Topsoil delivery to Himalayan rivers: the importance of sampling time

Abstract

Copyright © 2016 John Wiley & Sons, Ltd. Estimates of the amount of topsoil in river sediments can help constrain sediment budgets on decadal time scales. The tracers 137Cs and 210Pb(ex) are used to determine the proportion of topsoil in river sediments in two Himalayan catchments, a relatively simple but effective method that could be used in many catchments in this complex mountain range for management purposes. Different results are reached, apparently depending upon antecedent conditions, with a large component of topsoil in river sediments likely to be the result of rainfall that erodes hillslopes by sheet and rill processes, does not mobilize or mix with other sources of sediment such as from landslides, and does not generate high river flows to transport the topsoil downstream. These results show that sampling of tracers in sedimentary archives is essential to provide time series of topsoil input to Himalayan rivers to account for high temporal variability. Copyright © 2016 John Wiley & Sons, Ltd.