Progress in Physical Geography 29, 1 (2005) pp. 27–49
I Introduction
With the disappearance of the forest, all is
changed ... The soil is bared of its covering of
leaves, broken and loosened by the plough,
deprived of the fibrous rootlets which held it
together, dried and pulverized by sun and wind,
and at last exhausted by new combinations.
The face of the earth is no longer a sponge, but
a dust heap, and the floods which the waters of
the sky pour over it hurry swiftly along its
slopes, carrying in suspension vast quantities of
earthy particles which increase the abrading
power and mechanical force of the current,
and, augmented by the sand and gravel of
falling banks, fill the beds of the streams, divert
them into new channels and obstruct their
outlets ... From these causes, there is constant
© 2005 Edward Arnold (Publishers) Ltd 10.1191/0309133305pp433ra
Did humid-temperate rivers in the Old
and New Worlds respond differently to
clearance of riparian vegetation and
removal of woody debris?
Gary J. Brierley1,*, Andrew P. Brooks2, Kirstie Fryirs1
and Mark P. Taylor1
1 Department of Physical Geography, Macquarie University,
North Ryde NSW 2109, Australia
2 Centre for Riverine Landscapes, Griffith University,
Nathan Qld 4111, Australia
Abstract: Clearance of riparian vegetation and removal of woody debris are perhaps the most
pervasive of all forms of human disturbance to river courses. Geomorphic consequences of these
impacts have varied markedly from river system to river system, a result of variations in catchment
setting, climate, geology, sediment supply and evolutionary history. In this paper, geomorphic
responses of rivers to rapid, systematic clearance of riparian vegetation in New World (colonial)
societies are contrasted with changes associated with gradual, piecemeal, yet progressive
clearance of riparian forests in northern Europe (the Old World). It is postulated that the dramatic
nature of river metamorphosis experienced in landscapes such as southeastern Australia records
the breaching of fundamental geomorphic thresholds in a different manner to that experienced in
Old World landscapes.
Key words: channel changes, human disturbance, riparian vegetation, threshold, woody debris.
*Author for correspondence: School of Geography and Environmental Science, The University of
Auckland, Private Bag 92019, Auckland, New Zealand. E-mail: g.brierley@auckland.ac.nz
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28 Responses of Old/New World rivers to vegetation/woody debris clearance
degradation of the uplands, and a consequent
elevation of the beds of watercourses and of
lakes by the deposition of the mineral and
vegetable matter carried down by the waters.
The channels of great rivers become unnavigable,
their estuaries are choked up, and
harbours which once sheltered large navies are
shoaled by dangerous sandbars. (Marsh, 1864:
186–87)
The quote above, taken from George Perkins
Marsh’s (1864) classical treatise entitled Man
and nature, is indicative of the recognition in
the mid–late nineteenth century of the interrelated
set of impacts that vegetation
clearance induced on river courses. Building
on a series of case studies in northern Africa,
the French Alps and northern Italy, Marsh
(1864) demonstrated the critical nature of
ground cover as a control on geomorphic
processes, the linkages between hillslope and
river processes, and off-site impacts of disturbance
set within a catchment context. He
goes on to discuss how the nature, pattern
and rate of changes to these processes are
likely to vary in New World situations, where
the impacts can be more readily observed,
emphasizing that the best place to measure
the effects of land use impacts is in continents
such as Australia, where European settlement
occurred much later than elsewhere
(Marsh, 1864: 49). In this article, a synthesis is
provided of the lessons learnt from contemporary
research into the impacts of clearance
of riparian vegetation cover and removal of
woody debris along river courses in parts of
the New World, framing these observations
in context of prevailing debates on long-term
human-induced changes to rivers in the Old
World.
Most rivers throughout the world have
been subjected to some form of human disturbance
(Heede, 1972, 1981; Kellerhals and
Miles, 1996). Perhaps the most pervasive form
of disturbance is that associated with the
direct and indirect consequences of changes
to riparian vegetation cover and removal of
woody debris. Putting aside variations in scale,
climate, geology and topography, and the variable
geomorphic role of riparian vegetation
and woody debris in differing environmental
settings (e.g., Hupp and Osterkamp, 1996;
Gurnell et al., 2002), the question posed in this
paper is: were geomorphic responses of rivers
to the clearance of riparian vegetation and
removal of woody debris in the Old World
consistent with patterns and rates of river
metamorphosis following human disturbance
in the New World, where clearance was
much more rapid and systematic over large
areas? Prior to assessing the differential geomorphic
responses of rivers to changes to
riparian vegetation cover and removal of
woody debris in the Old and New Worlds,
their variable geomorphic roles in different
environmental settings are summarized. This
is followed by a brief review of the ways in
which riparian vegetation and woody debris
influence river morphology and an appraisal of
the longer-term evolutionary context that
underpins the geomorphological consequences
of human-induced changes to riparian
vegetation cover.
