## 2 Study Sites  
This work focuses on three study sites (Figure 1): (1) the San Vicente de la Barquera estuary (SVB), (2) the Guadiana River estuary (GUA) and (3) the Ebro River Delta mouth (EBR). These zones are located on different biogeographical regions along the Iberian coast: (1) the North Atlantic region, (2) the South Atlantic region, and (3) the Mediterranean region, which covers most of the environmental range and oceanographic conditions that occur on the Iberian Peninsula coasts. The first study site, San Vicente de la Barquera estuary (SVB; Figure 1 (1)), is located on the Cantabrian coast (N Spain, Gulf of Biscay). It is made up of two shallow estuarine subsystems and linked to a short river with a low hydrological regime [28]. Like most of the Cantabrian estuaries, it forms an incised valley and presents a sandy confining barrier, artificially channelled by two lateral jetties which change the morphodynamics of the entire estuary [28]. Regarding hydrodynamic conditions, SVB is a tide-dominated estuary, according to the classification proposed by [23]. It shows a semi-diurnal mesotidal regime with a mean spring tidal range (MSTR) of 3.94 m and a Highest Astronomical Tide (HAT) that reaches 4.85 m (Santander tide gauge, [29]). Tide is the most important dynamic factor in this zone, leading to well-developed tidal flats, extending upstream about 6 km from the river mouth. The predominant waves come from NW, with the most frequent significant wave height ranging between 0.5 and 1 m, peak periods greater than 9 s in most cases (frequency of 65%), and an eastward-dominant longshore transport. In this particular area, the coast orientation favours the refraction of the incident waves and forms a local coastal drift current moving westward. This region is considered a high-energy area, with strong and persistent winds from various directions but mainly blowing from W-NW [28]. The regional climate is oceanic, with no dry season. The second study site is the Guadiana River estuary (GUA, Figure 1 (2)). It is located on the southern border between Spain and Portugal, consisting of a single channel (50-700 m wide; [30]). It is also incised in a bedrock valley [31]. The marine area of this estuary, i.e., the lowest part of the estuary influenced by seawater, extends 10 km upstream from the mouth [32]. According to [23], it is a wave-dominated system. It has a mesotidal regime, with an MSTR of 3.06 m and a HAT of 3.87 m (Isla Canela tide gauge, [29]). Predominant waves come mainly from W and SW with an average significant height between 0.5 and 1 m [33], which produces a main littoral drift moving eastward. The winds in this region come from NW and SW, with an average wind speed of 3 to 4.5 m/s [29]. The regional climate is semi-arid, with the exception of the summer and winter seasons,  
Figure 1: Location of the study areas along the Iberian coast (central), with a closer look at the sites (**1**–**3**) and detail on the sequence of aerial orthophotos available for the study period (bottom). The legend of the dots matches colours in the central image: (**1**) SVB: blue, (**2**) GUA: green and (**3**) EBR: red. which are considered as arid and temperate-humid, respectively [31]. During the mid-20 th century, the Portuguese margin was transformed by the construction of jetties and revetments, leading to a severe rigidization of the shoreline; for this reason, in this study, only the more natural, Spanish margin will be evaluated. The Ebro River Delta mouth (EBR, Figure 1 (3)) is located on the Mediterranean coast of the Iberian Peninsula and is one of the most important deltas in the western Mediterranean [34]. The extension of the emerged area is 325 km\({}^{2}\), which represents only 15% of the total area of the delta [35]. It is a low-lying area, with elevations not exceeding 5-6 m, and around 50% of the total surface lies below 0.5 m above MSL [36]. The Ebro Delta has a microtidal regime, with a MSTR of about 0.20 m [37], which gives a greater importance to the meteorological tide component.