The Ebro basin is located at the NE of the Iberian Peninsula, with a surface of 85,550 km2 (17% of Spain). Climate is continental in most of the basin with a transition to mountain climate at the north (Pyrenees) and to Mediterranean climate at the lower catchment area and the coast. The main river is 928 km long and its main tributaries are the Segre, Aragon, Cinca and Gállego rivers. The mean annual flow near the mouth (Tortosa) was 592 m3s-1 at the beginning of the century. However, there is a continuous decreasing tendency since the 90’s (the average of the last 20 years is about 300 m3s-1).The population in the basin is about 3 million, with a density of 33 hab/km2. The main land use is agriculture, with more than 1 million hectares of irrigation (90% of water use in the basin). The basin has been strongly regulated by the construction of several dams (near 200), most of them built up between 1940 and 1970. There are two large dams in the lower Ebro River, Mequinença and Ribarroja, finished in 1964 and 1969 respectively.
The Ebro delta is located in Catalonia (NE Spain), being its seaward end at 40º 43’ N of latitude and 0º 53’ E of longitude. The delta plain has a surface of 320 km2, and the two existing bays account for another 68 km2 (Table 1). The width of the delta is 22 km and, like the surface area, they have remained rather constant during the last centuries (Figure 1). The delta started to undergo important human modifications by the mid of the 19th century, with the construction of irrigation canals and the transformation of wetlands to rice fields. Other major modifications affecting the delta during the 20th century (mostly the second half) have been the construction of dams in the watershed, the reduction and regularization of the river flow due to irrigation (and dams) in the watershed, the increasing eutrophication and pollution of river and coastal water bodies, the construction of pumping stations in the delta for agricultural purposes and the increasing impacts of tourism, among others. In the last decade there has been an improvement of the water quality of the river and delta, and several projects of environmental restoration are taking place in the framework of the Integral Plan for the Protection of the Ebro Delta (PIPDE).
Up to 80 % of the delta area has been developed (250 km2), mostly for rice field agriculture (210 km2), and there is only 56 km2 of wetlands left, as well as 14 km2 of lagoons and 82 km2 of bays. The total wetland loss is estimated to be about 200 km2. The Ebro delta is the second most Important Bird Area of Spain, and part of its surface (7.700 ha) is protected as a Natural Park, Special Protection Area (SPA) for birds and Ramsar site. Around 11.000 ha, including the Natural Park plus other wetlands and some rice fields, are included in the Natural 2000 Network of the European Union. Rice fields are nowadays the sustaining bases of most of the biodiversity of this important wetland area of the Mediterranean. 330 species of birds have been observed in the delta, from which 81 species breed regularly and other 28 do occasionally. Among breeding species 50 of them are aquatic, with 40,000 breeding pairs and a mean population of 180,000 birds in January. The Ebro delta has international importance for breeding of at least 24 species and for migration and wintering of 13 species, and occasionally for 14 more (SEO/BirdLife 1997).
The lower Ebro River used to be a strategic area for trade in Spain, at least since roman times, but from the end of the 19th century its importance started to decline, when the shipping rout through the river was abandoned. The Ebro River was the main trading route from the Mediterranean to Aragon and Castilla, and the fluvial port of Tortosa was important at a national level. On the other hand, the Ebro delta is important at national level for fisheries and aquaculture, as well as for rice cultivation and ecotourism. There is also oil extraction offshore that is important at the national level, though it is small in absolute terms. River flooding had in the past devastating effects in the lower Ebro valley towns, but in the delta plain the effects were no significant due to the topography and the low population density. Actually, the Ebro delta was created thanks to the big floods of the Ebro River delivering large amount of sediment to the coast.
