EROSION RATES, SEDIMENT BUDGETS, AND SEDIMENT EFFLUX FROM OROGEN-SCALE TROPICAL WATERSHEDS

Mountainous basins supply most of the sediment conveyed to rivers, and ultimately the ocean. Entire orogen-scale basins, regardless of size, are often treated as homogenous units, without considering whether they include internal zones of sediment erosion, transport, and deposition. By not distinguishing the zones of sediment production, transport, and deposition within orogen-scale basins, studies of sediment flux often do not resolve realistic process rates within these distinct meso-scale geomorphic process zones. I present results from studies of erosion within mountainous basins and sediment trapping within the adjacent foreland basin. A study of the geomorphic controls on decadal-scale denudation used mass fluxes measured within 50 erosional basins located throughout the Bolivian Andes. Sediment discharge is a strong function of lithology and mean hillslope gradient (or local relief), without a clear effect of climate. Another study of erosion rates used an intense sub-sampling of the cosmogenic isotopes in sediment from basins that drain into the Beni River (among the steepest basins in the first study). These millennium-scale erosion rates are not related to climate, but show only limited correlation to lithology and mean basin hillslope, but rather to normalized channel steepness index, a metric of the relative steepness of the channels in relation to their area. Therefore, the steep hillslopes in the Beni Basin may be at the threshold stability values of gradient and local relief for their lithological strength (and climate) and sediment supply is related to landsliding frequency, in turn a function of channel incision rate, itself a function of tectonic uplift or migrating knick-points. The final examples summarize how much sediment is trapped as it crosses foreland basins adjacent to the mountains. For the Beni River ~ 40% of the sediment is trapped, primarily within the foredeep. Both the timing of sediment trapping and the timing of landsliding in the Andean headwaters appear synchronized with ENSO climate cycles. Two other examples are the Strickland River, Papua New Guinea, and the Mamore River, Bolivia, where 20% and ~60% of the sediment is trapped within the foreland, respectively. This balance between erosion within and deposition adjacent to the orogen modulates the sediment efflux from mountains.