Dating Past Events on Fans and Cones – An Introduction

Alluvial fans and debris cones are fan- or cone-shaped sedimentary structures that are formed where a stream or torrent flattens, slows, and spreads. Moreover cones are also formed by scree deposits, rock avalanches, slope-type debris flows, mudflows, lahars, landslides, sheet floods and other forms of rapid mass movements. Alluvial fans and debris cones are typically located in mountain fronts and develop at the base of headwater basins. The apex of the depositional environment is usually located within a canyon or ravine mouth that serves as the outlet for a mountain drainage system or at the juncture of steeper hillslope tributaries with the main stream in a flatter plain (e.g. valley floor). Fans and cones can be seen as the expression of the ``history'' of their watersheds because they have been created by and represent a summary of hydrologic, geomorphic, climatic, biologic, and anthropogenic processes (e.g. agriculture, forestry) in the mountains and hillslopes upstream of fans and cones. Inherited and present-day processes in the watershed actively influence water and sediment regimes and can thus lead to significant changes in the timing, frequency, and magnitude of hydrogeomorphic events. The scarcity of event occurrence and the widespread absence of hydrogeomorphic disasters over several decades in the twentieth century represent a major handicap for an appropriate assessment of the frequency and magnitude of past and potential future events. Records of past hydrogeomorphic activity are stored in many different types of archives. Besides the classical anthropogenic archives, nature provides many sources of information that can be extracted using the appropriate methods. A broad spectrum of methods allowing the improvement of data series on past hydrogeomorphic events on fans and cones are introduced in this chapter and further described in this book.