## 4 Results
### Morphological Changes
#### 4.3.1 Downstream Morphological Changes
In the Neiva sub-basin, from 0 km to 90 km, the narrowing trend of the river was kept in the pre-dam and post-dam periods without important changes in sinuosity. After dam construction, the decrease in the channel width intensified. There is, however, no general trend, because the banks present contrasting lithologies. Two areas are highlighted with unique geological characteristics:  
* Between 14-18 km, there is an erodible volcanic lithology (EI = 1.5) on a high-confinement area. Then, an increase in the channel width was observed in the post-dam period. * From 18 to 25 km, the channel width was reduced by about \(-\)62% between pre- and post-dam periods. This is a Magdalena River transition area from high- to low-confinement. The transition area is close to where the Frio River flows into the Magdalena River. Figure 10: Morphological changes downstream of the Betania and Prado Dams: (**a**) confinement grade; (**b**) geological units near the Magdalena River; (**c**) Erodibility Index (EI) on the left and right sides of the river channel (RC) and faults (**—**); (**d**) sites near the Magdalena River, including affluent rivers and streams; (**e**) river sinuosity change as a percentage with respect to the pre-dam year 1969; and (**f**) river width change as a percentage with respect to the pre-dam year 1969. The transition zone, a valley with medium-to-high confinement, between 90 km and 158 km presents a thinner alluvial quaternary deposit. The general trend of the channel width is to decrease. That decreasing trend was enhanced in the post-dam period. Some sections are highlighted: For example, the river reach located between 103 km and 106 km exhibits the lowest channel width change throughout the Magdalena River in the post-dam period (\(-64\%\)) and a maximum sinuosity of 16%. This reach corresponds to the confluence zone of four streams into the Magdalena River. Between 112 km and 123 km, the channel width increases due to the presence of a fault zone (sheared rocks), which makes this zone erodible. In the Girardot sub-basin, with low confinement from 158 km to 244, there was a channel-widening trend during the pre-dam period. After dam construction, a general narrowing trend is observed until 192 km downstream of the dam. This trend is primarily associated with bars that decrease its channel width, delimited by the n4n6-SC CU formation. From 192 km to 244 km, quaternary formations, such as Q-t, alluvial terraces, and the Q2-Vc CU Guamo fan appear, which confine the river, generating an almost straight planform. In this area, there are no significant changes in sinuosity and width, but precisely between 215 km to 226 km, the channel width shows no change during the pre-dam period. The post-dam period maintains a value of \(-6\%\) for all years, defining the extent of the morphological impact due to dam construction, which occurs until 215 km downstream of the dam (near the city of Suarez). In this study site, it was not possible to differentiate the changes due to each dam (Betania and Prado), because the gauging station installation date does not coincide with the starting of the operation of the Prado Dam. The hydrological regime in this area presents increased discharges during the dry period, but those are not reflected in the observed changes in the channel width. This aspect also shows the relevance of the resistance of the riverbanks, which coincide with more consolidated formations.