 model. Right: CWatM model. 616 The agreement between the SIMPA model and the ISIMIP models on the annual time 617 scale is generally good. However, there is a significant discrepancy in their representation 618 of seasonality. Figure 12 illustrates the average monthly flow rates simulated by the three 619 models. The left-hand graph compares the mean flow values, while the right-hand graph 620 compares the mean potential sediment transport values derived from flow rates using the 621 estimated rating curve. As shown, the ISIMIP models simulate peak flows during the au622 tumn and winter months, whereas the SIMPA model simulates peak flows in spring. This 623 divergence is likely due to the inadequate representation of snow accumulation in the 624 ISIMIP models. The Ebro River exhibits a pluvio-nival regime, characterized by snow ac625 cumulation in the Pyrenees during winter, leading to peak flows in spring during snow626 melt. The SIMPA model accurately captures this behavior, whereas the ISIMIP models 627 appear to inadequately simulate the snow accumulation and melt processes. The right-628 hand graph indicates that this misrepresentation of the seasonal flow distribution results 629 in a significant bias in estimating the potential sediment transport capacity. Specifically, 630 the ISIMIP models produce disproportionately high sediment transport values in autumn 631 and spring compared to those obtained with the SIMPA model. The average potential 632 sediment transport produced by the SIMPA model in the period 1978-79 to 2018-19 is 26.76 633 Water 2025, 17, x FOR PEER REVIEW 19 of 32 Mt/yr. The result obtained with the H08 model is 17.74 Mt/yr, while the result with the 634 CWatM model is 46.89 Mt/yr. 635 636 Figure 12. Comparison of seasonality of flows and potential sediment transport in the Ebro River 637 produced by ISIMIP global hydrologic models and SIMPA model. Left: Seasonality of flows. Right: 638 Seasonality of potential sediment transport. 639 A bias correction procedure was applied to improve the alignment between the 640 ISIMIP models and the SIMPA model. This involved adjusting the monthly flow values 641 by multiplying them by the ratio of the average monthly values of the ISIMIP model to 642 those of the SIMPA model over the period 1978–79 to 2018–19. After applying this correc643 tion, the potential sediment transport obtained with H08 model is 32.79 Mt/yr, while the 644 result with CWatM model is 42.97 Mt/yr 645 The time series produced by the SIMPA model and the ISIMIP hydrological models 646 in all scenarios analyzed were processed using the sediment transport rating curve to ob647 tain projections of potential sediment transport under natural conditions. The analysis 648 was performed on the bias-corrected series. The results are presented in Table 2. The val649 ues correspond to the same periods shown in Table 1. Table 2 presents the values of mean 650 annual potential sediment transport obtained for the SIMPA and the average values ob651 tained with all climate drivers in each ISIMIP model and scenario. Rows and columns in 652 Table 2 follow the same structure as in Table 1. 653 Table 2. Mean potential sediment transport obtained in the simulations of the scenarios for the Ebro 654 basin, in Mt/yr. 655 Scenario obsclim obsclim historic ssp126 ssp370 ssp585 ssp126 ssp370 ssp585 Period 79-18 74-13 74-13 20-59 20-59 20-59 60-99 60-99 60-99 SIMPA 26.76 25.29 H08 gswp3 23.83 28.58 gfdl 28.40 20.23 13.99 16.40 19.99 11.77 8.00 mrl 30.36 24.91 15.41 32.79 31.05 25.74 14.54 ipsl 31.55 16.55 15.95 10.61 18.26 9.17 5.91 mpi 29.27 19.72 15.86 15.83 21.46 8.98 10.11 ukesm1 35.98 21.89 10.17 11.37 17.50 6.39 10.10 mean 23.83 28.58 31.11 20.66 14.28 17.40 21.65 12.41 9.73 CWatM gswp3 37.87 42.09 gfdl 43.13 39.51 30.99 36.15 43.36 32.44 27.52 mrl 45.06 48.22 42.06 55.62 50.38 48.93 38.74 ipsl 43.13 36.35 36.17 25.73 38.52 29.10 22.32 mpi 42.58 40.16 28.96 31.52 40.13 23.19 25.52 ukesm1 41.72 43.09 33.57 26.28 40.07 28.40 26.07 Water 2025, 17, x FOR PEER REVIEW 20 of 32 mean 37.87 42.09 43.12 41.47 34.35 35.06 42.49 32.41 28.03 656 The results show a significant difference between the mean potential sediment 657 transport estimated with the SIMPA model and those estimated with the two global hy658 drological models, despite bias correction. For the obsclim scenario, the estimate of the 659 SIMPA model is 26.76 Mt/yr. H08 provides a mean sediment transport for the Ebro River 660 of 23.83 Mt/yr (11% less than SIMPA), while the estimate of CWatM is 37.87 Mt/yr (41% 661 more than SIMPA). Therefore, these estimates are highly uncertain. The values for the 662 average over the five climate drivers of the historical scenario are 31.11 Mt/yr for the H08 663 model and 43.12 Mt/yr for the CWatM model. The corresponding values of the obsclim 664 scenario for the historical period, 1974-75 to 2013-2014, are 28.58 Mt/yr for the H08 model 665 and 42.09 Mt/yr for the CWatM model. The projections for future scenarios show more 666 variability than in the case of annual flows. The potential for sediment transport under 667 natural conditions is expected to decrease dramatically in some scenarios. 