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perhaps more importantly, attempts to determine the amount of sediment redistrib-
ution that has occurred during an individual event or short period of intense rainfall
requires that the spatial distribution of
7
Be be uniform across the area (as is true
for
137
Cs or
210
Pb
ex
). However, the dynamic nature of
7
Be inventories, which can
change in response to rainfall patterns or erosion/depositional processes during a
single event, means that
7
Be is unlikely to exhibit a homogenous spatial distribu-
tion unless the inventories inherited from previous events have been eliminated by
radioactive decay, or by means of cultivation and mixing of the soil profile (Walling
2013
). In the absence of cultivation, Walling (
2013
) suggests that about 5months
may be required to eliminate past patterns in
7
Be inventories. In other words, it may
be that
7
Be can only be applied as a tracer of sediment redistribution where signifi-
cant rainfall events (i.e., those that redistribute sediment) have not occurred for the
past 5 months, severely limiting its application.
3.4 Estimating Sediment Age, Residence Times, Transport
Distances, and Other Contemporary Sedimentation
Processes
Over the past 10-15years there has been a growing interest in the erosional and
depositional processes that operate along the channel bed, particularly with regards
to the magnitude of channel bed scour and fill during flood events, the 'age' and
residence time of the sediment that comprise the bed material, and distances and
rates that particles move over a specified period of time. Interest in these processes
stem from the fact that the exchange of particles with the channel bed as well as their
travel times and distances during a given event or set of events strongly influences
where the impact of sediment and its associated contaminants will be concentrated
and for how long (Bonniwell et al.
1999
). Long transport distances and short residence
times, for example, indicate that the sediment and any associated pollutants will be
passed rapidly downstream to receiving bodies (e.g., lakes, reservoirs, or estuaries),
whereas shorter travel distances and longer residence times may be associated with
more severe impacts on riverine habitats and biota. Quantifying these processes
has proven to be difficult using direct methods, leading investigators to explore the
use of FRNs to decipher such sedimentation processes. The majority of the work
conducted to date focuses on contemporary processes at the reach- or river-corridor
scale, thereby requiring the use of
7
Be (characterized by a short half-life).
The use of
7
Be (and other FRNs) to quantify channel bed sedimentation processes
is based on the postulated existence of a relatively simply system. Exposed sur-
faces within the catchment (e.g., the surface of upland soils, floodplains, and emer-
gent channel bars) are tagged by atmospherically deposited FRNs, including
7
Be
(Fig.
3.4
). Once the tagged sediments enter the flow, they can no longer receive
FRNs from the atmosphere. FRN activity will then begin to decrease as a result of
radioactive decay, and since the decay process is constant, the activity of the river
sediments can serve as a measure of how recently the particles entered the channel.
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