Flow events on a hyper-arid alluvial fan: Quebrada Tambores, Salar de Atacama, northern Chile.

The Tanibores alluvial fail is located within the hyper-arid Atacama Desert of northern Chile. We examine evidence of the range of flow processes operative ill this environment from a combination of Pleistocene-Holocene fail deposits and a recent (2001) flood event (16 m(3) s(-1)) in the fail feeder channel and upper alluvial-fan area. The field evidence suggests that peak flows recorded in the older deposits generated extensive sheetflood events dominated by antidune deposition in the upper fan area. These extreme, supercritical flows were generated by floods with sustained high sediment and water discharges and high stream power. Easily erodable alluvial source materials ensured high sediment discharge Could be maintained within flood events. High stream power was ensured is a function of the tectonically exacerbated gradients within the source area. The 2001 event indicates the rapid rheological changes that can occur within an individual flood event, ranging from hyperconcentrated streamflow to mudflow. The flow deposits vary little in maximum clast size either between the varying flood events in the upper fan area, or down the fail gradient. This is due to a limited calibre of sediment being produced front the source area. The study highlights: (I) the range of flow rheologies that call be generated from a hyper-arid catchment both within and between flood events of varying magnitude and the associated difficulties in generating, it reliable stratigraphy from the resultant deposits; (2) the high stream power and sediment discharge associated with major flood events and thus the nature of flood hazard in the catchment and oil the fan; and (3) the limitations of sedinientological information such as maximum clast size as an indicator of peak flow characteristics in ancient deposits.