ANALYSIS OF LANDCOVER SHIFTS IN TIME AND THEIR SIGNIFICANCE: AN EXAMPLE FROM THE MOUTH OF THE GUADIANA ESTUARY (SW IBERIA)

Many estuaries, in particular their mouths, are highly dynamic areas. The deposition and erosion of sedimentary bodies is dependent on many factors such as sediment supply (both from rivers and littoral drift), local setting, wave and tidal climate, storm frequency, floods, and, increasingly, human interventions.

In an attempt to better understand the temporal and spatial evolution and interrelationship of different inter- and supra-tidal sediment bodies related to estuaries, the well-documented example of the Guadiana Estuary mouth in SW Iberia was used as a case study. A series of aerial photographs from the 1940’s to present was analyzed utilizing GIS.

A statistical analysis of landcover shifts on the estuary margin, which has experienced continuous progradation throughout the analyzed period, shows that dune belts and marsh areas in the area evolved independently from each other. Both areas were found to react significantly to anthropogenic changes in the sedimentary regime. The dune belts on the western margin show an evolution from non-vegetated sandy areas, to areas with sparse, then dense vegetation, and finally areas covered with trees. The rate of change of these landcover successions was strongly influenced by the construction of jetties in the estuary in 1974. In the marsh belts, channel migration rates decreased significantly after the construction of these jetties, completely altering the sedimentary regime. At present, the dominant succession of landcover areas is from intratidal sands to channels and sandy marsh, to marsh (i.e. a fining upward succession). On the eastern estuary margin, which prograded into the main estuary channel halving its width, the construction of dams in the river basin is thought to the significant factor determining landcover evolution.

The spatial and temporal analysis of landcover shifts has allowed a better understanding of timing and magnitude of the environmental impact of anthropogenic changes in the Guadiana estuary mouth. This simple method can be applied to any similar environment with a high enough temporal density of aerial photographs.