Long-lasting impacts of a 20th century glacial lake outburst flood on a Patagonian fjord-river system (Pascua River)

Glacial Lake Outburst Floods (GLOFs) are an increasing threat to Patagonian environments and communities. Here, we investigate the geomorphological and hydrological impacts of a historical GLOF from Chile's third largest river (Pascua River), which discharges at the head of Baker Fjord (48°S). To do so, historical maps and satellite imagery of the past century and recent bathymetric data were examined, and a 1.4 m long sediment core taken ~4 km offshore of the Pascua River mouth was analyzed. Geomorphological data suggest that the two main subaerial river channels of the fjord-head delta extend subaqueously as submarine channels. The sediment core was taken on the flank of the largest submarine channel to evaluate changes in channel activity through time. Results show that the sediments are composed of two distinct units separated by a 6-cm thick sandy turbidite dated 1945₋₁₀⁺⁸ CE. Historical evidence suggests that the event deposit corresponds to a ~ 256 × 10⁶ m³ GLOF from the proglacial lake of Lucía Glacier (Bergues Lake) that discharged into Pascua River. Before 1945₋₁₀⁺⁸ CE, sedimentation at the coring site consisted of coarse silt and fine sand, likely representing sediment deposition from turbidity currents. After 1945₋₁₀⁺⁸ CE, sedimentation consisted of very fine silts and clays, likely representing settling from the surficial sediment plume. This switch in submarine channel activity corresponds in timing to the abandonment of the eastern distributary channel of Pascua River and likely represents a reorganization of the hydrology of the fjord-river system caused by the 1945₋₁₀⁺⁸ CE GLOF. This study provides the first report of a GLOF from the northeastern part of the Southern Patagonian Icefield, and it demonstrates that GLOFs can have long-lasting impacts on the hydrology of downstream fjord-river systems.