Fluvial environments - Environmental Sedimentology - page 87

a strong cooling event at c . 40 ka led to a change
from sandy anabranching to braided river con-
ditions (Kasse et al. 2003). With deglaciation at
the Pleistocene-Holocene transition came high
runoffs and sediment loads, and considerable
channel change. Thomas (1998) suggested that
the change from dry to much wetter climatic
conditions between c . 1200 and c . 9000 yr BP
resulted in high discharges in both mountainous
(e.g. Nile, Amazon) and lowland tropical rivers
(e.g. Niger) and accompanying landsliding and
changes in slope hydrology and processes. As
vegetation re-established, slopes stabilized and
erosion became more confined to bank areas.
During the Holocene, climate change has
continued to significantly affect fluvial activity.
Alluvial deposits in England are thought to have
formed during the Late Medieval Warm Period
and Late Medieval Deterioration in response to
coincident heavy rain and snowmelt, whereas
those from corresponding periods in Italy are
related to increased cyclonic activity (Brown
1998). Episodic flooding events have been shown
to play a major role in present-day sedimenta-
tion in the Eel River, California (Morehead et al.
2001), and in Scottish rivers, where rates of
lateral channel shift and extent of bare gravels
are related to changes in flood frequency since the
mid-nineteenth century (Werritty & Leys 2001).

tion in the Congo River arising from erosion
of the edge of the uplifted Angolan margin
(Lavier et al. 2001). In the Tejo river, Portugal,
successive regional uplift events during increased
intra-plate compression are thought to cause
dramatic fluvial incision (Cunha et al. 2005). In
this area, river terraces, previously thought to
be related mostly to glacial-interglacial cycles
superimposed upon uplift, were reinterpreted as
being caused mainly by these tectonic phases,
punctuated by relatively short periods of lateral
erosion and some aggradation, with climatic
and base-level change factors only acting as a
conditioning process.
Tectonic uplift can result in a short-term or
gradual change in longitudinal profile, particu-
larly when the uplift is domal (Knighton 1998).
In the eastern Carpathians, Radoane et al. (2003)
demonstrated that tectonic uplift at over 6 mm
per year, rather than age, was responsible for the
shape of the longitudinal profile and the types of
channel deposits present. Laboratory experi-
ments carried out by Ouchi (1985) have shown
meandering rivers respond to slow uplift by
increasing sinuosity (in the case of a steepening
valley slope), or by straightening or anastomos-
ing (in the case of a flattening slope).
In north-west Europe, uplift as a result of glacio-
isostatic rebound is thought to play a role in
controlling Early Pleistocene sedimentation in
rivers (Bridgland 2000; Westaway et al. 2002).
The basis of this theory is that, to maintain iso-
static equilibrium, a particular area will undergo
uplift when adjoining areas undergo repeated
and cyclical glacial surface loadings (Westaway
et al. 2002). This theory has been used to explain
the occurrence of thick ( c . 100 m) sequences of
gravel terraces, aggraded during glacial cycles.
Although climate is the major factor responsible
for these terraces, in that large-magnitude sea-
sonal flows and reduced vegetation cover cause
increased erosion and large-scale deposition
(Bridgland & Allen 1996), the shedding of this
sediment requires uplift during interglacial marine
highstands (Bridgland 2000; Westaway et al.
2002). There may also be an element of mantle
bulge or upwarp involved in response to crustal
unloading as a result of terrain erosion.

3.3.2 Tectonic uplift and glacio-isostatic rebound

Tectonic uplift often causes river incision and
shedding of sediment from upland areas to river
basins. It can result in considerable channel
change, particularly in valley slope, width, dis-
charge and, ultimately, denudation and alluvi-
ation downstream. The Bengal Basin in India and
Bangladesh, for example, is rapidly aggrading
due to Himalayan uplift and erosion, and sea-
level changes since the last glaciation (Allison
et al. 2003). Lave & Avouac (2001) used sus-
pended sediment loads in Himalayan rivers
to infer that contemporary hillslope erosion
over the whole Himalayan range in Nepal was
driven mainly by fluvial incision linked directly
to uplift. In the Miocene, the gradual tectonic
uplift of Africa has been linked to sedimenta-