## 4 Results
### Morphological Changes  
#### 4.3.1 Downstream Morphological Changes  
The channel width and sinuosity per section were calculated for all years, considering the pre- and post-dam periods. The values were averaged by river reach, and to identify the effects due to dam construction, the differences in the channel width and sinuosity were presented as a percentage, considering the pre-dam condition. A river reach is defined by the affluence of tributaries or changes in the river morphology. These values are schematically arranged in plots that include the confinement degree, the chronostrigraphic units, the erodibility index, and the main tributaries downstream of the dam. In the next sections, the main changes in the channel morphology of each selected dam are presented. * Morphological changes downstream of the Urra I Dam  
The spatial information available for this study site is distributed as follows: pre-dam years (1991 and 1998) and post-dam years (2007, 2018, and 2020). The percentage of change in the channel width and sinuosity referring to the pre-dam year of 1991 for this study site as a function of the downstream distance is presented in Figure 8. This figure emphasizes that downstream of the Urra I Dam, the general trend of the channel width after dam construction was to increase (see Figure 8f). After the construction of the dam, the Sinu River increased its width by 17% on average; meanwhile, sinuosity only decreased to \(-1.31\%\) on average. This general trend is consistent with the increase in low discharges and the sediment retention in the reservoir. However, these changes in the channel width are not correlated with the modifications to channel sinuosity. Figure 8e allows identifying areas with the changing channel width that almost maintain a constant sinuosity. These areas coincide with the location of the Betanci stream affluence and the Grande complex swamps (Figure 8d). In the high mountains, the river dynamic is driven by changes in lithology, because of the less thickness of the quaternary alluvium. This can be seen in the first 5 km downstream of the dam (see Figure 8), where one of the banks is dominated by rock, so the change in the channel width is less than that observed in quaternary-alluvium dominant areas. In Figure 8, it is also observed that in the year 2007, the channel width exhibited the maximum observed values (close to an increase of 20%). Areas with this high percentage of increase in the channel width are concordant with more erodible zones, where the quaternary alluvium is on both sides of the channel (see Figure 8b,c). In 2020 (the last year of comparison), some reaches had a reduced channel width in comparison with the maximum channel width observed during the study period; this negative trend indicates bed erosion and channel incision, a behavior that can be clearly identified near the city of Monteria. The morphological changes drastically reduced 219 km downstream of the dam, near the Cotoco site; therefore, this length can be assumed as the extent of the morphological impact due to dam construction. * Morphological changes downstream of the Salvajina Dam  
For this study site, the pre-dam years are 1961 (obtained from aerial photographs) and 1984, while the post-dam years are 2007, 2019, and 2020. The percentage of change in the channel width and sinuosity referring to the pre-dam year of 1991 for this study site as a function of the downstream distance is presented in Figure 9.  
Figure 8: Morphological changes downstream of the Urrá I Dam: (**a**) confinement grade; (**b**) geological units near the Sínū River; (**c**) Erodibility Index (EI) on the left and right sides of the river channel (RC); (**d**) sites of importance near the Sínū River, including affluent rivers and streams; (**e**) river sinuosity change as a percentage with respect to the pre-dam year 1991; and (**f**) river width change as a percentage with respect to the pre-dam year 1991.