Bedrock structural control on catchment scale connectivity and alluvial fan processes, High Atlas Mountains, Morocco.

Abstract

Lithology is acknowledged to be an important internal catchment control on flow processes to adjacent alluvial fans. However, the role of inherited structural configurations (e.g.bedrock attitude) in catchment connectivity and sediment transport is rarely considered. We examine four young (,100-year-old) active tributary junction alluvial fan systems from the Dade`s Valley in the High Atlas of Morocco in terms of their catchment-scale connectivity, sediment transfer and resulting alluvial fan processes. The catchments occur on the same lithologies (limestones and interbedded mudstones), but experience different passive structural configurations (tilted and structurally thickened beds). The fan systems react differently to historical peak discharges (20–172 m3 s21). Catchments containing tectonically thickened limestone units develop slot canyons, which compartmentalize the catchment by acting as barriers to sediment transfer, encouraging lower sediment to water flows on the fans. Syn-dip catchments boost connectivity and sediment delivery from translational bedrock landslides as a result of steep channel gradients, encouraging higher sediment to water flows. By contrast, translational landslides in strike-oriented drainages disrupt longitudinal connectivity by constricting the valley width, while the gradients of the main channels are supressed by the attitude of the limestone beds, encouraging localised backfilling. This diminishes the sediment to water content of the resulting flows.