## 5 Discussion
Thus, the sediment supply, the coastal circulation system, the areas of wave refraction and the resulting shore currents are crucial to achieve a coastal equilibrium and to maintain the balance of the Guadiana River estuary [17; 32]. The relationship between sandy and muddy environments is clearly illustrated in the SVB and the GUA estuaries. In the last one, the progradation of the sandy spits reduces the input of wave energy and leads to the generation of new sheltered muddy environments (indicated as the mudflat* in Figure 3). This also occurred in El Garxal in the EBR estuary (Figure 4). The oldest information available for the EBR estuary (Figure 5) allows dating the formation of El Garxal between 1956 and 1984 (no orthophotographs are available between those years), sheltered by sandy environments, namely sand bars and San Antonio Island. Together with the action of waves and winds, this island/hemi-delta, mainly formed by sandy features, facilitated the formation of littoral/dune ridges on the other side of the channel, where they were controlled by wave and wind action [59]. Nevertheless, this island has been gradually eroded through decades by wave action and also because of the lack of river sediment input, as the discharges of this river have been intensively regulated since the beginning of the 20 th century [35]. As a consequence, San Antonio Island is presently recording erosion rates of almost 30 m/year in the river mouth area [35], causing the strong negative rates in overwash deposits shown in Figure 8 (more than \(-\)40 ha/year), which were recorded only for San Antonio Island. The possible disappearance of San Antonio Island in future years could expose _El Garxal_ to eastern, energetic waves and very probably would cause its disappearance and that of its associated features and, as consequence, the elimination of the current river mouth complex, i.e., the two hemi-deltas. Sandy/muddy environment relationships have been also described in other estuarine systems [5], and it is widely described that the weakening of a protective dune system after external impacts, natural or artificial, has consequences for local saltmashes' functioning [70; 71]. Building and urbanizing on the upper zones of shoreline sandy environments and dunes interrupt their dynamics. This pressure, in addition to the effects of global sea-level rise [72] and other effects of climate change on the coasts [73], makes these features prone to disappearing [74; 75]. To summarize, understanding the processes connecting estuarine subsystems is important to enhance current schemes in ecosystem connectivity, as this is key to the long-term functioning of ecosystems [64] and references therein. Complex system transitions, self-organization and connectivity over the years are crucial concepts to understand the dynamics and adaptation of coastal habitats [5; 76; 77]. This work shows how small-scale features have effects on the functioning of the estuary [78] and mapping is the best tool for its monitoring. Thus, detailed maps, in addition to the adaptation to European and national classifications and regulations, support a better understanding on the functioning of the estuarine systems, including the inner processes occurring therein [16]. Reducing the scale of working features provides stakeholders with the basis for drawing up management plans adapted to their specific systems. This work demonstrates that the reduction of the spatial scale helps to understand the causes of change and facilitates pressures to be identified in the short, medium and long term, and therefore improves the efficiency of estuarine management practices [79]. At present, increasingly available high-quality image datasets combined with automatic classification, i.e., on a pixel-by-pixel basis, are used to analyse changes in different landscapes [18; 60; 80]. However, for retrospective studies, it is necessary to use aerial orthophotograph libraries.