II Geographic variability in the
geomorphic role of riparian vegetation
and woody debris
The evolution of forests and the associated
input of woody debris into rivers extends back
over the last 400 million years (Schumm,
1968). Indeed, the first appearance of meandering
stream deposits in the geologic record
coincides with the evolution of land plants,
marking a transition from braided depositional
environments characterized by rapidly
shifting channels over largely unimpeded
valley floors (Schumm, 1968; Cotter, 1978).
Late Cenozoic changes in global vegetation
patterns imparted substantial variability to
the role of riparian forests and associated
woody debris in rivers in differing parts of the
world. For example, whereas parts of eastern
Australia have been continuously forested for
over 100 million years, only a few European
rivers maintained forest refugia during glacial
maxima (Montgomery and Piégay, 2003).
Modern forests cover around one-third of
the Earth’s land surface. In some settings,
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G.J. Brierley et al. 29
significant human-induced changes to forests
have occurred over at least the last 6000
years. Forests in southern Europe were
already confined to mountainous areas by
classical times (Marsh, 1864; Darby, 1956). In
historic times, humans have reduced global
forest cover to about half its maximum
Holocene extent, and eliminated all but a
fraction of the world’s aboriginal forests
(Montgomery and Piégay, 2003).
The geomorphic role of riparian vegetation
and woody debris is quite different in humidtemperate
or tropical settings, given the
nature and abundance of riparian forests
relative to, say, arid and semi-arid climates,
whether hot or cold (arctic) desert conditions.
Notable differences in geomorphic
process activity are observed in river systems
characterized by no vegetation or systems
with grass, shrub or tree cover (whether in
the form of a corridor or a fully developed
riparian forest) or in systems with swampy
vegetation. Hence, disturbances to riparian
vegetation cover may bring about profound
changes to the pattern and rate of geomorphic
activity, impacting on the nature and
extent of geomorphic adjustments and the
capacity of river systems to recover (e.g.,
Wolman and Gerson, 1978). Further complications
are presented by differences in
tree/forest structure and related issues of
vegetation size, root structure and whether,
as a function of its density, the wood floats
once it falls into a river (e.g., Abernethy and
Rutherfurd, 1998). For example, widely
spreading or multiple-stemmed hardwoods
are more prone to forming snags than accumulating
as racked members of large log jams
because they extend laterally as well as
beyond their bole diameter (Montgomery and
Piégay, 2003). In contrast, coniferous wood
debris tends to produce cylindrical pieces that
are more readily transported through river
systems, resulting in local concentrations of
log jams. While the decay rate of wood in
tropical rivers is rapid, temperate streams
may retain the same pieces of wood for several
thousand years (Nanson et al., 1995).
This paper does not focus on the geomorphic
role of riparian vegetation and woody
debris in different environmental settings. To
minimize this acknowledged variability,
emphasis in our analysis focuses on river
responses to variable rates of clearance of
riparian vegetation and removal of woody
debris in systems that previously were characterized
by near-continuous riparian forests
in humid-temperate settings of the Old and
New World, focusing on Europe and North
America/Australia, respectively. As noted in
the following section, until recently the geomorphic
role of riparian vegetation cover as a
determinant of river forms and processes in
these settings has been barely appreciated or
examined.
III The geomorphic role of riparian
vegetation and woody debris
Although geomorphic responses to vegetation
clearance have been noted for some time
(e.g., Marsh, 1864), our understanding of the
direct process-based roles played by riparian
vegetation and woody debris as controls on
river forms and processes has emerged only in
the past two decades. Prior to the 1980s, only
a few studies had highlighted linkages
between riparian vegetation and river morphology
(e.g., Mackin, 1956; Zimmerman et
al., 1967; Nanson and Beach, 1977; Graf,
1978; Charlton et al., 1978). Indeed, Hickin
(1984) suggested that the science of fluvial
geomorphology was flawed because of its
failure to incorporate the vegetation component
into fluid hydraulics and geomorphic
theory. In a similar vein, Montgomery and
Piégay (2003: 1) highlight the neglected significance
of the geomorphic role of woody
debris in river systems in their recent editorial,
in which they comment:
No doubt about it, wood complicates fluvial
geomorphology. It messes up nice tidy streams,
complicates quantitative analyses, invalidates
convenient assumptions, and opens new
questions about how different contemporary
channels are from their pristine state. It is no
coincidence that modern fluvial geomorphology
developed through the study of channels
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lacking a substantial load of wood debris
(Leopold et al., 1964) and there is little mystery
as to why geomorphologists sought to study
rivers or river reaches where wood does not
exert a significant influence on channel
morphology and processes. As a consequence,
many of the empirical relationships used to
describe various components of river system
behaviour must be viewed as characterising
modified channel morphology.
