Channel morphology in an active volcanic complex under humid tropical conditions

The morphology of rivers is the result of complex relationships between sediment supply, hydrological regime, geological and vegetation conditions, and human disturbances. Numerous river channel classifications have been developed in different geographical contexts over the last few decades. Nevertheless, channel characterization in active volcanic environments under humid tropical conditions is almost completely absent in geomorphological research.

We carried out a detailed characterization of river morphology in an active volcanic complex, the Irazú-Turrialba, characterized by extreme rainfall conditions (>7000 mm/yr.), frequent volcanic eruptions (>10/100 yrs.), high-magnitude earthquakes (>Mw5), and dense tropical vegetation. This volcanic complex is located in the central volcanic chain of Costa Rica and at its foothills, most of the country’s population and economic activities (>60%) can be found. A total length of 166.5km of the river network was mapped to understand the occurrence and distribution of channel morphologies in this high-energy and dynamic environment.

Using remote sensing techniques (RGB and multispectral satellite imagery, digital terrain models, spaceborne imaging radar products, and unmanned aerial vehicles), 74 river reaches located on four rivers within the volcanic complex were analyzed using 13 morphometric variables, including channel slope, channel width, confinement index, braided index, and others. Then, channel morphology for each reach was defined referring to four internationally recognized classification schemes, examining how such schemes adapt to the active tropical volcanoes. Further on, to characterize with more detail the river reaches and the volcanic complex, a morphometric index was developed to identify sediment sources and erosion-sedimentation areas. The morphometric index relies on vegetation height, terrain roughness index, slope degree, and average precipitation

Results allow a novel understanding of river morphology and processes in active volcanic environments under humid tropical conditions. The main outcomes are:  (i) channel morphology in this volcanic complex is dominated by steep, confined and coarse sediment river reaches; (ii) there is a strong difference in channel morphology and processes between the north and south parts of the volcanic complex due to climatological, geological, and tectonic aspects; (iii) established classification schemes partially failed when applied in this specific environment which is characterized by very high energy and a large amount of sediments; (iv) the morphometric index developed to analyze the volcanic complex and river reaches turned out to be useful for mapping sediment sources and detecting landforms such as lava flows, debris avalanches, landslides, and volcanic cones. Overall, this study provides novel insights about river morphology under highly dynamic and active volcanoes with extreme rainfall events, resulting in steep, confined, coarse sediment channel morphologies that are quite uncommon in other environments (e.g., boulder and cobble-bed braided rivers).