668 Figure 13 shows a summary of projected changes in mean annual flow and potential 669 sediment transport under natural conditions. The projected reductions in mean annual 670 flow will dramatically reduce the potential sediment transport capacity of the Ebro River 671 under natural conditions, due to the non-linearity of the sediment transport rating curve. 672 Although the H08 and CWatM models produce different absolute figures, the relative 673 projected evolution is similar in both models. In the first time period, 2020-2059, the ex674 pected reduction projected by H08 ranges between 14% and 22% for mean annual flow 675 and between 34% and 54% for sediment transport. The CWatM model projects a reduction 676 between 5% and 11% for mean annual flow and between 4% and 20% for sediment 677 transport. 678 679 Figure 13. Projected changes in mean annual flow and potential sediment transport in the Ebro River 680 under ISIMIP climate scenarios. Left: Changes in mean annual flow. Right: Changes in potential 681 sediment transport. 682 3.3 Sediment transport under altered conditions 683 The impact of projected reductions on the potential sediment transport capacity of 684 the Ebro River under altered conditions will depend on how the Ebro River is managed 685 in the future. Under the current management strategy, roughly 6 km3/yr are abstracted 686 from the 15 km3/yr of natural flow, leaving 9 km3/yr in Tortosa. This reduces the potential 687 sediment transport capacity from 27 Mt/yr under natural conditions to 9 Mt/yr, but this 688 figure is still enough to transport the sediment required by the delta. The projected reduc689 tions suggest a future mean annual flow between 12 km3/yr and 14 km3/yr. The potential 690 sediment transport capacity for the natural regime is expected to be reduced to between 691 14 Mt/yr and 25 Mt/yr. If the potential sediment transport under altered conditions was 692 similarly reduced, it would fall to a range between 4.5 km3/yr and 8 km3/yr and some 693 carrying capacity would still remain in the river. However, if current abstractions of 6 694 Water 2025, 17, x FOR PEER REVIEW 21 of 32 km3/yr were maintained in the future, available flow would be reduced to a range between 695 6 km3/yr and 8 km3/yr, which represents a larger reduction than that of natural flow. 696 Therefore, it is very likely that the potential for sediment transport under altered condi697 tions will be reduced more than under natural conditions. 698 The results produced by the AgWaMed project for the mean annual flow under the 699 hypothesis of maximum use of water resources under each climate scenario are presented 700 in Table 3, following the same structure as in previous tables. It should be noted that the 701 water demand is different in each of the scenarios, since it has been assumed that the water 702 resources corresponding to each scenario are used to the maximum. 703 Table 3. Mean annual flow obtained under the hypothesis of maximum use of water resources in 704 the Ebro basin, in km3/yr. 705 Scenario obsclim obsclim historic ssp126 ssp370 ssp585 ssp126 ssp370 ssp585 Period 79-18 74-13 74-13 20-59 20-59 20-59 60-99 60-99 60-99 Tortosa 8.96 9.66 SIMPA 6.57 6.75 H08 gswp3 6.21 6.72 gfdl 7.08 5.54 5.33 6.57 5.63 5.00 4.36 mrl 6.50 6.03 6.08 7.93 7.04 6.31 5.17 ipsl 6.94 5.78 6.69 6.79 5.50 4.54 4.47 mpi 6.97 5.79 5.88 7.38 6.10 4.45 5.43 ukesm1 7.27 6.03 5.93 5.92 4.99 4.18 4.72 mean 6.21 6.72 6.95 5.84 5.98 6.92 5.85 4.90 4.83 CWatM gswp3 8.91 9.35 gfdl 9.02 8.64 8.08 9.61 8.83 8.33 7.52 mrl 9.66 9.31 9.17 11.55 10.19 9.75 8.85 ipsl 9.02 8.53 9.00 8.44 8.50 6.92 6.53 mpi 8.97 8.56 7.78 9.59 8.84 6.67 7.85 ukesm1 9.11 9.35 8.36 8.24 8.61 6.55 7.47 mean 8.91 9.35 9.15 8.88 8.48 9.49 8.99 7.64 7.64 706 The results show that the estimate of the CWatM model fits the observed flow in 707 Tortosa better than that of the H08 model, but H08 is closer to the results provided by the 708 SIMPA model. For the obsclim scenario, H08 provides a mean annual flow for the Ebro 709 River of 6.21 km3/yr, while the estimate of CWatM is 8.91 km3/yr. The estimate of the 710 SIMPA model, 6.57 km3/yr, is similar to that of the H08 model, while the observed flow in 711 Tortosa, 8.96 km3/yr is almost coincident with that of the CWatM model. The values for 712 the average over the five climate drivers of the historical scenario are 6.72 km3/yr for the 713 H08 model and 9.35 km3/yr for the CWatM model. 