What is it about?
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 mobilise 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.
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Why is it important?
• Estimates of the amount of topsoil in river sediments can help constrain sediment budgets on decadal time scales. • Sampling of tracers in sedimentary archives is essential to provide time series of topsoil input to Himalayan rivers to account for high temporal variability. • In the Jhelum catchment spatial analysis shows that temporal variability of erosive events and river discharges can deliver to rivers large amounts of fine sediment from topsoil erosion, with likely impacts on aquatic ecosystems.
Perspectives
The approach adopted in this study is relatively simple and could be applied in other high mountains for management purposes. It should also be tested using a wider range of tracers. The topsoil tracers 137Cs and 210Pb(ex) have been shown to be detectable in two Himayalan catchments, both in topsoils and river sediments. Aspects of the sediment budget can therefore be constrained, but with some limitations. From the results presented here it is plain that Eckholm’s [1975] hypothesis is not supported, and the bulk of sediment in the sampled rivers is unlikely to come from the erosion of topsoils by sheet and rill processes. In the Jhelum catchment, it is also clear that this conclusion applies to sediment that reaches the plain at the foot of the mountains. This result can be added to that of Wasson et al., [2008] and Starkel and Basu [2000] to conclude that all of the available evidence contradicts Eckholm. In the Jhelum catchment spatial analysis shows that temporal variability of erosive events and river discharges can deliver to rivers large amounts of fine sediment from topsoil erosion, with likely impacts on aquatic ecosystems. This conclusion implies that further analysis of this spatially and temporally complex system requires analysis of sediment sources over much more than a few years. This will only be possible where there are reservoirs that can be sampled or where there are alluvial deposits of fine sediment, the latter being the least available.
Muhammad Nawaz
National University of Singapore
Read the Original
This page is a summary of: Topsoil delivery to Himalayan rivers: the importance of sampling time, Hydrological Processes, August 2016, Wiley,
DOI: 10.1002/hyp.10949.
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