*3.2. Vertical Land Motion (VLM) and Line of Sight (LOS) Velocities*

For both ALOS and Sentinel-1A/B data, vegetation coverage and the high dynamics of the coastline resulted in areas below the coherence threshold of 0.65 established in this work. Thus, there is a lack of information westward of Cuatro Bocas lagoon, where wetlands and vegetation are widespread (Figures 5 and 6). However, bare and sparsely vegetated terrain along the barrier island, particularly in the eastern and central areas where salt plains, stabilized dunes, and paleo-beaches are typical, provides a continuous strip of sufficient return signal, achieving the coherence threshold.

VLM velocities, calculated by combining the LOS velocity vectors of Sentinel-1A (i.e., ascending track) and Sentinel-1B (i.e., descending track) are shown in Figure 5. The Supplementary Materials show the same results of Figure 5 after aggregating the data in 1.5 and 5 km sections. In the coastal town of Ciénaga and northeast from this location, it is observed that VLM velocities are larger than −0.5 cm/yr, and decrease (i.e., become more negative) around the CGSM. West of Ciénaga and seaward from the CGSM, the signal is patchy. Subsidence rates are the largest westward from the location known as Kangarú (see Figure 2b for location), reaching values of up to −1.0 cm/yr along the stretch of coast between Kangarú and Cuatro Bocas lagoon, a sector characterized by erosion rates larger than 5 m/yr (see yellow and orange transects in Figure 5). In contrast, velocities larger than +0.5 cm/yr were observed west of the CGSM mouth (see dark blue in Figure 5).

**Figure 5.** VLM rates for 2017–2021 as derived from Sentinel 1A/B data and coastline changes for 2010–2020 displayed with an arbitrary offset from the actual coastline for presentation purposes. Positive velocities (white and cold colors) represent stable areas and displacements toward the satellite, while negative velocities (warm colors) indicate displacement away from the satellite.

**Figure 6.** LOS velocities for (**a**) Sentinel-1A ascending track for 2017–2021; (**b**) Sentinel-1B descending track for 2017–2021, and (**c**) ALOS ascending track for 2007–2011. Positive velocities (white and cold colors) represent stable areas and displacements toward the satellite, while negative velocities (warm colors) indicate displacements away from the satellite. The location of the four time series described below is shown in (**a**).

LOS velocities from Sentinel-1A and B show the same trend as the overall VLM (Figure 5) along the deltaic barrier island, suggesting that the vertical component of land displacement dominates over the horizontal component at this incident angle (Figure 6a,b). LOS velocities related to scatterers in the coastal town of Ciénaga were around zero for all three tracks illustrated in Figure 6, and became smaller (i.e., subsidence increased), southward from the town, towards the eastern margin of the CGSM. Westward from

the town Ciénaga, specifically in the stretch of coast between the mouth of the CGSM to Kangarú (see Figure 2b for locations), LOS velocities of Sentinel-1A showed a zone ranging from 0 to +0.5 cm/yr (white colors in Figure 6a), whereas both ALOS and Sentinel-1B indicated LOS velocities ranging between −0.5 and −1.5 cm/yr (see orange and brown colors in Figure 6b,c). Westward of Kangarú (close to site 2 in Figure 6a), the LOS velocities of ALOS, Sentinel-1A, and Sentinel-1B revealed negative values along the coastline and around the wetlands.

The standard deviation (S.D.) of the LOS velocities, illustrated in Figure 7, indicates that the smallest spread of the LOS velocities, given by the standard deviation values closer to zero, is found for the Sentinel-1B data (Figure 7b). For all tracks, the smallest standard deviation values occur in the town of Ciénaga (Figure 7), reflecting higher coherence values in urban settings. In the undeveloped areas, the largest S.D. values occur on the CGSM shores (see red colors in Figure 7), where the most negative LOS velocities within the study site occur (Figures 6 and 7). In general, it is observed that extreme LOS velocities, either positive or negative, coincide with high S.D. values (red colors in Figure 7).

**Figure 7.** Standard deviation (S.D.) for each pixel of the estimated LOS velocities for (**a**) Sentinel-1A ascending track for 2017–2021; (**b**) Sentinel-1B descending track for 2017–2021, and (**c**) ALOS ascending track for 2007–2011. Red/green colors indicate high/low variability in the LOS velocities over time.
