## 5 Discussion  
### Downstream Dam Effects  
Although geology is considered essential in influencing river morphology, a few studies have considered the riverbank's composition in the study of the morphological effects of dams. It is more common to use the geological information to define the right place to build a dam, analyzing, for instance, the rock type and geological structure [86]. Studies dealing with the morphological effects of dams use geologic information to obtain a context of the area of interest with lithological descriptions at a regional scale, but this information is not included in the morphodynamic analysis. Geology determines the spatial variation of riverbank materials (formations) and their associated resistance [73]. [PERSON] et al. [30], for example, distinguished the importance of geology as a primary control, determining valley and landscape configurations and creating a relationship between sediment flows and post-dam changes. It is important to mention that due to the Colombian topography, the selected dams are built on the fall line (with an upstream bedrock reach and a downstream alluvial bed). Therefore, our analyses focused on this high-low intervention type. Downstream of a dam, it is expected to find narrower channels through time [2; 33]; however, a few authors have reported other trends during the post-dam period. [PERSON] and [PERSON] [8] reported contrasting trends based on data from 21 dams in the United States. They explained these different responses due to the resistance of the alluvial bank materials, identified through field campaigns. In the Snake River, [PERSON] et al. [87] reported wider channel widths after dam construction, associated with large tributary floods. The results of our study suggest that including the geological framework and confinement degree of the selected area may clarify the possible channel width response, allowing identification of the more susceptible areas. This information is valuable in defining the extent of the biotic compensation measures in environmental impact studies. In mountainous areas (regions with high-to-medium confinement) near a dam, there is a general trend to have increased widths, even though the thickness of the alluvial quaternary area is thinner than in the alluvial valley. Moreover, in some sectors, the riverbanks can be in direct contact with the geological formations. Just downstream of the dams of the selected projects, a high-confinement zone is found. However, the high mountain area downstream of the Betania Dam (Magdalena River) is composed of more erodible materials (feldespathic sandstones and conglomerates rich in lithics). These low-resistance riverbanks explain the wider channels observed after dam construction (see Figure 10). In the other two cases (Salvajina and Ura I projects), narrower channels are present, an aspect that can be attributed to incision processes induced by high-resistance banks (see Figures 8 and 9). In contrast, in the alluvial valley region (area with low confinement), quaternary formations predominate, and it is there where the trend may vary. The valley composition of the selected sites determines the channel width response. The Sinu and Cauca River valleys present the quaternary alluvium in both riverbanks; however, downstream of the Salvajina project, a graben structure (i.e., distinct escarpment on each riverbank caused by downward displacement) composed of terraces and colluvial exists. These material sequences generate channel confinement, a configuration that is not present downstream of the Ura I project. This difference in the geological structure generates channel narrowing in the Cauca River valley and channel widening in the Sinu River valley (see Figures 8 and 9). Downstream of the Betania project, the Magdalena River exhibits decreased channel widths through the valley up to 160 km, where the alluvial material interacts with consolidate rocks.