The estuarine part of the Ebro River is 32 km long, and has a mean depth of 6.8 m and mean width of 237 m. Only one mouth is currently open, but under high flows, an extra mouth (Migjorn) can remain temporally open. The low tidal range causes the existence of a salt wedge estuary, with a maximum saline intrusion of 32 km. Its hydrological dynamics is mainly controlled by the river discharge. The mean annual river flow is, approximately, the critical flow that determines the formation and the break up of the salt wedge. The topography of the estuarine bed is also important for the advance and retreat of the salt wedge. There are several steady positions determined by shallow reaches in the estuary. When the river discharge is between 400 and 300 m3s-1 the salt wedge can occupy the last 5 km of the estuary, but with discharges lower than 300 m3s-1 the salt wedge advances quickly until a shallow sill at Gracia Island (18 km from the mouth). The salt wedge remains in this position until the river discharge is less than 100 m3s-1, and then it advances quickly to its maximum extent (a shallow sill 32 km from the mouth) (Ibàñez et al., 1997).
Before the construction of the irrigation system in the 19th century, the fresh water inputs to the delta plain were associated to river floods and to the springs of the Ullals area at the inner border. The most distal part of the delta plain (about 47 % of the surface) was occupied by lagoons and salt marshes affected by weak tides and storm surges. Only the higher areas located close to the river channels and to the inner border presented fresh water vegetation and a permanent superficial fresh water aquifer. Present superficial fresh water inputs to de delta come from the river through the irrigation system and from the springs of the Ullals, drained also through the drainage system to the sea. The hydrology of the delta has been strongly modified by the establishment of the irrigation-drainage networks for rice cultivation. There are drastic seasonal changes associated to the cycle of rice cultivation. From April to December (wet period) the irrigation canals are open and deliver 45 m3s-1 to the rice fields. During the rest of the year, the irrigation canals are closed (dry period) and the rice fields are dried out from January to April, in order to prepare land for cultivation.
River sediment transport into the delta at the end of the 19th century was estimated to be in the range of 30 million m3yr-1 (Gorría 1877, Ibáñez et al. 1996). At present, after the construction of about 200 dams in the basin, the solid discharge is only about 0.1 million m3yr-1 (Rovira and Ibáñez 2007, Tena et al. 2010), so more than 99 % of the sediment flow is retained in the reservoirs (Table 2). The bed transport was measured in one of these studies, giving values between 1,600 and 33,000 Mt yr-1 (Guillén et al., 1992). The same authors have estimated that at the beginning of the present century the bed transport was comprised between 0.4 and 2 million Mt yr-1.
The construction of dams began during the 20th century (the first one in 1913), but most of them were built in the period 1940-1975. The capacity of the reservoirs of the Ebro basin is around 7 km3 with and an area of more than 300 km2. The two large dams in the lower Ebro River, Mequinença and Ribarroja, were finished in 1964 and 1969, respectively. The initial capacity of the Mequinença reservoir was 1.53 km3, whereas for the Ribarroja reservoir was 0.22 km3. The drastic reduction in sediment transport due to dam construction has implied the suppression of peaks because of flood regulation. The last big flood in the lower Ebro took place in 1937, causing the change of the river mouth. The other big flood of the last century took place in 1907, the only one in which the river discharge was measured. Its maximum was reached on October 23, when a peak of 23,484 m3s-1 was recorded in Tortosa (40 km from the mouth). During the last decades, only small floods of 2,000 – 3,000 m3s-1 have occurred from time to time, whereas in the past they occurred almost every year.
Climate is continental in most of the basin, except in the Pyrenees (mountain climate) and in the coast (Mediterranean). In the delta, annual rainfall is close to 500 mm and mean annual air temperature is around 17 ºC. Tidal regime in the delta is semi-diurnal. The Mediterranean Sea is characterized by very weak tides, mostly in the range 20-30 cm. In the Ebro delta, mean and maximum tidal ranges are 16 and 25 cm, respectively. Meteorological tides are higher than astronomical tides, and monthly maximum surge height due to meteorological tides is about 1 m in the Ebro delta (Jiménez 1996). Minimum sea level is usually recorded in winter or in summer, especially under atmospheric high pressures. Maximum sea level and rainfall are reached in fall, with a secondary maximum in spring.
The evolution of the Ebro Delta during past centuries clearly shows a relative equilibrium between river-dominated patterns and wave-dominated patterns, which is a response to a very irregular river flow and a variable marine wave climate. Before massive dam construction, maximum floods reached over 20,000 m3s-1 whereas minimum flows of about 50 m3s-1 occurred in summer (Maldonado 1972). The marine climate use to be mild, although during occasional days strong easterly winds from the sea might occur. East winds of up to 117 km h-1 and offshore waves as much as 8 m high were recorded in 1990. However, ocean borne storms are almost never destructive.
