We used Google Earth Pro historic images of the coastal sandbars immediately adjacent to the mouth of the four rivers to compare coastal change around the estuaries of the four rivers (Santiago, November 2010–April 2013; San Pedro, August 2010–March 2013; Acaponeta, October 2003–May 2013; and Fuerte, February 2004–March 2014). The time span of these images is variable because it depends on the availability of historic imagery but is always much shorter than that of the Landsat images, encompassing in all cases less than a decade and never reaching earlier than 2004. The pixel resolution, however, is higher (1.3 m × 1.3 m for all images available for the study sites) and allows for a more detailed assessment of coastal change. In all four rivers, we selected images at the same scale, encompassing roughly 1 km of ocean coast. Using the same procedure as with the Landsat images, we transformed each image into a binary representation of marine versus terrestrial environments, selecting in each image the marine pixels formed by ocean surface and unvegetated beach strands, and separating them from the remaining pixels, classified as terrestrial environments. By subtracting both images, we could identify areas that corresponded to terrestrial environments in the first date and had been eroded into the ocean later or, conversely, areas that were occupied by seawater or unvegetated beaches in the first image and had accreted into a vegetated terrestrial environment later. The area of lost/gained terrestrial pixels was divided by the length of the coast to obtain a linear measure of coastal advancement or retreat. Last, by numerically resampling the image with 10 random transects perpendicular to the coastline, we got an estimate of the standard error of the linear rate.

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