Morphological changes due to a high-mangitude flood in a steep Andean stream

Steep mountain streams are usually narrow, confined, and characterized by coarse sediments, and cascade or step-pool morphology. Step sequences are quite stable geomorphic units, but high magnitude floods can destroy and reform these features and lead to significant channel changes. However, our current knowledge of channel changes due to floods of different magnitude is still limited to few field sites, and limited evidence are available for high-gradient streams. However, the increased use of unmanned aerial systems to collect photos, and structure-from-motion algorithms able to generate high resolution point clouds from photos, provided unprecedented chances for acquiring multi-temporal sets of digital terrain models. The study was conducted on the Estero Morales, a 27 km² glacierized Andean catchment located in central Chile. Runoff is dominated by snowmelt in late spring, and glacier melt from December to March. Autumn rainfall events can also generate infrequent but high-magnitude flood events. The study site is a 150-m long, step-pool/cascade reach. A high-magnitude flood generated by an ENSO event occurred in April 2016. The flood destroyed a bridge and caused considerable damages downstream. Detailed topographical surveys of the reach were taken before and after the flood and geomorphological changes due to the flood event were assessed by comparing the pre- and post-flood DEMs. The flood event, which is estimated to have recurrence interval higher than 30 years, was able to move boulders up to 2 m, and caused remarkable changes in the study sites. The channel avulsed in several points, and the morphology changed considerably. The number of steps along the study sited were 25 and 17 before and after the flood event, respectively. However, only 4 of the original 25 steps remained stable in the channel. The number of pools remained constant before and after the flood, but only 9 of the 26 pools remained in their place. The DoD revealed that the study reach experienced vertical changes up to 1m, especially due to bank erosion. Overall, the volume eroded and deposited within the study reach was about 359 and 680 m³, respectively, for a net volumetric change of 321 m³.