*Study Site*

The study area is located in the Colombian Caribbean, between the municipalities of Ciénaga and Barranquilla (Figure 2). The area extends 70 km along the coastline eastward

from the mouth of the Magdalena River. As the largest basin in Colombia, the Magdalena River and its tributaries cover 257,438 km<sup>2</sup> [19] and its discharge averages 10,287 and 4068 m3/s during high and low flows, respectively [20]. Eight major shifts in delta location have been reported for the river since the Pliocene, the latest of which has occurred since the mid-Holocene, by way of a westward migration of a former river mouth located by the Ciénaga Grande de Santa Marta (hereafter CGSM) to its present location [21] (Figure 2). Adding to the natural displacement of the delta, human-made structures associated with a harbor built in the 1920s near the mouth of the river have confined its mouth to a single channel, thus further reducing the present influence of the Magdalena River in the study area [22]. Moreover, a highway built next to the coastline in the 1950s to connect the cities of Barranquilla and Santa Marta formed a causeway for the otherwise free interchange of water between ocean and lagoons, producing accelerated degradation and mortality of mangrove forest due to hypersalinization [23].

**Figure 2.** *Cont.*

**Figure 2.** Study area and its surrounding wetlands and lagoons; (**a**) the epicenters of earthquakes and the coverage of ALOS and the Sentinel-1A/B ascending and descending radar data are included. The study site extends from the River Magdalena mouth (west) to the town of Ciénaga (east); (**b**) detail of the study area indicating the locations referred to in the manuscript. The wind rose displays the distribution of wind speed and direction for the study site.

The conditions described above have favored a transition of the landscape from fluvial to littoral-dominated process, portrayed by a palimpsest of fluvial and marine landforms such as channels, lagoons, oxbow lakes, salt plains, beaches, and dunes shaped by the predominant northeasterly trade winds [24] (Figure 2). Following Oertel [25], the landforms required to make up a barrier island system are present in the study area (i.e., mainland, backbarrier lagoon, barrier island, barrier platform, shoreface, inlets and inlet deltas). The sediments associated with these landforms, originated in marine and fluvio-lacustrine environments [26], range from poorly consolidated sands along the shorefront to laminated muds within lagoons and in the backshore. These Holocene sediments are bounded on the east side by the Santa Marta Bucaramanga Fault (SMBF), a regional-scale striking left-lateral structure that extends 374 km from the Caribbean coast to the eastern range of Colombia (Figure 2a) [27,28]. The fault marks a contrasting relief change between the Sierra Nevada de Santa Marta range east of the structure, and the flat topography of the Magdalena Delta floodplain to the west. According to the records of seismological activity in the area, there is no evidence of significant earthquakes associated with the SMBF with magnitudes (mb) larger than 4.9 since instrumental records are available (i.e., 1984). The few earthquakes registered for the observational period have magnitudes (mb) of less than 4.6 [29] (location of epicenters shown in Figure 2a).

Rapid coastline changes have been identified in the study area since the 1950s, affecting natural and human resources; specifically, coastal erosion rates increase alongshore in the same direction as the littoral drift (east–west) [24]. Chronic erosion is threatening the highway that connects the cities of Barranquilla and Santa Marta and, consequently, a rock rip-rap structure was built in 2014 to protect the highway from continuous erosive processes in a point known as the 20th km (the 0 km is located at the outskirts of Barranquilla) [30] (Figure 2). This hard structure has fixed the coastline in that area, but adjacent sectors keep rapidly retreating landward. The causes of coastal retreat have not been fully identified. A regional analysis of the sea-level trends for the Caribbean coast between 1950 and 2001 found a statistically significant sea-level rise trend of approximately 2 mm/yr [31]. Moreover, a sea-level rise of 5.3 mm/yr between 1950 and 2010 was measured at a tide gauge station located in Cartagena, approximately 100 km southeast of the study site [32], which indicates that, in addition to regional sea-level rise, there might be additional drivers contributing to the observed coastal retreat. Based on data from a global navigation satellite system (GNSS) station, subsidence velocities between −1.78 and −1.82 mm/yr have been quantified for Cartagena [33]. The influence of the rate of RSL change on barrier behavior

is not completely understood, but some of the known effects of RSL rise include reworking of sand eroded from barrier fronts into transgressive dunes and shifts in sediment supply due to the modification of slope and course of rivers [34].
