Erosion rates driven by channel network incision in the Bolivian Andes

The Bolivian Andes flank one of Earth's major topographic features and dominate sediment input into the Amazon Basin. Millennial-scale erosion rates and dominant controls on erosion patterns in this range are poorly known. To define these patterns, we present 48 erosion rate estimates, derived from analysis of in situ ¹⁰Be in quartz-bearing alluvium collected from the Upper Beni River basin. Erosion rates, corrected for the non-uniform distribution of quartz in the sample basins, range from 0·04 mm a^-1 to 1·35 mm a^-1 and thus integrate over 10²-10⁴ years. Mean and modal values are 0·42 (standard deviation: 0·29) and 0·2-0·4 mm a^-1 respectively, within the range of long-term average erosion rates in this area derived from apatite fission track thermochronology (0·1-0·6 mm a^-1). Hence, our data do not record any significant variation in erosion rate over the last several million years. Mean and modal short-term erosion rates for the Andes are an order of magnitude lower than rates in the Ganges River headwaters in the High Himalaya and an order of magnitude greater than rates typical of the European Alps. In the Upper Beni River region of the Bolivian Andes, short-term, basin-averaged erosion rates correlate with normalized channel steepness index, a metric of relative channel gradient corrected for drainage area. Neither normalized channel steepness index nor basin-averaged erosion rate shows strong correlation with mean basin hillslope gradient or mean basin local relief because many hillslopes in the Upper Beni River region are at threshold values of slope and local relief. Patterns of normalized channel steepness index appear primarily to reflect tectonic patterns and transient adjustment to those patterns by channel networks. Climate and lithology do not appear to exert first-order controls on patterns of basin-averaged erosion rates in the Bolivian Andes.