714 The time series produced by the water resources simulation model in all scenarios 715 analyzed were processed using the sediment transport rating curve to obtain projections 716 of potential sediment transport in altered conditions. The results are shown in Table 4, 717 which presents the values of mean annual potential sediment transport obtained for each 718 scenario, following the same structure as in previous tables. The results show a significant 719 difference between the mean potential sediment transport estimated with the results of 720 the water resources model under the hypothesis of maximum water use and those esti721 mated with the observed flow in Tortosa. The differences are due to uncertainty in mod722 eling and to the fact that current water use in the Ebro basin is not maximum. 723 Water 2025, 17, x FOR PEER REVIEW 22 of 32 Table 4. Mean potential sediment transport obtained under the hypothesis of maximum use of wa724 ter resources in the Ebro basin, in Mt/yr. 725 Scenario obsclim obsclim historic ssp126 ssp370 ssp585 ssp126 ssp370 ssp585 Period 79-18 74-13 74-13 20-59 20-59 20-59 60-99 60-99 60-99 Tortosa 8.91 11.26 SIMPA 6.26 6.01 H08 gswp3 4.49 6.46 gfdl 6.95 4.33 3.99 5.45 5.21 1.46 1.71 mrl 7.19 8.54 3.11 16.32 10.53 9.97 3.30 ipsl 9.32 3.74 7.80 3.76 5.32 3.81 2.24 mpi 8.18 5.57 5.34 8.07 6.48 1.60 4.48 ukesm1 9.74 6.50 3.11 4.26 3.82 2.33 4.96 mean 4.49 6.46 8.27 5.74 4.67 7.57 6.27 3.83 3.34 CWatM gswp3 8.09 9.30 gfdl 8.31 6.84 6.27 8.20 8.81 4.92 6.25 mrl 9.88 13.91 8.36 19.16 12.76 14.34 9.42 ipsl 8.31 6.93 10.01 5.12 10.65 6.77 5.59 mpi 10.81 8.93 6.65 10.56 9.58 4.60 7.39 ukesm1 9.30 12.76 7.49 6.44 12.12 8.94 7.94 mean 8.09 9.30 9.32 9.87 7.75 9.90 10.78 7.91 7.32 726 The results obtained in the analysis of altered flow in future projections are summa727 rized in Figure 14. The effects are similar to those shown in Figure 13, but with a larger 728 spread between the hydrological models and the climate scenarios. In the first time period, 729 2020-2059, the expected reductions projected by the H08 model range between 0% and 730 16% for mean annual flow and between 8% and 44% for sediment transport. The changes 731 projected by the CWatM model for mean annual flow range between an increase of 3% 732 and a decrease of 3%. These changes result in a range between an increase of 6% and a 733 decrease of 17% for potential sediment transport. 734 735 736 Figure 14. Projected changes in mean annual flow and potential sediment transport in the Ebro River 737 under altered conditions in future climate scenarios. Left: Changes in mean annual flow. Right: 738 Changes in potential sediment transport. 739 3.4 Relationship between annual flow and sediment transport 740 The analyses conducted in this study allow us to characterize the relationship be741 tween average annual river flow and potential sediment transport capacity. Figure 15 syn742 thesizes the results of all simulations, showing the mean potential sediment transport 743 Water 2025, 17, x FOR PEER REVIEW 23 of 32 capacity as a function of the mean annual flow across the various scenarios and models 744 evaluated. The figure clearly illustrates a non-linear relationship between these two vari745 ables. The most reliable data correspond to the observed flow at Tortosa and the results 746 derived from the natural and altered flow scenarios simulated with the SIMPA model. 747 In particular, the SIMPA estimate for natural flow deviates from the trend suggested 748 by the ISIMIP model outputs. This discrepancy arises from differences in the seasonal 749 distribution of flow: in the ISIMIP models, discharge is concentrated in just a few months 750 of the year, whereas in the SIMPA model, flow is more evenly distributed throughout the 751 year. Since sediment transport is most effective during high-flow periods, ISIMIP models 752 yield a higher potential transport capacity than that of SIMPA under equivalent annual 753 flow volumes. 754 The comparison between SIMPA and Tortosa results under altered conditions indi755 cates that the assumption of maximum water use adopted in the modeling is conservative 756 for the observed period. Both the observed flow and sediment transport at Tortosa are 757 higher than those estimated by the SIMPA model. However, given that full utilization of 758 available water resources in the Ebro basin is likely to occur in the future, it is appropriate 759 to base the analysis on this conservative assumption. 