In recent years, however, an increasing body
of theoretical and empirical work has demonstrated
the significance of riparian vegetation
and woody debris as controls on river morphology
and behaviour.
Through controls on increased bank
shear strength, and reduced boundary layer
shear stress, riparian vegetation exerts a
significant control on channel size and shape
(Zimmerman et al., 1967; Smith, 1976;Thorne,
1990; Millar and Quick, 1993; Abernethy and
Rutherfurd, 1998; Millar, 2000). Empirical
studies have shown that channels with dense
bank vegetation (i.e., trees and shrubs) are on
average between 0.5 and 0.7 times the width
of an equivalent channel vegetated only by
grass (Charlton et al., 1978; Hey andThorne,
1983, 1986; Andrews, 1984). Theoretical
modelling has confirmed these empirical
relationships (e.g., Ikeda and Izumi, 1990;
Millar and Quick, 1993, 1998; Huang and
Nanson, 1997, 1998). Other research has
demonstrated the role of vegetation in enhancing
bar and bank stability (Smith, 1976;
Hughes et al., 1997; Brooks and Brierley,
2000), while aquatic plants (including mosses
and macrophytes), algae and even freshwater
sponges may promote the biological stabilization
of channel beds (Brown and Quine,
1999). Li and Shen (1973) demonstrated from
flume studies that flow resistance associated
with in-channel vegetation can reduce bedload
transport rates by more than 90%.
Alternatively, vegetation incursions into
channels may increase flow roughness,
reduce flow velocities, induce wash-load
deposition and stabilize in-channel sediment
deposits (e.g., Petts, 1984; Thorne, 1990;
Abt et al., 1994). A nonlinear relationship has
been demonstrated between hydraulic resistance
due to vegetation and flow stage
(Li and Shen, 1973; Petryk and Bosmajian,
1975; Kouwen and Li, 1980), reflecting flexural
rigidity and foliage distribution of the
vegetation, among many factors.
Numerous authors have reported channel
narrowing and planform adjustments following
the colonization of the riparian zone of
formerly unvegetated rivers by exotic vegetation
(e.g., Hadley, 1961; Nevins, 1969;
Schumm, 1969; Burkham, 1972, 1976; Graf,
1977, 1978; Friedman et al., 1996; Brooks and
Brierley, 2000). Indeed, vegetative controls
on bank strength may influence channel planform
and the associated geomorphic
structure of rivers (e.g. Mackin, 1956; Brice,
1964; Ferguson, 1987; Marston et al., 1995;
Millar, 2000). Hickin (1984) noted that lateral
migration rates of channels with vegetated
banks were half those of channels with
cleared or cultivated riparian zones. Beeson
and Doyle (1995) supported these findings,
observing that nonvegetated banks were
nearly five times more likely to undergo
notable erosion compared with vegetated
banks. Variations in the height, density and
flexibility of aquatic vegetation can also influence
reach-scale flow resistance. As such,
changes to riparian vegetation may alter
floodplain roughness and the surface area
exposed to the flow and sedimentation,
potentially modifying patterns and rates of
depositional processes and the capacity for
floodplain reworking (e.g., Brown and
Brookes, 1997).
As a general rule, the proportion of vegetation
occupying a channel cross-section
decreases downstream as the channel
becomes larger (Abernethy and Rutherfurd,
1998). Zimmerman et al. (1967) suggested
that in very small catchments, up to approximately
2 km2, grass- and sedge-dominated
channels are smaller than channels having
similar catchment area (or discharge) that are
dominated by trees. However, moving downstream,
channels dominated by trees are
comparatively smaller than channels with
30 Responses of Old/New World rivers to vegetation/woody debris clearance
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equivalent catchment area but only grass and
sedge on the banks and floodplain. Similar
patterns have been reported elsewhere (e.g.,
Keller and Tally, 1979; Gregory and Gurnell,
1988; Thorne, 1990; Fetherston et al., 1995;
Abernethy and Rutherfurd, 1996, 1998),
while Millar and Quick (1993, 1998) acknowledge
this phenomenon from a theoretical
perspective.
