Bio-climate affects hillslope and fluvial sediment grain size along the Chilean Coastal Cordillera.

Sediment dynamics in river catchments are controlled by tectonics, climate, and biota effecting material production on hillslopes and transport of sediment from their source to the catchment outlet. Tectonics create topography and control erodibility of the bedrock material, whereas climate controls the efficiency of weathering and transporting processes. In contrast, the effects of biota (i.e. vegetation) are more ambiguous. Vegetation accelerates bio-chemical weathering and effects mass wasting through uprooting, trapping and stabilizing hillslope material. Furthermore, vegetation exerts a strong control on the grain size distribution of hillslope sediments and thus on sediment rooting through catchments. In this study we compare grain size distributions of hillslope and channel sediments collected in four headwater catchments located in the Chilean Coastal Cordillera covering a strong bio-climatic gradient. 76 volumetric bulk samples were taken with grain sizes ranging from clay (d = 0.3 μm) to boulders (d > 300 mm). Results show that the production of fine material is strongly dependent on bio-chemical weathering intensity and hence humidity. The coarse fraction in hillslope material and channel sediments increases from arid to sub-humid conditions, presumably reflecting a higher intensity of mass wasting processes. Channels show varying degrees of armouring reflecting nonselective and thus transport-limited conditions under arid climate that are contrasted by relatively well sorted and armoured channel sediments in the humid and vegetated catchments, indicating size selective and supply-limited transport conditions. Channel grain sizes generally show high similarity to hillslope grain sizes of the same catchment, revealing the strong dependency of channel sediments on hillslope supply, at least in headwater catchments, transferring a bio-climatic control from the hillslopes to the channel system. • Bulk sampling (covering clay to boulder) reveals differences in transport conditions along the sediment flow path • Grain size sorting between hillslope and river sediment shows complex response along the Chilean bio-climatic gradient • Hillslope vegetation affects channel dynamics via grain size in headwaters of the Chilean Coastal Cordillera [ABSTRACT FROM AUTHOR]