Influence of Lithology and Biota on Stream Erosivity and Drainage Density in a Semi-Arid Landscape, Central Chile

Abstract

Drainage density is a fundamental landscape feature that determines the length scale for hillslope sediment transport and results from the competition of diffusive hillslope and advective stream incision processes, whose efficiencies are known to vary with rock type but are notoriously difficult to quantify. Here, we present a comprehensive analysis of a catchment in semi-arid Central Chile, where landscapes with different drainage densities, but equal tectonic and climatic conditions, have formed on three neighboring granitoid plutons (a monzogranite and two diorites). We combined topographic analysis of a 1-m digital elevation model with 10Be-derived denudation rates to estimate stream erosivity and soil diffusivity in the different landscapes. We find that the higher drainage density in the monzogranite is primarily due to higher stream erosivity, whereas soil diffusivity is similar between rock types. Remote sensing data from Landsat imagery confirm field observations of higher vegetation cover in the diorites, especially with regard to deeper-rooted shrubs, which may result in increased infiltration. Based on geochemical and compositional analyses, we link vegetation differences to a relatively higher abundance of plant-essential elements in the diorite bedrock. Additionally, the monzogranite's composition and crystal grain size supports more intense physical weathering and leads to a smaller observed hillslope grain size, which increases its erodibility. We conclude that subtle differences in composition and grain size can have a significant impact on stream erosivity and drainage density. Our results demonstrate the importance of taking lithology into account when interpreting fluvial networks and topographic metrics in slowly eroding landscapes.