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In-Depth Information
progressive weathered and mobilized (Farella
et al. 2001). Recently deposited alluvium is thus
more humified and N-rich than older material.
Many authors (e.g. Macklin et al. 1992;
Macklin & Lewin 1993; Brown 1997; Coulthard
et al. 2000) have stressed that anthropogenic
activity and climate change act in tandem to
affect sedimentation in river basins. In a numer-
ical (computer) modelling study, Coulthard et al.
(2000) showed that, although deforestation
alone increased sediment discharges by 80% in
an upland basin in northern England, a change
in both climate and vegetation cover resulted in
a 1300% increase, confirming field-based studies
in the area. River basins are therefore more
sensitive to climate change when the soils are
destabilized by human activities such as defore-
station and agricultural practices (Coulthard
et al. 2000). These activities expose relatively
fresh, unweathered material, making it more
susceptible to, and possibly inflating the rates
of, chemical and physical weathering.
Afforestation, particularly in upland portions
of basins, can also have significant effects on
sedimentation in rivers, owing to soil distur-
bance when planting. Among the effects docu-
mented are (i) elevated rates of bank erosion
(Painter et al. 1974), increased suspended
sediment yields, increased bed load yields, par-
ticularly upstream (Newson 1980), and develop-
ment of a flashy hydrological regime (Leeks &
Roberts 1987). Mount et al. (2005) constructed
sediment budgets to evaluate the impacts of
upland afforestation between 1948 and 1978 in
the Afon Trannon, mid-Wales. These budgets
demonstrated that upland basin bed load yields
of between 2 and 3 t km
−2
yr
−1
were equivalent
to localized gravel inputs from bank erosion,
and thus were probably not responsible for
increased sedimentation downstream. Rather,
the nature of the bank material, and increases in
flood magnitude and frequency since 1988, pos-
sibly as a result of climate change, were deemed
to be responsible. This study again demon-
strates the interplay between different natural
and anthropogenic processes in influencing river
sedimentation, and the need for interdisciplin-
ary study to understand these processes.
3.4.1.2 Mining
Present-day and historic mining activities have
released large quantities of metal-bearing mineral
particles into fluvial environments (Fig. 3.15a;
Lewin et al. 1977, 1983; Lewin & Macklin
1987; Macklin et al. 1994; Hudson-Edwards
et al. 1996, 1999b, 2001; Miller 1997). Mining-
related sources of metal-bearing sediments to
rivers include discharge of mine or processing
waste, tailings dam failures, erosion of tailings
and waste rock piles, remobilization of mining-
contaminated alluvium, and mine drainage.
Direct discharge of mine tailings, effluent and
waste rock is one of the most common and
significant sources of metal-bearing particles
to river systems. This has been an important
process in the past (e.g. north-east England;
Hudson-Edwards et al. 1996), but is still
ongoing today (e.g. Río Pilcomayo, Bolivia;
Hudson-Edwards et al. 2001; Fig. 3.15b). Ore
processing activities (e.g. amalgam treatment,
cyanide leaching) also contaminate rivers with
metals and metalloids such as Ag, Bi, Cd,
Cu, Hg, Pb, Sb and Zn which, over time,
can accumulate in significant amounts (Fig.
3.16).
Failures of tailings dams can release very large
quantities of metal-bearing particles to river
systems. The discharge of these tailings often
greatly exceeds the sediment-transport capacity
of a river and results in considerable channel
and floodplain aggradation ( James 1989). Phys-
ical remobilization of abandoned tailings or
waste piles, and of channel beds and mining-
contaminated floodplain alluvium (formed dur-
ing historic mining activity), also provides large
amounts of metal contaminants to rivers (Macklin
et al. 1992; Miller 1997; Miller et al. 1998).
Merrington & Alloway (1994), for example,
showed that the wash load being transferred
from abandoned tailings heaps in two mines in
Cornwall and Wales, UK, was very high for Cu
(38 kg yr
−1
) and Pb (74 kg yr
−1
). Erosion occurs
during lateral channel migration and exposure
of bank sediment (Miller et al. 1998), or channel
bed, tailings, waste pile or cut-bank incision
(Macklin & Lewin 1989).
