## 2 Alhajuela Lake  
In the center of the Isthmus of Panama, within the Panama Canal Watershed, is located the study area of this work: the sub-basin of Alhajuela Lake. The selected area is of great economic importance at the national level because of its link to the Panama Canal economic zone. The Alhajuela Lake sub-basin, with a surface area of 1026 km\({}^{2}\), is the second largest within the Panama Canal Watershed and drains its waters to the reservoir of the same name. Alhajuela Lake is, together with Gatun Lake, one of the reservoirs used to supply water to Panama Canal operation. Gatun Lake was created in 1910 to reduce the excavation and for flood protection against the flooding of the rivers that poured into the navigation channel. Alhajuela Lake was created in 1935 to support Gatun Lake and for hydro-generation. Figure 1 shows Gatun and Alhajuela Lakes with their respective sub-basins. Since its construction, Alhajuela Lake has experienced sedimentation of eroded material from its upstream drainage. Figure 2 shows the sediment accumulation at the confluence of the Boqueron and Pequeni rivers. The climate in this area is sub-Equatorial with a dry season; this is the predominant climate throughout the Alajuela sub-basin and is characterized by annual temperature averages between 26.5 and 27.5 \({}^{\circ}\)C in the lowlands (at 0-200 m above sea level [m.a.s.l. ]), and as elevation increases, the temperature decreases to 20 \({}^{\circ}\)C at 1000 m.a.s.l. Rainfall is high and ranges between 2500 and 3500 mm/year. In low regions such as Alajuela Lake, it reaches 2454 mm/year and increases towards higher elevations, exceeding 3000 mm/year. The climate has a short dry season of 3 to 4 months, lasting somewhat longer and more accentuated in the mountains and foothills [16]. Large rainfall events produce a peak in erosion and sedimentation processes. A particularly intense and long rainy event on 8 December 2010 produced landslides in the upper part of the Alhajuela sub-basin that completely filled the lake with suspended sediment for several days. Figure 3 shows the landslides, especially in areas of steep slopes, caused by the 8 December 2010 storm, and how these landslides filled the rivers with sediment. Figure 1: Sub-basins of the Panama Canal Watershed (From: [[https://agua.micanaldepanama.com/index.php/portfolio/maps/](https://agua.micanaldepanama.com/index.php/portfolio/maps/)]([https://agua.micanaldepanama.com/index.php/portfolio/maps/](https://agua.micanaldepanama.com/index.php/portfolio/maps/)) (accessed on 1 June 2023)). Figure 2: Alhajuela Lake: Confluence of the Boquerón and Pequeni rivers (From: [15]). Comparing the 69 m contour lines, the evolution of sedimentation over the years can be observed (Figure 4). As shown in Figure 4, sedimentation over time has been concentrated mainly in the northern area of Alhajuela Lake, at the confluence of the Boqueron and Pequeni rivers. Figure 4: Alhajuela Lake: Evolution of the 69 m contour line from 1928 to 2012 (From: [15]). Figure 3: Landslides and sediment transport in the rivers of the Alhajuela Lake sub-basin after the 8 December 2010 storm. The study area is located in the Alhajuela Lake sub-basin (1026 km\({}^{2}\)) within the Panama Canal Watershed (Figure 1). For the application of the CSSI technique, the main tributary rivers of Alhajuela Lake were sampled: Boqueron, Pequeni, Chagres and Indio Este. Other points associated with small tributaries such as the Monocongo, La Fea, Benitez, La Palma and Quebrada Ancha rivers were also included. The representative relief of the study area is undulating to slightly mountainous, with hills to the northwest and to the southeast of the area, between which there is an intra-mountainous depression of low altitude, as shown in Figure 5, which shows the differences in height and the relative position of the sampling points. The topography of the sub-basin is characterized by steep slopes and short riverbeds. It is made up of a mountainous terrain (500-1000 m.a.s.l.)