Publication Date:
2013-03-05
Description:
Cosmogenic 7 Be is a natural tracer of short-term hydrological processes, but its distribution in upland fluvial environments over different temporal and spatial scales has not been well described. We measured 7 Be in 450 sediment samples collected from perennial channels draining the middle of the Connecticut River Basin, an environment that is predominantly well-sorted sand. By sampling tributaries that have natural and managed fluctuations in discharge, we find that the 7 Be activity in thalweg sediments is not necessarily limited by the supply of new or fine-grained sediment, but is controlled seasonally by atmospheric flux variations and the magnitude and frequency of bed mobilizing events. In late winter, 7 Be concentrations in transitional bedload are lowest, typically 1 to 3 Bq kg -1 as 7 Be is lost from watersheds via radioactive decay in the snowpack. In mid-summer, however, 7 Be concentrations are at least twice as high because of increased convective storm activity which delivers high 7 Be fluxes directly to the fluvial system. A mixed layer of sediment at least 8 cm thick is maintained for months in channels during persistent low rainfall and flow conditions, indicating that stationary sediments can be recharged with 7 Be. However, bed mobilizing rain on snowmelt events in late Spring can “reset” 7 Be amounts and concentrations in the channel as previously buried “old” sediment with low 7 Be is mixed into the thalweg. We conclude that given proper temporal and spatial sampling, 7 Be is a valuable tracer of seasonal-timescale mass transport and exchange in coarse-grained fluvial systems. Copyright © 2013 John Wiley & Sons, Ltd.
Print ISSN:
0885-6087
Electronic ISSN:
1099-1085
Topics:
Architecture, Civil Engineering, Surveying
,
Geography
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