Geomorphological analysis of the active processes in the Derrumbe creek valley and its alluvial fan, Turbio river - Puelo lake basin, northwestern of Chubut province, Argentina

Abstract

Alluvial fans and their eventual records of debris flows are environmental archives of recent climatic and tectonic events. This paper studies the Derrumbe´s Creek avulsion process, a tributary of the turbio river-lake Puelo (northpatagonian andean range) where the channel that flowed towards lake Puelo was abandoned after the Valdivia mega-earthquake of 1960. The contribution characterizes the geomorphology and the active processes of the Derrumbe´s creek basin, analyzes the morphodynamics of its terminal alluvial fan in relation to its patterns of recolonization of forest vegetation and describes the geomorphic evolution pre and post avulsion event. The entire valley is dominated by pleistocene glacial landforms. Among the main mass movement processes, which modify the landscape by deepening its slopes, wedge-shaped landslides, debris avalanches and channelized debris flows were identified. These flows have the capacity to dam the creek in its middle basin. In turn, the fan was divided into four zones: inactive sector with mature forest vegetation, inactive sector with young forest vegetation, abandoned channel, and active channel. Processes linked to mass movement and avulsión in the terminal fan are discussed and it is concluded, that the most probable hypothesis is that the Derrumbe´s Creek avulsión of the stream would have been associated with erosion during an outburst flood produced by the undamming of a temporal water body located in the middle basin.