Geochemical Fingerprinting - Application of Geochemical Tracers to Fluvial Sediment - page 21

that sediment loads typically vary throughout the event, with larger loads tending to be
associated with higher flows. The relationships between sediment load and discharge
is not always perfect, however, as larger sediment loads may be associated with the
rising, peak, or falling stages of a hydrograph. For example, the episodic erosion of
easily eroded sediment during the onset of a runoff event often leads to larger loads
during the rising stage of the flood, in comparison to the same discharge conditions
during the falling stage, producing a phenomena referred to as the first flush (Miller
and Orbock Miller 2007 ). This 'first flush' phenomenon not only demonstrates that
the rates of soil erosion vary through the flood, but that erosion varies from one
sediment source to another over the landscape at any one time. Thus, the source
contributions determined for an instantaneously collected sample will apply only to
the portion of the flood that was sampled, rather than for the entire storm (Collins
et al. 2001 ; Massoudieh et al. 2013 ). To address this issue it is now common to
collect an integrated sample in which sediments are obtained continuously or semi-
continuously over a longer time span, such as the entire flood. The sediments within
these samples can be expected to reflect the averaged contribution fromeach sediment
source within the watershed (Fox and Papanicolaou 2008b ). While such integrated
samples may be collected using automated, pump-type sampling devices, the need
for relatively large sediment sample sizes for geochemical analysis has led to the use
of passive samplers or sediment traps (e.g., Phillips et al. 2000 ; Russell et al. 2001 )
that collect materials representing the entire storm hydrograph (Massoudieh et al.
2013 ).
An alternative to the use of these time-integrated sediment traps is to sample the
channel bed material (Evrard et al. 2013 ; Collins et al. 2013 ) as recent studies have
shown that such bed sediments serve as an effective surrogate of continuously col-
lected material over multiple flood events (Miller and Orbock Miller 2007 ; Horowitz
et al. 2012 ; Collins et al. 2013 ). Two additional advantages of sampling the channel
bed sediment is that it is not necessary to wait for a flood event to conduct the sam-
pling, nor does one have to sample over an extended period of time (Mukundan et al.
2012 ). The sediment stored in the channel bed may change, however, over time and
at an unknown rate. Thus, bed sediment may need to be sampled on more than a
single occasion to assess the relative contributions from key sources over, say, an
entire year (Collins et al. 2013 ).
Some investigators have sampled the surface of floodplain deposits (e.g., Collins
et al. 2010a , b , 2012 ). This particular sampling scheme does not assess the sediment
loads during low to moderate flood events contained within the channel banks, but
rather is used to assess sediment provenance during events capable of inundating the
floodplain. The assumption inherent in this approach is that these overbank events
transport a majority of the sediment within the catchment, a conclusion reached by
studies dating back to the 1960s (e.g., Wolman and Miller 1960 ). Thus, the results
provide a reasonable assessment of sediment source contributions within the catch-
ment by flows that transport, on average, the most sediment (Collins et al. 2012 ).
While most early studies were aimed at documenting contemporary sediment
sources, Mukundan et al. ( 2012 ) point out that the same basic approach has been
applied to floodplain, reservoir, wetland, and lake deposits to determine the changes