760 761 Figure 15. Relationship between mean annual flow and mean potential sediment transport for all 762 scenarios examined. 763 3.5 Limitations of the analysis 764 As with any complex modeling and scenario-based study, our analysis presents cer765 tain limitations that should be acknowledged. These limitations relate to methodological 766 assumptions, data availability, spatial and temporal resolution, and the inherent uncer767 tainties associated with climate projections and hydrological modeling. Recognizing these 768 constraints is essential to properly contextualize the results and guide future improve769 ments. 770 One limitation of this study is the use of monthly flow averages in sediment transport 771 estimations, which introduces a nonlinear bias due to the shape of the transport rating 772 curve. A comparative analysis using daily and monthly observed data at the Tortosa sta773 tion showed an average underestimation of 11% in annual sediment transport when using 774 monthly data. This effect is systematic and reflects the loss of information on flow varia775 bility, particularly during short-duration high-flow events that are critical for sediment 776 mobilization. Although this bias is relatively small in the context of long-term average 777 transport capacity, it could become significant over decadal projections, particularly if 778 Water 2025, 17, x FOR PEER REVIEW 24 of 32 absolute values are required for infrastructure design or sediment management planning. 779 In our case, since the same time resolution is applied across all scenarios, the relative com780 parisons and trends remain consistent and reliable. Future studies could incorporate 781 event-based or daily-resolution modeling, particularly focused on flood events or con782 trolled releases, to better capture episodic sediment pulses that dominate delta sediment 783 budgets. Such approaches would be especially relevant to evaluate the feasibility and ef784 ficiency of sediment flushing or augmentation strategies. 785 The analysis presented in this study focuses on the transport of suspended sediment, 786 using a calibrated median grain size of D₅₀ = 0.0005 mm (clay), which was found to be the 787 largest particle size that could be transported to the delta without significant deposition 788 under current flow conditions. Simulations carried out with coarser fractions (silt with D₅₀ 789 = 0.005 mm) resulted in substantial sedimentation within the first kilometers downstream 790 of the Flix dam, confirming that current discharges are insufficient to transport such ma791 terial over long distances. This modeling choice reflects the dominant transport dynamics 792 in the current hydrological regime of the Lower Ebro River, where coarse sediment is 793 largely trapped in reservoirs and where downstream sediment delivery is predominantly 794 composed of fine suspended material. As noted by [21], the interruption of coarse sedi795 ment flow has significantly altered the morphology of the riverbed and the patterns of 796 delta accretion. 797 Although this focus allows for a reliable assessment of the present and future sus798 pended sediment transport potential, it constitutes a conservative estimate of total sedi799 ment flux, as bedload transport is not considered. Coarse fractions (silt and sand), alt800 hough largely immobilized today, are crucial for long-term delta morphodynamics and 801 restoration strategies. Future work should integrate bedload transport modeling and ex802 plore the feasibility of restoring coarse sediment connectivity through sediment augmen803 tation or engineered flushing schemes [25]. These approaches are critical to address the 804 sediment deficit in the delta and to recover the full range of sedimentary processes neces805 sary for the sustainability of the delta. 806 4. Conclusions 807 This study provides a comprehensive assessment of the sediment transport potential 808 of the Ebro River under historical, current, and future hydrological conditions. Using a 809 sediment transport rating curve developed through hydrodynamic modeling, the analysis 810 quantifies the impacts of flow regulation and climate change on the river capacity to de811 liver sediment to the delta 812 The results reveal a significant decline in sediment transport capacity due to anthro813 pogenic interventions. Compared to late 19th century natural reference conditions, when 814 the river transported an estimated 28.1 Mt of sediment annually, the current capacity has 815 declined by about 67%, with present day flows able to convey only around 9 Mt/yr. This 816 reduction results from a 41% drop in mean annual flow, primarily attributed to water 817 abstractions and flow regulation throughout the basin. 