Because of such riverine and marine conditions, there have always been strong marine redistribution and reshaping processes on the continuous sandy coast, and consequently, a relatively continuous delta front stratum has developed under the superficial riverine and lagoonal clays of the delta plain. As a consequence of the nearly complete retention of sediment discharge due to the dam construction, the growth at the present river mouth has virtually stopped and the delta is now wave-dominated, tending to smooth the coast line by processes of coastal erosion, transport, and re-sedimentation. The existence of spits at both extremes of the outer coast causes a nearly closed system with a sand budget without net losses to the emerged surface (Canicio and Ibáñez 1999; Ibáñez et al. 1997).
Around 40 % of the delta plain is below 0.5 m above mean sea level and part of the southern margin of the delta is at mean sea level in an area protected by small dikes. Preliminary estimates of mean rates of secular subsidence in the Ebro Delta range between 1 and 6 mm yr-1 whereas eustatic sea level rise is nowadays about 3 mm yr-1. Thus, the present relative sea level rise (RSLR) rate in the Ebro Delta is at least 4 mm yr-1. Measured accretion rates in the delta range from 4 mm yr-1 in the wetlands surrounding the river mouth to less than 0.1 mm yr-1 in some salt marshes (Ibáñez et al. 2010). The annual sediment deficit in the delta plain to offset RSLR is at least 1 million m3 yr-1. Accretion rates in the rice fields before the construction of large dams in the Ebro watershed were higher than RSLR rates, ranging between 3 and 15 mm yr-1 (Ibañez et al. 1997).
Water quality of the Ebro River and the delta have changed during the last decades due to sediment reduction and increased eutrophication and pollution. Eutrophication strongly increased during the 70’s and 80’, and mean annual ortophosphate and nitrate concentration increased in the lower Ebro river from 0.2 mg l-1 and 3.0 mg l-1 in the 70’ to 0.9 mg l-1 and 9.0 mg l-1 at the beginning of the 90’s, respectively (Ibañez et al. 1995). Among the main causes are the development of intensive farming, increased population in the basin and industrial development, as well as dam construction and reduced discharge of the river. During the 90’s the values have tend to stabilize due to the construction of water treatment plants in the towns of the basin. Fluxes of nutrients into the coastal system from the river basin in the period 1986-87 were estimated to be 15,700 Mt yr-1 for dissolved inorganic nitrogen (DIN) and 900 Mt yr-1 for dissolved inorganic phosphorus (DIP) (Muñoz 1989). During the last decade a process of olgotrophication is taking place in the lower Ebro River, due to the decrease in phosphorus that has lead to an increase in water transparency and the proliferation of submerged macrophytes (Ibáñez et al. 2007).
The ecological functioning of the Ebro Delta at present is largely dependent on and affected by human activities because of modification of the natural hydrological regime. There have been changes in the temporal and spatial patterns of salinity, nutrients and organic matter concentrations, pesticide pollution, and changes in the spatial and temporal patterns of sediment transport and deposition in the deltaic plain. Rice cultivation, because its surface extent and freshwater needs, has become the crucial element in the hydrology of the Ebro Delta. All the aquatic ecosystems of the delta are influenced by water coming from rice fields. From April to October, approximately 45 m3 s-1 of river water are diverted to the irrigation canals to continuously feed the rice fields with freshwater. Although river water is rich in dissolved and particulate nutrients (Muñoz and Prat 1989), the farmers add large amounts of fertilizers to enhance rice production, as well as several types of pesticides, mainly during spring and early summer (Forés and Comín, 1987). Drainage canals carry water coming from the rice fields to the bays and the sea.
To summarize, the modern history of the Ebro Delta can be divided in four periods reflecting human impacts and management practices in the river and delta:
Until the first half of the 19th century, when the irrigation network of the south hemi delta was developed, there was a quite natural low-population delta, with large river floods and high-suspended sediment flux, high accretion rates, large wetland surface and low eutrophication.
