Coupling tree-ring and geomorphic analyses to reconstruct the 1950s massive Glacier Lake Outburst Flood at Grosse Glacier, Chilean Patagonia

Abstract

Over recent decades, global warming has led to sustained glacier mass reduction and the formation of glacier lakes dammed by potentially unstable moraines. When such dams break, devastating Glacial Lake Outburst Floods (GLOFs) can occur in high mountain environments with catastrophic effects on populations and infrastructure. To understand the occurrence of GLOFs in space and time, build frequency-magnitude relationships for disaster risk reduction or identify regional links between GLOF frequency and climate warming, comprehensive databases are critically needed. GLOF inventories have thus been compiled at both regional and global scales. While these accounts offer valuable data for hazard assessment, many GLOF events are omitted because they occurred before the satellite era or prior to the installation of river gauges, went unreported or were only mentioned in local media but not documented in the scientific literature. In addition, while geomorphic evidence of past GLOFs exist, their timing oftentimes remains unknown. These discontinuities preclude the identification of return periods for the most disastrous events and makes the link between GLOFs and ongoing climate warming remain inconclusive. To overcome these limitations, long-term records have been developed using a wide variety of proxies. Here, we explore the potential of coupling dendrogeomorphology and geomorphic analyses to reconstruct a GLOF event that occurred before the satellite era. The Grosse Glacier outlet, in San Rafael National Park, Chilean Patagonia, was selected based on satellite imagery, historical photographs and local testimonies indicating a previous lake drainage. Tree-ring analysis allowed identification of a massive GLOF and five smaller floods from 105 disturbed Nothofagus and Podocarpus trees. Unmanned aerial vehicle (UAV) imagery and object-based image analysis (OBIA) were used to identify boulders and to estimate flow direction in the glacio-fluvial floodplain and point to the occurrence of at least one high-energy flow event. Aerial photo analysis also revealed the formation of a 200-m-wide breach in the frontal moraine and the disappearance of a lateral lake, estimated to be 1.8 km² in size, sometimes after 1944-45. An in-depth interview with an eyewitness pinpointed the date of the GLOF to October 1957, in line with the estimated date of damage in the tree-ring records and precipitation records at Puerto Aysén. When used in conjunction, the different approaches contribute valuable insights into the timing and consequences of past extreme events, thus aiding in future assessments of GLOF hazards, not only at the Grosse Glacier but also in other high-mountain environments.

Keywords: Dendrogeomorphology; GLOF reconstruction; OBIA; Remote sensing.