## 4 Results
### Morphological Changes
#### 4.3.1 Downstream Morphological Changes
Figure 9: Morphological changes downstream of the Salvajina Dam: (**a**) confinement grade; (**b**) geological units near the Cauca River; (**c**) Erodibility Index (EI) on the left and right banks of the river channel (RC); (**d**) sites near the Cauca River, including affluent rivers and streams; (**e**) river sinuosity change as a percentage with respect to the pre-dam year 1961; and (**f**) river width change as a percentage with respect to the pre-dam year 1961. Figure 9 shows that downstream of this dam, the morphological changes had two contrasting behaviors, observed in the high mountain area and the alluvial valley. In the high mountain area (between 0 km and 39 km) with high-to-medium confinement, the tendency of the channel was to increase its width, with some changes in sinuosity, during the post-dam period, with a short reach in which the channel width decreased between 1984 and 2019. The most recent year, 2020, presented an increase in the channel width in most areas. The first 17 km of the Cauca River downstream of the Salvajina Dam exhibit a similar behavior to that reported for the Sinu River: the channel width and sinuosity respond to the bank composition; fewer changes are observed in areas with low erodibility, and vice versa. The transition from high mountain to alluvial valley (see Figures 4 and 9a,b) occurs between 17 km and 23 km, where the channel width increases but sinuosity remains constant. From 23 km to 39 km, the channel width increases, which coincides with the transition zone toward the vast valley, where medium confinement is found with quaternary materials (Q- Vc and Q-ca CUs). In the alluvial valley area, from 39 km to 123 km, low confinement and quaternary lithology are observed. In this zone, the Erodibility Index is the highest for the studied river reach (with values up to 3.2). However, there are different quaternary alluvium types (Q-Vc, Q-Ca, Qt, and Q-al) and the river is sinuous. The general tendency of the channel width is to decrease, increasing channel sinuosity. This increase in sinuosity implies a decrease in the river slope, which is reflected in a reduction in the erosive capacity. The quaternary terraces (Q-t CU) dominate on the right side of the valley, between 70 km and 123 km, confining the river to the left side or pulling it toward the Western Andes. The river stops being sinuous exactly in confluence with the Desbaratado River, approximately 118 km downstream of the dam. From 118 km to 123 km, the channel width and sinuosity did not significantly change in the post-dam period. This narrowing trend of the river channel combined with an increase in discharges during the dry period may indicate the existence of an incision process in this area. This aspect is contrary to the expected increase in bank erosion processes, which emphasizes the relevance of the composition of the alluvial valley materials. In general, before the construction of the dam, the Cauca River width increased by 3.5 m (4.33%) and its sinuosity by 0.1 (6.5%). During the post-dam period, the channel width decreased by \(-\)0.17 m (\(-\)0.21%) and the sinuosity by 0.09 (4.74%). The morphological changes drastically reduced 118 km downstream of the dam, near the Desbaratado River; therefore, this length can be assumed as the extent of the morphological impact due to dam construction. * Morphological changes downstream of the Betania and Prado Dams The pre-dam years are 1969 (obtained from aerial photographs) and 1984 for the Betania and Prado Dams, respectively, while the post-dam years are 1988, 1998, 2015, and 2020. The percentage of change in the channel width and sinuosity referring to the pre-dam year of 1969 for this case as a function of the downstream distance is presented in Figure 10.  
The Magdalena River crosses different confinements and lithologies, separated into three areas according to the area's geology: the Neiva sub-basin, the Girardot sub-basin, and the transition zone between them (valley).