An equivalent set of downstream relationships
has been described for the geomorphic
role of woody debris. The stability of woody
debris and its influence on channel forms and
processes reflects the relative size of key
wood elements compared with channel size
(Montgomery and Piégay, 2003). In loworder
channels, woody debris may induce
channel blockage ratios as high as 80% (Keller
and Tally, 1979; Bilby and Ward, 1989;
Thorne, 1990; Nakamura and Swanson,
1993; Keller et al., 1995; Abernethy and
Rutherfurd, 1996, 1998). Moving downstream,
woody debris tends to be rotated
subparallel to the flow (Kochel et al., 1987;
Fetherston et al., 1995; Abbe and Montgomery,
1996, 2003), minimizing the blockage
ratio but maximizing its role in bar accretion
and bank toe protection (Nanson, 1980;
Hickin, 1984; Abbe and Montgomery, 1996).
Indeed, various studies have demonstrated
the influence of woody debris as a control on
channel width (e.g., Beschta, 1979; Keller and
Tally, 1979; Lisle, 1986, 1995; Nakamura and
Swanson, 1993; Keller et al., 1995; Montgomery
et al., 1995, 1996; Woodsmith and
Buffington, 1996; Hogan et al., 1998; Curran
and Wohl, 2003). In wider channels, woody
debris may be transported beyond the fall
point, and become incorporated into log
jams, potentially causing local bank erosion,
triggering channel avulsion or cut-off development
(e.g., Swanson et al., 1976; Keller and
Swanson, 1979; Heede, 1981; Marston, 1982;
Hickin, 1984; Gregory and Gurnell, 1988;
Piégay, 1993; Nakamura and Swanson, 1993;
Fetherston et al., 1995; Montgomery et al.,
1995, 1996; Abbe and Montgomery, 1996,
2003; Piégay and Gurnell, 1997; Piégay and
Marston, 1998; Cohen and Brierley, 2000;
Daniels and Rhoads, 2003). Indeed, the
influence of woody debris may even shape the
formation of floodplains and valley-bottom
landforms, as exemplified by island development
(Harwood and Brown, 1993; Abbe
and Montgomery, 1996, 2003; Piégay and
Gurnell, 1997; Gurnell et al., 2001; Brooks
et al., 2003; Jeffries et al., 2003; O’Connor
et al., 2003). In addition, woody debris may
enhance geomorphic recovery following a
major disturbance by increasing roughness
and helping to stabilize in-channel sediment
(e.g., Cohen and Brierley, 2000).
Many of the geomorphic effects of wood
in rivers arise from its influence as an obstruction
to local flow hydraulics, as the capacity
to create significant hydraulic roughness influences
flow velocity, shear stress, bedload
transport rates, and reach-average surface
grain sizes (Keller and Tally, 1979; Mosley,
1981; Macdonald and Keller, 1987; Shields and
Smith, 1992; Smith et al., 1993a, b; Lisle,
1995; Assani and Petit, 1995; Shields and
Gippel, 1995; Keller et al., 1995; Buffington
and Montgomery, 1999; Manga and Kirchner,
2000; Montgomery and Piégay, 2003).
Numerous studies have demonstrated how
woody debris, whether as individual pieces,
log jams or through its influence as a determinant
of the size, type and evolution of
in-channel features such as pools, bars and
steps, can impart significant hydraulic resistance
(Beven et al., 1979; Gregory et al., 1985;
Gippel et al., 1994, 1996; Gippel, 1995; Shields
and Gippel, 1995; Abernethy and Rutherfurd,
1996; Brooks and Brierley, 2002).
These recent developments in our understanding
of vegetative controls on river
character and behaviour present an intriguing
context with which to reappraise our interpretations
of river responses to environmental
change, enabling us to more accurately
decipher the long-term evolution of river
systems and their responses to human disturbance.
