## 2 Materials and Methods
### Data
#### 2.2.3 Remote Sensing Data  
To define the morphological changes in the selected river reaches, it is important to gather information before and after dam construction. For this purpose, three different types of spatial information were collected: aerial photographs, spectral images, and radar images. For the dams built between 1972 and 1986 (i.e., Prado, Salvajina, and Betania), the acquisition of aerial photographs supplied by the _Instituto Geografico Agustin Codazzi_ (IGAC; [[https://www.igac.gov.co/](https://www.igac.gov.co/)]([https://www.igac.gov.co/](https://www.igac.gov.co/)); accessed on 15 April 2023) was required. A total of 85 aerial photographs were purchased to characterize the pre-dam morphology (see Appendix B). Aerial photographs are acquired following flight lines that coincide with the river alignment, but several flights of the same year or nearly a year (not more than 5 years) may be needed. In the case of the Betania and Prado projects, the predominant spatial information is from 1969 (most photographs), so this year was used as a general reference. For the Salvajina project, a similar behavior was observed for the year 1961.  
The satellite images chosen for this study were obtained from (1) Landsat missions provided by the United States Geological Survey (USGS; [[https://earthexplorer.usgs.gov/](https://earthexplorer.usgs.gov/)]([https://earthexplorer.usgs.gov/](https://earthexplorer.usgs.gov/)); accessed on 30 March 2023), with a spatial resolution of 30 m (Landsat 4-5 TM, between 1982 and 2012; Landsat 7 ETM+, between 1999 and 2003; and Landsat 8, from 2013 to the present), and (2) Aster images provided by the National Aeronautics and Space Administration (NASA; [[https://earthdata.nasa.gov/](https://earthdata.nasa.gov/)]([https://earthdata.nasa.gov/](https://earthdata.nasa.gov/)); accessed on 30 March 2023), with a spatial resolution between 10 and 30 m, available from 2000 to the present. In areas with high cloudiness, Sentinel 1-GDR radar images from the European Spatial Agency (ESA; [[https://scihub.opernicus.eu/](https://scihub.opernicus.eu/)]([https://scihub.opernicus.eu/](https://scihub.opernicus.eu/)); accessed on 30 March 2023), with 10 m spatial resolution, were used.  
Spatial uncertainty in measurements of the river width depends on adequate delimitation of the water body borders [65]. These uncertainties appear when channels are digitized from aerial photographs and satellite images with vegetation, clouds, and shadows. The images used in this study are of a different type and year; therefore, river alignments may exhibit considerable changes. These errors are, however, not easy to estimate; therefore, only the uncertainty in the spatial resolution of each image and how this affects the river width estimation [66] were considered. Because most images considered in this study were Landsat images with a 30 m pixel size, all spatial information was worked on with this resolution to avoid error propagation. From our selected sites, the Cauca River is most sensitive to this aspect, with a width of 86 m (36% of possible variation), followed by the Sinu River with a width of 112 m (26.7% of possible variation). As the Magdalena River is the widest (channel width from 236 m to 302 m), it exhibits less uncertainty, from 10% to 13%.