Deciphering the Morphologic Change in the Radial Drainage System of the Calbuco Volcano Caused by the 2015 Eruption

The eruption of the Calbuco volcano on April 22, 2015 presented three eruptive pulses, the second eruptive pulse being the main responsible for the most considerable impacts both in the natural and anthropic environment. This pulse generated several lahars due to the interaction of pyroclastic flows with glaciers and snow close to the summit of the volcano, causing alterations in the morphology of the volcano’s radial drainage system. The most affected basins were those of the Blanco Este, Tepu, Blanco Sur y Este rivers, altered by the lahars´ geomorphic work resulting in a remarkable river widening on alluvial deposits and producing geomorphic changes in the typology and configuration of several rivers reaches. To retrace the geomorphic changes occurred in the Blanco Este, Tepu and Blanco Sur rivers, the IDRAIM method was used, which allows for a thorough hydro-morphological analysis encompassing the rivers past geomorphic evolution, the characterization of their current dynamics and the exploration of their possible future trajectories. Following this method, river basins were characterized according to their physiography, then the rivers were segmented into river reaches based on their confinement, their geomorphic units and hydro-morphological typologies were identified by calculating a set of geomorphic indices and morphometric parameters. With this information we assessed the geomorphic signatures left in the affected rivers by comparing the elaborated cartographies referring to the geomorphic configurations before and after the eruption, respectively. The affectation of the basins occurred mainly in the headwaters, generating remarkable changes, thus altering the geography of the place. The basins of the Blanco Este, Tepu, Blanco Sur, and Este rivers were the ones that suffered the greatest degree of alteration featuring on average wider active channels and an increased braiding tendency in the lower water courses. These rivers, affected by large sediment injections, will most probably continue to exhibit major adjustments in the years to come.