There is now sufficient, unequivocal
evidence to assert that riparian vegetation
and woody debris represent fundamental
G.J. Brierley et al. 31
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controls on geomorphic river forms and
processes. However, depending on an array
of factors such as the type of river under
investigation, its within-catchment position,
its sensitivity to change, and the nature of the
vegetation and woody debris, the geomorphological
influence exerted by riparian
vegetation and responses to its removal, can
be remarkably variable. Prior to assessing
geomorphic river responses to differing patterns
and rates of riparian vegetation
clearance in humid-temperate settings of the
Old and New Worlds, changes to river
morphology induced by altered riparian vegetation
cover are placed in their longer-term
evolutionary context.
IV Placing geomorphic river responses
to vegetation change in a broader
context
A series of inherent limitations will always be
encountered in making generalized arguments
about, or comparisons of, landscape
behaviour, as a multitude of factors drive
system change over differing spatial and
temporal scales. Ultimately, system responses
to disturbance are catchment-specific,
reflecting the spatial configuration of any particular
system, the history, patterns and rates
of response to disturbance events, system
sensitivity to change and the persistence of
factors from the past that continue to exert
an influence on contemporary system forms
and processes, among many other considerations.
In framing the argument presented in
this paper, two broad-based aspects of longerterm
evolutionary history must be considered
in appraisal of system responses to riparian
vegetation disturbance in Old World northern
Europe relative to the New World in parts of
North America and Australia. First, the
impacts of glacial history are starkly different
in the northern and southern hemispheres,
impacting profoundly on the volume and calibre
of sediment made available for rivers to
transport through the Holocene. Secondly,
sediment availability is not solely determined
by differences in glacial history. Longer-term
geological history and associated topography,
as influenced by tectonic setting, present
stark differences in landscape setting when
comparing system responses to disturbance
in Australia relative to Europe or North
America. These boundary conditions, along
with influences exerted by climate variability
and differences in substrate (and other variables),
frame the context in which appraisals
of system response to clearance of riparian
vegetation and removal of woody debris must
be made. Throughout the discussion that follows,
it should be remembered that reference
is being made to differences in the sensitivity
of response to disturbance in riparian forests
in humid-temperate settings.
Changes to channel dimensions and morphology
are brought about by any factor that
unsettles the pre-existing balance in impelling
and resisting forces along a reach. Adjustments
that modify the water-sediment
regime or riparian vegetation cover bring
about changes to either the driving forces that
promote change, or the resisting forces that
inhibit change. Ultimately, changes to river
morphology reflect river responses to differing
forms of disturbance events, however
induced. Natural disturbances may be external
(e.g., climate change or tectonic activity)
or internal (e.g., slope adjustments along
longitudinal profiles). Human-induced disturbances
may be direct (i.e., generally
purposeful in-channel disturbances) or indirect
(i.e., accidental or unintended disturbances,
often associated with changes in ground
cover at the catchment scale).
Recognizing that a multitude of factors
may promote river change, unravelling
causality requires that adjustments are placed
in their longer-term, late Quaternary, context.
In general terms, large shifts in climate
over the timescale of glacial cycles bring
about profound river changes. However, the
sensitivity of alluvial channels, especially
reaches that are close to a threshold condition,
may ensure that relatively modest
climatic changes can trigger major episodes of
fluvial adjustment (see Knox, 1993, 1995;
32 Responses of Old/New World rivers to vegetation/woody debris clearance
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Elliot et al., 1999). At particular stages in
their evolution, concatenations of natural
processes, such as major bushfires followed
by extreme flood events, may conspire to
bring about river metamorphosis. It is in this
context that human-induced changes to river
forms and processes must be viewed.
Knox (1995) identified three primary
phases of fluvial activity over the past 20 000
years:
1. 20–14 ka BP. Major continental ice sheets
dominated temperate latitudes, reaching
their maximum extent by around 18 ka BP.
Large volumes of glacial meltwater and
sediment were contributed to proglacial
areas, where extensive braided river
systems developed.
2. 14–9 ka BP. Major climatic change saw the
transition from glacial to post-glacial conditions.
Initially rates of sediment supply
were very high during deglaciation, but
they progressively subsided as vegetation
cover increased. Meltwater sources and
increasing precipitation maintained high
river discharges. Over time, the reduced
sediment load coupled with relatively
high discharges brought about a transition
from an aggrading to an erosional regime,
and an associated shift from braided to
meandering in many temperate valleys.
