SEDIMENTOLOGY AND GEOMORPHOLOGY OF LLACA LAKE IN THE CORDILLERA BLANCA, PERÚ – INVESTIGATION OF A TROPICAL SUPRAGLACIAL LAKE LANDSYSTEM

The Cordillera Blanca, Perú contains the largest number of tropical glaciers in the world which are currently experiencing rapid thinning and retreat as a result of climate warming. As these glaciers retreat, the number and size of moraine-dammed glacial lakes has increased, and the risk of catastrophic release of water as glacial lake outburst floods (GLOFs) is also increasing. Little is understood about processes operating to create and infill these glacial lakes but this information is essential for local communities threatened by future GLOFs. This study focuses on investigating the sedimentology and geomorphology of an enlarging supraglacial lake formed on the margins of Llaca Glacier, a debris-covered glacier, in the Cordillera Blanca, close to the city of Huaraz.

Llaca Lake lies on the downvalley margin of Llaca Glacier and is a supraglacial moraine-dammed lake over 1 km long with an area of over 65,000 m², and a maximum depth of ~ 20 m. The lake partially covers the glacier margin which is melting from beneath a relatively thick cover of supraglacial material. Sedimentological logs were taken from lacustrine sediment exposed on upstanding ice-cored hummocks. Eight facies types were identified including massive gravels, rippled, horizontally laminated, graded and deformed sands, and laminated and deformed silts and clays. These sediments were delivered to the lake by traction and density underflow currents generated by sub and supraglacial meltwater streams and valley side gullies. Abundant soft sediment deformation structures suggest rapid deposition on unstable substrates and interbedded gravel horizons record past flood or iceberg calving events. Abrupt vertical changes in facies types may record lake level changes caused by the release of floodwaters. Unmanned Aerial Vehicle (UAV) derived photogrammetry of the glacier margin and lake system has allowed a high resolution DEM to be created which, in combination with field observations, provides data on the spatial distribution of landforms and sediments found within the supraglacial lake system. This information can be applied to the study of lake genesis and development on the margins of other retreating glaciers in the Cordillera Blanca and to the better understanding of GLOF events.