Ebro Delta: Evolution
Physical History of the delta.
The Ebro delta is the most important aquatic environment in the western Mediterranean after the Camargue and the second largest in Spain after the Coto Doņana National Park. Its 320 km2 form the most important wetland in Catalonia.
Evolution of the Delta alluvial plain
The current delta dates from the end of the last ice age when eustatic changes lead to its growth. Sediment was transported by the Ebro river and acted upon by marine, wind and river processes while successive changes in sea level allowed progressive and rapid growth over the sediment platform. Thus the delta is a relatively recent feature on the Mediterranean coastline. Indeed, the old town of Amposta which is thought to have existed from Neolithic times was a sea port in the fourth century. The period of most accelerated growth was in the fifteenth and sixteenth centuries.
As the diagrams above show, the delta has continually changed its form and many river mouths or 'goles' have been created and abandoned. Many of the previous distributaries provided the points from which the sediment used to build the two extensive sand bars reached the sea and after their abandonment these subsequently formed the freshwater lagoons.
Parts of the delta are now eroding as shown below and the 15% fall in sediment delivered from upstream, as a result of dam and reservoir construction further up the river Ebro, has lead to widespread concern about possible inundation by the sea. A certain amount of disagreement exists, however, over the threat of coastal erosion.
The modern delta has been and is intensely transformed by human activities and most of the area is now dedicated to agriculture -15,000ha. of rice and 9,000ha. of market gardening and fruit orchards. Large expanses of beaches, marshes and salt pans and lagoons still exist. Over 90km of unspoiled beach remain in addition to two large open sea bays enclosed by sand bars. Both of these sand bars enclose vast expanses of shallow water which are important for waders and gulls. This last type of environment is rare in the Mediterranean due to the low tidal range and is therefore of great importance.
Map of Ebro Delta showing the location of the Coastal Lagoons and the Land Use
All of these environments suffer from intense hunting, fishing and recently fish farming pressures.
Factors affecting Delta form
Where rivers reach the sea and are checked by the coastal processes deposition may occur. In conditions where coastal erosion is slight or sea level changes are taking place the formation of a delta may result. Such coastal wetlands are environmentally diverse and have dynamic hydrological systems where freshwater, brackish and sea water all contribute to the complex underground layers of the groundwater. Zonation is common as are the processes of natural vegetation succession and colonisation.
This is a diagram of the deltaic formations in accordance with the relative importance of the river deposits (rios), the force of the waves (olas) and the influence of tides (mareas). The shape of the Ebro River Delta would be the consequence of a position of relative balance between the river deposits and wave erosion, in an area uninfluenced by tides. This position can change in time. This would be the case if, for example, the solid river deposits diminish, due to the use of water for hydroelectric and irrigation purposes.