River dynamics in an active volcano with tropical humid conditions

Abstract

 Active volcanic catchments within low-latitude tropical humid climates constitute some of the most dynamic and sediment-rich fluvial systems globally. The combination of factors such as active explosive vulcanism, frequent high-magnitude earthquakes, dense biodiverse vegetation, and intense rainfall leads to very high sediment supply and very active channel morphodynamics within such fluvial systems. Our research focused on channel dynamics in three remote and challenging-to-access reaches of the Sucio River, situated within the Irazú stratovolcanic structure in Costa Rica's central volcanic chain. This unique setting experiences extreme conditions, including rainfall exceeding 8000 mm annually, infrequent but significant volcanic eruptions (occurring over three times per century), high-magnitude earthquakes (>Mw5), and dense pristine vegetation. The mapped river reaches within this active volcano exhibit a distinctive confined multi-thread channel morphology predominantly comprised of coarse sediments, notably boulders, displaying exceptional dynamism. These reaches showcase rapid shifts between braided, island-braided, and anabranching morphologies in relatively short periods (<20 yrs.). Additionally, the primary sediment sources located in crateric areas have undergone rapid and substantial changes, resulting in the emergence of large landslides and drastic alterations in vegetation. Employing remote sensing techniques, geostatistical analysis, and fieldwork, we investigated the impacts of eruptions, earthquakes, and rainfall on the Sucio River's channel morphology and its primary sediment sources (craters) from 1961 to 2023. Over 65 images were processed to generate various derived raster products, including supervised classification datasets, change detection outputs, and morphometric parameters (such as channel width, braided index, anabranching index, and area of bars and islands). Moreover, we constructed a precipitation database spanning the study period to assess the frequency, magnitude, and duration of extreme rainfall events. Historical seismic data was utilized to compile a database of relevant earthquakes that might have affected the catchment, given the river's proximity to several active faults. Subsequently, exploratory statistical analysis employing linear regression models helped discern the influential factors behind channel dynamics and changes. Our findings provide a novel understanding of how this specific fluvial volcanic environment responds to external perturbations and adapts its channel morphology over time. Key outcomes include the rarity of the multithread boulder morphologies observed in these reaches, rapid morphological transformations occurring within this multithread system in short intervals, the significant role of dense pristine vegetation in stabilizing banks and islands, and the cyclic stability-instability phases (erosion-deposition) triggered by pivotal events like eruptions, hurricanes, and high-magnitude earthquakes.   This study presents novel insights into channel morphology dynamics in one of Central America's most extensively studied active volcanoes, likely having the river transporting the most sediments in Costa Rica's volcanic regions.