Linking erosion-deposition to geomorphic units changes in a high-gradient stream in the Central Chilean Andes

Geomorphic changes in rivers often happen after either single high magnitude floods or several following ordinary flood events. Erosion and deposition have been well documented in all types of rivers, as well as the formation and destruction of step-pool sequences. However, there are less evidence available on the link between erosion-deposition at the reach scale and the formation-destruction of geomorphic units. This work is based on a series of field surveys carried out in a small glacierized basin in the central Chilean Andes. The location and extent of erosion/deposition were quantified using the photogrammetric technique with a drone before and after a high magnitude flood occurred during autumn 2016. High-resolution Digital Elevation Models (DEMs) were computed to generate erosion-deposition maps (DoD; Difference of DEMs). Also, orthomosaics were used to derive maps of geomorphic units for a 100-m long study reach, before and after the studied flood event. Results show an overall deposition of sediments in the study reach, but a decrease in the number of step-pool sequences from 20 to 14. Step-pool destruction is linked to depositional patterns, whereas the formation of new step-pool sequences is more likely to occur in erosional zones. Rapids and cascades also change in number, increasing from 1 to 4 units, and their formation was related to the deposition of sediments. These results may have larger implications in terms of ecological habitat dynamics and are also important for planning and management in civil projects like bridges and hydropower water intakes