From the first half of the 19th century to the 60’s, when traditional rice agriculture was developed, there were no big dams in the lower Ebro River, and moderate and high floods were still frequent. Enough accretion rates in the rice fields took place despite of different kind of dikes (roads, canals, rice fences, etc.), essentially because the suspended solids in the irrigation water were still high. The wetland surface was drastically reduced (mostly from 1900 to 1950), and eutrophication was low.
From the 60’s to the 90’s, when two big dams (Mequinença and Ribarroja) were constructed in the lower Ebro River, modern agriculture arrived and population increased. Solid river discharge and accretion rates in the delta plain became insignificant. The wetland surface decreased slightly, and eutrophication and pesticides increased markedly.
From de 90’s to the present, during which an improvement of the ecological status of the river and delta have occurred, mostly due to the creation of the Natural Park and the application of the European environmental legislation.
Human settlements within the delta were originally dispersing, but those located in the levees at the central part of the delta (Deltebre and Sant Jaume) became real towns in the second part of the 19th century. However, in the inner border of the delta there are some old towns like Amposta, the main city of the delta (20.000 inhabitants nowadays). Another important city is Sant Carles de la Ràpita (15.000 inhabitants), funded in the 18th century by the king Charles III with the idea of creating the Ebre valley port at the Alfacs Bay. Nowadays is mainly an important fishing port. The present population of the Ebro Delta is close to 60,000 inhabitants, with seven towns.
The main human activities before rice cultivation were salt production, glasswort harvest for soap production, artisanal fishing and hunting, cattle and some farming on the higher zones along the river levees and the inner border. At present, agriculture is the main human activity of the delta and rice fields play a crucial role in its economy and its ecology. Moreover, other activities depending on the environment like tourism, fishing, aquaculture, hunting or those directly related to nature conservation, have an increasing importance.
The primary control over planning, management and development of the delta is in charge of the Government of Catalonia (Generalitat de Catalunya). This institution has exclusive attributions in land use planning and infrastructures, environment and nature conservation, tourism, agriculture and coastal fisheries, among other issues. The Catalan Government has also most of the attributions in water management (Agència Catalana de l’Aigua), except those concerning watershed management (Confederación Hidrográfica del Ebro) and water transfers between different watersheds (Ministerio de Medio Ambiente), which are in charge of the central government (Gobierno de España). The central government is also in charge of the coastal management in all the public land, mostly the coastal fringe (Dirección General de Costas). The city councils are also in charge of land use planning, but always under the criteria determined by the regional government. The Natural Park of the Ebro Delta is in charge of nature conservation of the protected areas.
There is no organization for the integrated planning and management of the entire delta. Such a plan would need the agreement of the regional and central governments. Planning and management is not really integrated nowadays, and there is little coordination between the different administrations and between the different departments of each administration. The only existing planning of the entire delta is the “Pla Director del Delta de l’Ebre”, which is a land use planning (mostly urban planning) elaborated by the regional government in 1995. The most recent Plan of Integral Protection of the Ebro Delta (2006) is mostly an environmental restoration plan, but not a proper management plan. On the other hand, the Natural Park is the institution able to do some vertical integration of management from the local to the national level, but it is only in charge of 25 % of the delta surface (the protected areas) and has no management plan for its own territory.
At the watershed, level the situation is similar, with little coordination between administrations. However, there is a water management plan of the watershed (Plan Hidrológico de la Cuenca del Ebro) elaborated by the basin authority (Confederación Hidrográfica del Ebro), that has been updated in 2010, which is mostly devoted to develop hydraulic infrastructures (dams, irrigation canals, water treatment plants, etc.) and control of water quality and uses. This plan has not included an environmental flow regime for the lower Ebro River and delta, and this subject is a matter of conflict between the lower basin and the upper basin stakeholders and administrations.
