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zone several metres above the adjacent valley floor
(Hudson-Edwards et al. 1999a). Contaminants
associated with fine-grained sediments also pre-
ferentially accumulate in topographically low
areas on floodplains (Wolfenden & Lewin 1977;
Bradley & Cox 1990). In floodplains affected
by overbank deposition, sediment contaminant
concentrations are either highest immediately
adjacent to the active channel, dropping sharply
with increasing distance from the river (Macklin
1988; Leigh 1997; Chesnokov et al. 2000; see
Case Study 3.3), or are more uniform across the
floodplain (Bradley & Cox 1990). Macklin (1996)
suggested that these differences were due to grain-
size controls on contaminant concentrations.
contaminant inputs from mining or industrial
areas (e.g. Knox 1987; Swennen et al. 1994).
3.4.2.5 Contaminant remobilization
Contaminants stored on floodplains may be
remobilized through chemical processes (changes
in redox and pH; Hudson-Edwards et al. 1998)
and through physical erosion, as shown by Brunet
& Astin (2000), who reported that elevated dis-
charges of inorganic N and P were associated
with increased autumn rainfall and sediment con-
veyance in the River Adour, south-west France.
Physical remobilization may take place long after
the primary contaminating activity (e.g. mining)
has ceased (Miller et al. 1998). In the Carson
River basin, west-central Nevada, USA, Miller
et al. (1998) demonstrated that lateral instabil-
ity, coupled with channel-bed incision, resulted
in the exposure and erosion of mining-related,
Hg-contaminated sediment from bank sediment,
and suggested that the valley fill was the prim-
ary contemporary source of Hg to the river.
Contaminant remobilization is often initiated
by natural (climate) or anthropogenic changes
(land use) that cause modifications to sediment
load and delivery, and ultimately erosion and
deposition (Lewin & Macklin 1987; Macklin &
Lewin 1989; Macklin 1996; Miller 1997).
3.4.2.4 Uses of overbank sediment profiles
Overbank sediment profiles have many uses
in contaminated river systems. Because over-
bank sediments record both the natural and
anthropogenic geochemical evolution of flood-
plains, one of their main uses is as sampling and
mapping media to assess metal contamination
caused by mining and industrial activity (Lewin
& Macklin 1987; Macklin et al. 1992, 1994;
Miller 1997; Hudson-Edwards et al. 1999a).
This is generally carried out by comparing near-
surface overbank contaminant concentrations
to those in sediments buried deeper in the
sediment profile. Macklin et al. (1994) stressed
that this type of assessment should be carried
out only in conjunction with geomorphological
mapping and dating of representative floodplain
overbank profiles at a number of reaches, to
take into account any lateral variations in metal
concentrations and to cover as wide an age range
as possible. Metal concentrations and ratios
in overbank sediments also have been used as
stratigraphical markers for provenancing (e.g.
Passmore & Macklin 1994), dating (Davies &
Lewin 1974; Lewin et al. 1977; Macklin & Lewin
1989; Macklin et al. 1992) and examining the
contaminant sedimentation histories of vertically
accreted fine-grained overbank deposits (Swennen
et al. 1994; Hudson-Edwards et al. 1999a). This
is possible because metal concentrations and
ratios vary systematically with respect to the
3.5
MANAGEMENT AND RESTORATION OF
FLUVIAL SYSTEMS
River systems are continually being brought under
management owing to human requirements for
water, agriculture, industry and urbanization.
Indeed, there are few rivers in the world today
that are not regulated or managed to some degree.
River management is carried out for a large num-
ber of reasons, based on perspectives that rivers
are either potential hazards or benefits to
anthropogenic activity (Table 3.4).
In many cases, river engineering and man-
agement has not been carried out with a full
understanding of fluvial geomorphology. Gilvear
(1999) has identified fundamental areas where
fluvial geomorphologists can contribute to the
