LATE HOLOCENE FLUVIAL DYNAMICS IN THE RIO GRANDE DE NAZCA CATCHMENT, SOUTHERN PERU

Along the pacific coast of Peru variation in rainfall and fluvial dynamics is well correlated to ENSO phases. Only during El Niño events high precipitation activates the fluvial system; often in the form of severe floods. The link of El Niño and fluvial dynamics is well established for northern Peru. In southern Peru, precipitation pattern and response of the fluvial system is more complex. Precipitation is limited to very strong El Niños, and to the coastal lowlands, while drought prevails in upland areas.

The paper presents investigations of flood deposits in the Rio Grande de Nazca catchment. The study area is located close to the village of Palpa (14°30’ S), west of the Andes. The Rio Grande and two major tributaries, Rio Palpa and Rio Viscas, head in the mountains and represent allogenic, perennial or intermittent drainage systems. In addition, several smaller catchments with endogenic, episodic drainage are present. The aim is to establish local and regional flood deposit chronologies, taking into account the hydrological character of the catchments.

First results are available from the Rio Palpa alluvial plain and the alluvial fan of the La Muña creek, an endogenic tributary. Mapping a 120 m long and 3 m high river terrace along the Rio Palpa and a transect of 3 pits on the alluvial plain, revealed two distinguished stratigraphical units: A basal unit with gravel beds and a top unit, which varies strongly in thickness and consists of several beds of sandy and silty material. Traces of a long lasting human utilisation of the alluvial plain, probably starting as early as 2 ka BP, are present. A transect of pits in the La Muña alluvial fan revealed a 4 m thick sequence of well layered deposits, including several organic layers, laminated silty and sandy beds, and debris flow deposits. Preliminary dating of the lowest debris flow, based on pottery finds, suggests that the upper 2,5 m of this sequence documents endogenic fluvial dynamics in this part of southern Peru for the last 1.5 ka.

Radiocarbon dating of organic layers and charcoal, analysis of sediment samples, including grain size composition, soil organic matter and calcium carbonate content, and palynological investigations will be presented in this paper.