Suspended sediment dynamics of the Biobío River in response to water routing along the catchment

Abstract:

Supply-transport feedbacks in catchments involve processes with fundamental consequence for river management, land use and for the prediction of climate-driven sediment fluxes in rivers that are yet little understand. We address the spatial variability in the dynamic of the suspended sediment discharge (Qs, M/T) in a mountain-piedmont system located in the southern-central Andes (Biobío River Basin, 37-39°S). In steep catchments, the Q vs. Qs diagrams show a marked seasonal hysteresis curve. In the piedmont, Qs is proportional to Q, without showing hysteresis. The contrast in the hysteresis pattern between catchments with different topographies can be explained by differences in the routing of rainfall-derived water. In the piedmont, most of the rainfall is converted into surface runoff because the water table level is near the surface. In the mountain, groundwater storage imposes large seasonal variations in the fraction of Q that flows at the surface and transports sediment from the hillslopes, which produces hysteresis. By separating the total Q between two frequency components (direct discharge, Qd and base-flow, Qb), we observe the response of Qs to the fraction of water that quickly leaves the catchment after a rainfall event (Qd). Similar results between the mountain and piedmont and the absence of hysteresis simplify the behavior of Qs by a linear relationship with Qd over the entire catchment. Our finding highlight the importance of an adequate hydrological model to understand the erosion and transport processes of a catchment and lead us to propose that the main erosion process along the Biobío catchment is overland flow.