Geoscience Reference
In-Depth Information
Climatic Optimums such as the Little Ice Age in
the sixteenth to nineteenth centuries), to 10
1
years
(global warming trends in the late 1990s and
early 2000s). These climatic changes are related
to factors such as Milankovitch cycles (Lewis
et al. 2001), changes in meridional circulation
(Knox 1995) and global warming, and they
play a major role in influencing weathering pro-
cesses and, ultimately, channel morphology and
channel and floodplain sedimentation.
Many investigators have demonstrated that
rivers are particularly sensitive to changes in
climate, and several have sought to define the
relationships between these changes and sedi-
mentation in river systems. Early work suggested
a direct relationship between sediment yield and
precipitation (Fig. 3.10a) (Langbein & Schumm
1958), in that the highest yields occurred dur-
ing peak periods of precipitation. Later work by
Walling & Kleo (1979) showed that this rela-
tionship was more complex on a global scale
(Fig. 3.10b), with a three-peak average sediment-
yield-precipitation plot that reflected distinct
global climatic zones. Walling & Kleo (1979)
did, however, suggest that this might be too
simplistic, and that other factors, such as sea-
sonal effects on precipitation and temperature,
relief, soil and rock type, and land use, may
be responsible for the three peaks on Fig. 3.10b
(Hooke 2000). In terms of floodplain deposition,
increased discharges have been related directly
to the area of floodplain that is inundated by
suspended-sediment-bearing water (Hamilton
1999). The situation is not always as simple as
this: Aalto et al. (2003), for example, showed
that crevasse-splay deposits in the Bolivian
Beni and Mamore river basins in Bolivia could
be grouped temporally, and that the deposi-
tion pattern was punctuated in a stop-and-
start manner. Aalto et al. (2003) related these
crevasse-splay deposit groupings to the cold (La
Niña) phases of the ENSO (El Niño-Southern
Oscillation) cycle, in that rapidly rising floods
during these phases destabilize and deposit
colossal volumes of Andean sediment. Inman
& Jenkins (1999) demonstrated that the wet
periods of alternating, decadal-scale ENSO-
induced climate changes were responsible for
annual sediment fluxes that were up to 27 times
greater than those in dry periods.
Flood frequency plays a major role in defining
patterns of river channel erosion and deposi-
tion downstream (Rumsby & Macklin 1994).
Correlations of alluvial chronologies in the UK,
Europe and North America have revealed major
discontinuities in the Holocene alluvial record,
marked by alternating wetter and drier phases,
and alternating higher and lower frequency
of extreme flood events (Starkel 1983). This
clustering has been linked mainly to climatically
driven changes (Macklin & Lewin 1993, 2003;
Rumsby & Macklin 1994; Knox 1995). The high
flood-frequency clusters resulted in landslides,
increased rates of deposition and lateral channel
change and avulsion (Starkel 1983). Because of
the intimate link between fluvial sedimentation
and climate, many investigators have used the
fluvial sedimentary record to reconstruct past
climates (despite the sometimes sparse nature of
the climate record), particularly in contemporary
times when global warming is of such concern.
(a)
Desert
shrub
800
Grassland
Forest
600
Reservoir data
400
200
Stream data
0
500
0
1000
1500
Effective precipitation, mm
(b)
1200
800
400
0
0 800 1600 2400 3200 4000
Mean annual precipitation, mm
Fig. 3.10
Relationship of sediment yield to (a) effective
precipitation (after Langbein & Schumm 1958; Knighton 1998)
and (b) mean annual precipitation (after Walling & Kleo 1979;
Knighton 1998), based on measured data.
