Alluvial fan-deltas of the Comau Fjord, North Patagonian Andean Basin (Chile): a classification proposal

Abstract Fjordlands dominate Southern Chile, where deltaic river mouths are commonly found. In Comau Fjord, Northern Patagonia, we identified several previously undocumented alluvial fan-deltas within the meso-tidal estuaries. As these environments are highly sensitive to climate change, our primary objective was better describe and classify the alluvial fan-deltas in this region, focusing specifically on the Huinay River mouth. We analyzed this system using multi-decadal orbital and aerial imagery, along with daily and monthly tidal variations records, and multi-year fieldwork campaigns. Geomorphological mapping under varying tidal conditions was employed to characterize both the subaerial and subaqueous features of the fan-delta. Field surveys revealed significant alluvial activity, evidenced by active deltaic lobes with braided patterns, high sediment loads, and smaller-scale features, such as bars and channels, composed of fluvial gravels reworked by marine processes. Our results indicate that tidal currents are the primary driver of morphological reworking at the Huinay River mouth. Furthermore, we observed that glacial and snow-pluvial valley basins draining into the fjord typically develop confined fan-deltas, whereas adjacent non-glacial basins exhibit unconfined systems at their mouths. Based on these findings, we propose a new classification for the delta at the Huinay River mouth: ‘Tidally Modified Alluvial Fan-Delta.’ Further research on these dynamic landforms is needed, particularly given the ongoing environmental changes, including shifts in precipitation, sea-level rise, increasing temperatures, and direct anthropogenic landscape modifications, such as the planned extension of the " Carretera Austral " highway construction through the Comau Fjord.