An example of the Ebro Delta coastline’s evolution in the 20th century, using registry and overlay of maps, aerial photographs and satellite images.

 

 

Contents

Example prepared for the tutorial by L. Solé-Sugrañes and Oscar Miralles. Earth Sciences Institute "Jaume Almera", CSIC:

The tools used in image processing and analysis, commonly applied in Remote Sensing, can be highly useful in the comparison of maps and images from very different sources in an attempt to analyze the changes that have taken place in a given area over a period of time. This example is a demonstration of the use of these methods to analyze and evaluate Ebro Delta evolution throughout the 20th century

Geographical location

The Ebro River Delta is in the northeast coast of the Iberian Peninsula, lying midway between the cities of Barcelona and Valencia. In 1995 its area totaled some 331 km², making it the third largest of the Mediterranean, after the Nile (Egypt) and Rhone (France). Within the deltaic area, the river’s course is approximately 31 km in length.

Delta population is relatively low. Nevertheless, it is a region of key economic and ecological importance. LANDSAT satellite images show that 56% (18,614 ha) of its surface consists of irrigated crops, mainly rice. Roughly 15% is occupied by other species of plant life, essentially swamp and marshland vegetation. Freshwater marshes and shallow lacunes constitute yet another 9% (3,000 ha) of the delta area. These are the most important wetlands of the Spanish Mediterranean coast, and a vital natural reserve where migratory birds stop and nest. The remaining area (20%) consists of a combination of sandy soil, towns channels and roadways. The delta’s most important industry, after from rice farming and fishing, is the extraction of salt in salt marshes. Arabs introduced the activity in the 9th century.

During the 1940’s, a significant recession began to be observed in the delta’s frontal part (the river mouth area). This retreat is partly due to the use of river water for irrigation, as well as the construction of some 140 reservoirs along its tributaries and the Ebro itself. These changes tend to affect the river’s flow, causing the solid components it carries to be retained. This shows the importance of having access to systems that allow researchers to analyze and compare different maps, aerial photographs and satellite images, in order to precisely define coastal evolutionary trends. These processes can then be monitored economically, quickly and precisely, without requiring expensive on-site topographic surveys.

 

The formation of the delta

Deltaic constructions are formed at the mouth of the river during periods of stability in the level of the sea, when there is an excess supply of stream-borne sediments which can not be taken into the sea by the surf, coastal currents or tides. The dynamic balance between these factors is determinant in shaping a river mouth and its deltaic plain.

In the Western Mediterranean sea, there was a certain stabilization of the level of the sea, at some 100 m below its current level during the last glacial period approximately 18,000 years ago. During this era, it is likely that the river ran up to the outer edge of the continental shelf (the platform surrounding the continent, between 80 and 100 m below sea level). The sediments carried by the river were probably scattered around the continental slope to greater depths.

When the thaw of the last glacial period began, the sea rose progressively until it reached its current level, approximately 6,000 years ago. It was at this time when the delta as we know it today began to be formed.

Although formation of the delta began some 6,000 years ago, for a number of millenniums the sediments were dispersed on top of the continental shelf, without ever constituting an emerged surface. The subaqueous delta (or delta front) is estimated to have a surface area of 2,000 km², compared to the 331 km² of emerged surface.

The first Iberian inhabitants settled near the mouth of the Ebro river, near Amposta, 24 km inland from the present-day mouth. The Phoenicians and later the Romans constructed an important river port in the city of Tortosa (17 km upriver from Amposta). There are reports from this epoch which describe the river mouth as an enormous estuary, 2 km wide, between Amposta and Tortosa. In fact, throughout the entire Moorish occupation and until the end of the Middle Ages, Tortosa was a major river and sea port, through which most of the trade between Zaragoza and other cities of the Mediterranean was channeled.

The Arabian geographers were the first to record information relative to the existence of a deltaic plain, where great salt works were founded. In the 12th century, the river mouth was probably where Gracia Island is today. There are 14th century reports of the mouth being some 20 km downstream from Amposta, next to the Al-Kebtil (La Cava) seaport. Today, the mouth is 30.9 km downstream from Amposta.

During this period, it is likely that the delta was developing into a trilobate shape, with three mouths pointing to the north, east and south. There exists geomorphological evidence which leads to the reconstruction of the delta in this shape (Maldonado and Riba, 1972). Yet, there are many historical records on the river mouths starting in the Middle Ages. In Muntaner’s chronicles on the reign of James I, there is mention of the Port of Fangós, located at one of the three river mouths with a southern orientation. This port was of great commercial (salt transport) and military importance starting in the 12th and until the middle of the 14th century, when it was almost completely filled in and had to abandoned. In 1577 the Port of Fangós was within the limits of a lagoon called "lo pantà" (the swamp). It is likely that the Trabucador bar and "Els Alfacs" are the results of the erosion and redistribution of sediments by the coastal currents from this bygone southern port. Starting in the 15th century, the Port of Els Alfacs substituted El Fangós. This port remained open until the more modern San Carles de la Ràpita (1780) facility was built, during the reign of Carlos III.

In a map from 1777 the current geometry of the delta can be clearly seen, with Els Alfacs and El Fangar bays. The river’s course seems to have taken advantage of what must have been the eastward lobe (the shorter way to the sea). A short ways before its mouth, the river forks, birthing Buda Island and the Mitjorn and Llevant mouths, which were active until the first half of the 20th century.

 

The Delta in the 20th century

The evolution of the Ebro Delta has been well documented throughout the 20th century through precise maps dating from 1918 with a 1:50,000 scale, vertical aerial photographs since 1956, high-resolution satellite images since 1972 (the LANDSAT MSS series) and 1984 (the LANDSAT TM series) and several studies on river flow and coastal dynamics which began to be conducted at the turn of the century.

During this century, the most significant changes have taken place in the river mouth area, but there have also been far-reaching changes in the El Fangar and Els Alfacs bars or horns. In the delta’s interior, the swamp and marsh zones have also undergone profound changes. These have been mostly intentional, in order to make rice farming possible.

In 1940, Carreras Candi estimated that in the river mouth region called the "Gola Est", next to the Cape of Tortosa, an annual delta progradation of approximately 7-8 m was taking place. The 1918 map (1:50,000 scale) shows the prominent shape of the Cape of Tortosa. This morphology had barely begun at the end of the 18th century, as can be clearly seen in a map from 1777.

One of the landmark events in the river’s recent evolution occurred in 1937, when a major flood (the second-worst of the 20th century) broke through the septentrional bank, opening alternative mouths toward the north. The topographical map from 1950 includes these three mouths, but the main eastward mouth (Gola Est) is still in use. At that time, the recession of the Cape of Tortosa was still insignificant.

However, the new mouth cut back the river’s course and created a more efficient route. Thus, a few years later (1957 aerial photograph, 1958 map) the northern mouth had become the most active, the eastward mouth definitively abandoned and a significant recession of the Cape of Tortosa had begun. The satellite images from 1984, 1991 and 1995 clearly demonstrate how this delta progradation process is manifested toward the north from the new mouth as well as the recession at the old mouth at the Cape of Tortosa. These movements are due in part to the fact that the river sediment is deposited next to the northern mouth, and partly because the eroded sediments from the Cape of Tortosa are dragged northward by coastal currents.

The redistribution of sediments by the sea currents, from the frontal area toward the north and south, is what determines the minor modifications of the coast, as can be observed on its meridional "Alfacs" and septentrional "Fangar" limits.

 

Materials used

 

For the analysis of the Ebro coastline, mainly during the 20th century, the following cartographic material and images have been used.

 

 

Methodology and evolution of coastal line (next page)