## 4 Results
### Changes in Occupied Surface  
Changes on the perimeter of occupied surfaces can be used as a proxy for net size fluctuations with time (Figure 6). This reveals that the SVB estuary increased its total occupied area with respect to 1957 (Figure 6). This was mainly due to an increase in anthropic features, which grew until the 2000s and stabilized afterwards. As expected for its geographical constraints (see the study sites section), the rest of the SVB subsystems mostly maintained their surfaces, as the possibilities to change their perimeters are limited. The GUA estuary, although it seems to be a fluctuating system (Figure 6), was the most stable case, showing a similar surface at the initial and the final years. The changes were related to variations in the sand bars in the intertidal zone, which are very dynamic features, the emergence of which depends on tidal and seasonal conditions. Lastly, the Ebro River Delta seems to be the most vulnerable, showing a decrease in the total occupied surface larger than 10%. This trend was supported by strong changes in the erosion process on San Antonio Island, which was counterbalanced with growth on the left side of the estuary.  
When analysing more specifically by Category 1 (Table 1), the percentage of changes allows for the identification of the most dynamic subsystems (Figure 7). To facilitate this identification, the tidal flat/wetland category was divided into Category 2 units (saltmarashes/vegetated wetland and mudflats/submerged meadows). The analysis revealed that, in the SVB estuary, the main changes affected the muddy environments, whereas the saltmarashes' extension decreased >20% (corresponding to approximately 85 ha), which gradually became mudflats (Figure 7). For the GUA and EBR estuaries, the changes mostly affected the sandy environments. In particular, the GUA estuary showed a stable saltmarsh, with a slight increase with time due to the protective role played by the sandy hooks upstream, and the mudflats almost maintained their initial surface. In the natural part of the estuary, new sandy features, especially dunes, increased. However, this effect was masked by the loss associated with anthropogenic constructions on the Isla Canela side (Figure 7).  
Figure 5: Eco-geomorphological map of the Ebro Delta mouth in 1956.  
The shoreline sandy environments in the GUA estuary include large areas of beach and sand bars (Category 2). These features experienced huge variations, what explains the corresponding temporal curve in the general trend of the system (Figure 6). In the case of the EBR estuary, the sandy subsystems (shoreline sandy environments and dunes) increased in the last few years by almost 3% (Figure 7). As described for the GUA estuary, this gain was partially masked and counterbalanced by the strong erosion suffered on the eastern side of the estuarine system. Despite this, the sandy environments increased by between 10 and 20 ha during the last 5 years (Figure 8).  
Figure 6: Temporal changes in the total occupied surface of the three study zones, referred to the initial area (ha) considered in the study period. Note the difference in reference time for the Ebro case.  
Figure 7: Temporal changes in the surface occupied by the main subsystems (Category 1) for the three study sites (% referred to the reference surface for each study site). Note the temporal scale changes on the x-axis.  
The channels are also very dynamic features in the three estuarine systems (Figure 7), whose differences due to the tidal regimes (meso- and microtidal) seem to affect the magnitude of the oscillating pattern. For the mesotidal sites, these variations were related to the effect of the tidal level on the periodical emergence of certain features (especially in the GUA estuary) at the moment the orthophoto was taken.