818 Projections for the midand late-21st century indicate further declines in both natural 819 and altered flow regimes. Between 2020 and 2060, natural flows are expected to decrease 820 by 10–15%, leading to reductions of 15–30% in sediment transport capacity. Under altered 821 flow conditions, assuming maximum water use, an additional 10% reduction in flow 822 could result in up to 30% further decline in sediment transport. These trends become more 823 pronounced between 2060 and 2100, with potential decreases of up to 30% in natural flows 824 and up to 50% in transport capacity. 825 Despite these reductions, current flow conditions still offer sufficient capacity to 826 transport more sediment than is currently observed reaching the delta, suggesting that 827 improving sediment supply (through bypass systems or sediment releases) could be 828 Water 2025, 17, x FOR PEER REVIEW 25 of 32 effective if it is synchronized with appropriate flow management. However, the strong 829 non-linear relationship between discharge and transport underscores the need for strate830 gic planning, ensuring that flow releases are timed and scaled to activate sediment mobi831 lization. 832 The presented analysis should be taken only as a first approximation to the problem, 833 since future projections show a highly uncertain scenario. The main sources of uncertainty 834 of the analysis carried out are the variations between the emission scenarios and between 835 the climate models, the limitations of the hydrological models to capture the real hydrol836 ogy of the basin and the non-linearity of the relationship between flow and sediment 837 transport capacity. 838 In general, the findings highlight the urgent need for integrated sediment and flow 839 restoration strategies in the Ebro River basin. Such efforts are critical to restoring sediment 840 continuity, protecting the Ebro delta, and enhancing resilience to ongoing and future cli841 mate pressures. 842 5. Patents 843 Supplementary Materials: The following supporting information can be downloaded at: 844 https://www.mdpi.com/article/doi/s1, Table S1: Projections of monthly flows in the Ebro River un845 der natural conditions. Table S2: Projections of monthly flows in the Ebro River under maximum 846 water use 847 Author Contributions: Conceptualization, L.G. and F.M.; methodology, L.G.; software, D.S. and 848 D.G.; validation, A.I., and D.S.; data curation, D.S. and D.G.; writing—original draft preparation, 849 F.M.; writing—review and editing, L.G.; supervision, A.I.; funding acquisition, A.I. and L.G. All 850 authors have read and agreed to the published version of the manuscript. 851 Funding: This research was funded by the European Union’s Horizon 2020 research and innovation 852 program under grant agreement No 101037097 (REST-COAST project). 853 Data Availability Statement: Daily flow data in the Tortosa gauging station may be downloaded 854 from https://ceh.cedex.es/anuarioaforos/afo/estaf-datos.asp?indroea=9027. The monthly series of 855 flow in the Ebro River used in the analysis are shared as supplementary material. 856 Acknowledgments: We gratefully acknowledge Carles Ibàñez of the Institute of Agrifood Research 857 and Technology for his insightful suggestion to perform the analyses presented in this paper and to 858 Miguel Ángel García Vera of the Ebro River Basin Authority for sharing the bathymetric data of the 859 Ebro River. We also extend our sincere thanks to the rest of the REST-COAST team for their fruitful 860 discussions, which played a significant role in refining our ideas and shaping the final work. 861 Conflicts of Interest: The authors declare no conflicts of interest. 862 Abbreviations 863 The following abbreviations are used in this manuscript: 864 AR6 6 th Assessment Report CHE Confederación Hidrográfica del Ebro (Ebro Basin Authority) CWatM Community Water Model ICOLD International Commission on Large Dams IPCC Intergovernmental Panel on Climate Change ISIMIP Inter-Sectoral Impact Model Intercomparison Project SAIH Automated Hydrologic Information System SIMPA Integrated System for Rainfall-Runoff Modelling SSP Shared Socioeconomic Pathway SWE Shallow water equations Water 2025, 17, x FOR PEER REVIEW 32 of 32 59. CHE (2022). Plan Hidrológico de la parte española de la Demarcación Hidrográfica del Ebro Revisión para el tercer ciclo: 2022-1093 2027. Memoria. 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