Abstract
Ecuador lies within the Northern Andes of South America, a mountain chain driven by tectonic processes. Several studies have documented sediment mass transfer between the Andean chain and the Amazon Basin. It has been estimated that up to 95% of the sediment discharged into the Atlantic Ocean by the Amazon River originates from the Andean Range. Within this geological framework, the Ecuadorian Government constructed the Coca Codo Sinclair Hydroelectric Plant (CCSHP), a run-of-river dam, in the Coca River Watershed (CRW). However, in February 2020, the San Rafael Waterfall, located at 19.1 km downstream of the diversion dam, collapsed catastrophically, triggering a rapid headward erosion process that has damaged strategic infrastructure and continues threatening watershed stability. This study assesses how human activity has impacted hydrosedimentary dynamics, accelerating natural erosion processes and altering sediment transport and deposition patterns, with significant consequences for fluvial geomorphology and infrastructure. A multidisciplinary approach integrating hydrology, geomorphology and geology was applied. Hydrological modeling indicates that legacy sediments from the 1987 earthquake and 2002 volcanic eruption increased erosion rates by approximately 28% in the undammed Salado watershed. However, dam construction accounts for approximately 42% of the intensification of erosion observed between 2008 and 2016 in the Coca River reach between the diversion dam and the former San Rafael waterfall, mainly due to disrupted sediment connectivity and the loss of bed armoring. Historically, the Coca River experienced periodic natural damming associated with volcanic eruptions and landslides, responding through erosional adjustment. In contrast, hydroelectric infrastructure permanently altered its discharge and sediment regimes, producing profound geomorphological changes: sediment aggradation formed an unexpected mid-catchment delta upstream of the diversion dam, while the release of sediment-starved waters downstream triggered severe degradation with erosion advancing 15.5 km over a 5-year period through July 2025.
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