Exploring lithological controls on drainage density in Santa Gracia, Central Chile

The controls of drainage density remain a mystery, despite it being a fundamental attribute of landscape morphology. Studies have shown that dry climates with less vegetation or less-consolidated sediment often have higher drainage densities. It remains unclear, however, how lithology effects drainage density, and existing studies have mixed results. Here, we investigate a landscape in Central Chile where two adjacent granitoid plutons (a monzogranite and a granodiorite) have different drainage densities. Since the tectonic setting and climate are the same between the two lithologies, we hypothesized that lithological differences control drainage density through modulating groundwater infiltration and runoff. To test this, for each lithology, we quantified the density of streams, channel heads, and vegetation using a 1-m resolution LiDAR digital elevation model. In the field, we measured infiltration rates and surficial sediment grain size distributions. In the lab, we obtained major oxide compositions from bedrock samples using X-Ray Fluorescence, identified mineralogy within thin sections, measured the hydraulic conductivity of the bedrock, and obtained in-situ cosmogenic 10Be-derived denudation rates from hillslope and stream sediment samples. Our results show that the granodiorite has a lower drainage density, a more weatherable mineralogy (more abundant biotite, hornblende and plagioclase) and composition (lower levels of SiO2 and K2O and higher levels of Na2O and CaO), denser vegetation, and a larger average grain size. Infiltration rates and hydraulic conductivity are similar between the two rock types, and 10Be results suggest similar erosion rates between the two lithologies. Our results suggest that differences in grain size and/or vegetation cause the observed differences in drainage density. In addition, there may be differences in infiltration rates that were undetected by our measurements or were different in the past.