The environmental crisis of the Ebro Delta started in the 60’s, when the mechanisation and the chemical agriculture arrived and they strongly increased the impact of human activities on the environment. Formerly, the transformation of wetlands into rice fields implied a severe reduction of natural habitats, but the fact that rice fields are wetlands in many aspects and the use of traditional techniques of farming allowed the conservation of many aquatic species, though those linked to freshwater environments were favoured against the halophilous ones. The future possibility of transformation of rice fields into other non-flooded crops because of a decrease of rice price is a considerable threat to the deltaic environment. This transformation would imply a large loss of aquatic habitats and a most intensive drainage of the land that produces salt stress and drainage waters of low quality. Ensuring the continuity of rice cultivation and introducing changes in its practices tending to minimise the impact on the environment are priority goals for the conservation and sustainability of the deltaic system. In this sense, the application of the agro-environmental measures of the European Union and the organic farming represent an important step forward.
The fragmentation and small surface of the natural areas, as well as the absence of an institution for a global management and protection of the entire delta, are important problems. The reduction of the marshes to a narrow belt around the lagoons and along the coast diminishes the ability of these areas to contain fauna, to conserve the natural gradients of plant communities and to accomplish the function of nutrient filtering. Consequently, the biodiversity and productivity of the lagoons is also affected, because the ecological functioning of both environments is closely linked.
The reduction of sediment transport of the Ebro River is about 99 % of that existing before the construction of reservoirs in the basin. The construction of the Mequinenza and Ribarroja reservoirs at the end of the years 60 drastically culminated this process of reduction. Under these conditions, the Delta has stopped its growth and the coast is being strongly reshaped by waves, though there is no net loss of surface so far. Additionally, the sediment deficit and the relative sea level rise (eustatic rise + subsidence) imply a loss of land elevation of the deltaic plain. This means that approximately 50 % of the emerged plain will likely be under sea level at the end of the present century, causing the formation of polders and wetland degradation due to salt stress and water logging (Ibáñez 2009).
The extension of tourism during the last decades has caused an increasing environmental impact that should be minimised in the future through a careful planning and the development of ecotourism as an alternative to the growth of the classical tourism of sun and beach. The lack of a global land use planning has lead to a strong degradation of the landscape of the Delta. The almost complete deforestation, the loss of traditional architecture, the excessive proliferation of electricity lines and the cementation of the irrigation canals, are some examples of this degradation.
The effective protection and restoration of the river margins is not only an important conservation goal in the Delta but also a necessary condition for a sustainable use of the river for recreation purposes. A tour along the river reveals the situation of strong degradation of its margins that now is being partially reversed through some restoration efforts. The erosion of the margins due to navigation causing the fall of trees and the accumulation of many types of waste are the main examples of this degradation.
After the creation of the Natural Park in 1986, this institution started a process of regulation of the hunting activity with two main goals: to reduce hunting pressure and to rationalise the hunting practice. All the areas of common exploitation have been suppressed, and a network of hunting reserves and refuges has been created (they occupy 4,400 ha, a 57 % of the Natural Park surface). As a result, the populations of hunting species show in general terms an increasing tendency. However, the Ebro Delta presents the maximum concentration of lead pellets found in world wetlands (up to 2,500,000 pellets per ha in some areas). Because of the high presence of pellets in the Delta, the levels of lead poisoning in birds are also high. The mallard (Anas platyrhyncos), a good indicator species due to its worldwide distribution, presents a 27.35 % of the population of the Ebro Delta affected by lead poisoning. The figure is high if compared with other Mediterranean areas, where values range from 1 to 18 %, being only comparable with the Camargue (Rhone Delta).
The degradation and overexploitation of the lagoons have led to a strong reduction of fish captures in the four lagoons where fishing is carried out. In the case of marine fishing, the total captures of the last years are stabilised around 6,000 tones per year despite of the increase of power and tonnage of the boats, thus indicating an overexploitation of the resource. The habitat degradation because of fishing practices is a worrying question, especially when dragging fishing is concerned. The aquaculture had a big development in the years 80, mainly in the bays (Fangar and Alfacs), where mussel and oyster cultures were developed. However, water quality problems (high temperature and low oxygen in summer) and the excess of exploitation permissions with no consideration of the loading capacity of the bays, caused a decrease of the production and problems of mussel mortality.
Table 1: Topographic distribution of the delta plain and surface of the different units.
Table 2: Historical data on sediment transport (million Mt yr-1) in the lower Ebro River and reservoir capacity (Km3) in the basin.
Figure 1. Map of the lower Ebro River and delta.