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Seasonal controls on meteoric 7 Be in coarse‐grained river channels
Author(s) -
Kaste James M.,
Magilligan Francis J.,
Renshaw Carl E.,
Burch Fisher G.,
Brian Dade W.
Publication year - 2013
Publication title -
hydrological processes
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.222
H-Index - 161
eISSN - 1099-1085
pISSN - 0885-6087
DOI - 10.1002/hyp.9800
Subject(s) - snowmelt , fluvial , hydrology (agriculture) , thalweg , tributary , sediment , environmental science , meltwater , geology , streams , snowpack , flux (metallurgy) , storm , discharge , tracer , bedform , sediment transport , drainage basin , geomorphology , snow , structural basin , oceanography , geotechnical engineering , computer science , physics , nuclear physics , computer network , materials science , cartography , geography , metallurgy
Abstract 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.