3. 9–0 ka BP. Sea level has risen by around
130 m since the last glacial maximum. The
Holocene marine transgression, which
was largely accomplished by around 6 ka
BP with relatively minor oscillations in sea
level thereafter, brought about different
sets of river adjustment in upland and lowland
zones of catchments. The global
warming that characterized the commencement
of the Holocene around 10 ka
BP marked the start of a more climatically
stable period. During the Climatic Optimum,
from 9–4 ka BP, temperatures were
higher than those of today across much of
the northern temperate zone. This was
followed by an episode of climatic deterioration,
which became most marked around
3–2.5 ka BP. Two subsequent climatic
fluctuations are of note in this period: the
Little Optimum centred on approximately
1200 AD and the Little Ice Age around
1550–1700 AD, which was characterized
by marked increases in flood activity in
some areas.
The extent and impacts of glacial activity
were very different in the northern and
southern hemispheres. A large proportion of
the northern hemisphere was glaciated at
glacial maxima. In contrast, across the
Australian continent the effects of Quaternary
glaciation were marked by changes to
sea level and rainfall and temperature
regimes, but they were not accompanied by
glacial erosion and reworking of materials on a
scale comparable with landscape settings in
the northern hemisphere. The legacy
imposed by sediment stores generated in
response to glacial activity continues to be a
major factor influencing contemporary landscape
forms and processes across much of the
northern hemisphere (Church and Slaymaker,
1989). Geomorphic responses of landscapes
are particularly pronounced in the‘paraglacial
interval’, when climate variability is pronounced
and vegetation cover is yet to be
fully established (Church and Ryder, 1972).
The pattern and rate of system adjustment to
these enhanced sediment loadings draws into
question whether these river systems had
established an ‘equilibrium’ configuration
prior to the impacts of human disturbance, as
highlighted in the following section.
Nanson et al. (2003) present a picture of
cyclical but generally declining episodes of
fluvial activity for rivers across southeastern
Australia during the past full glacial cycle, in
direct response to climate and flow regime
changes. By the early Holocene, conditions
were such that highly sinuous, slowly migrating,
suspended load rivers had replaced the much
larger, actively migrating mixed- or bedloaddominated
channels that had characterized
the Murrumbidgee River over the previous
100 000 years (Page and Nanson, 1996).
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In this catchment, relatively little change to
river morphology is inferred throughout the
Holocene. Roughly parallel longer-term climateinduced
phases of river adjustment are
evident along the lower Wollondilly-Nepean
River (Nanson et al., 2003). However, alluvial
fills of smaller, relatively confined coastal valleys
have preserved a slightly different record
of river changes during the Holocene. In part
this reflects the impacts of base level adjustments
associated with sea level changes. In
these systems, the late Pleistocene and early
to mid-Holocene (12 000–3000 years ago)
was marked by much lower flows than those
experienced earlier in the Pleistocene, and
was characterized, in broad terms, by lateral
reworking and terrace formation (Nanson
et al., 2003). Other than a few remnants,
the early to mid-Holocene alluvium was
reworked, aided by the confining effect of the
Pleistocene terraces. A shift in climate in the
mid–late Holocene, which ranged from
around 4000 to 2500 years ago in different
valleys, led to an increase in sediment preservation
with the development of relatively
stable vertically accreting floodplains alongside
well-vegetated channels (Nanson et al.,
2003). It was these low-energy, laterally stable
rivers that European land clearance so
dramatically destabilized (Nanson and Doyle,
1999). However, Brooks and Brierley (2002)
and Brooks et al. (2003) note that the Thurra
and Cann Rivers, adjacent systems in Eastern
Gippsland, Victoria have been actively migrating
and occasionally avulsing over at least the
last 20 000 years. The Thurra River remains
remarkably intact today (Brooks and Brierley,
2002), while riparian vegetation removal and a
desnagging programme along the Cann River
brought about dramatic metamorphosis
within a few decades of disturbance (Brooks
et al., 2003).
The tectonic stability and generally low
relief of the Australian land mass, along with
the lack of a glacial history of notable extent
and the fact that climate changes along
the eastern highlands and coast were not
sufficiently extreme as to have caused the
transformation of forests, clearly goes a long
way to explaining some key differences
between channel response in Australia and
their glaciated counterparts of the northern
hemisphere–whether in Old World countries
of northern Europe or New World countries
such as the USA. It is in the context of this
longer-term perspective that the rate and
magnitude of human-induced disturbance
must be gauged. This presents a template
upon which the nature, rate and consequences
of geomorphic river adjustments to
different patterns and rates of riparian vegetation
clearance in the Old and New World can
be contrasted. To address this issue, available
literature on river responses to land use
changes in Europe is contrasted with river
responses to human disturbance in New
World (colonial) settings in North America
and Australia.
V The imprint of land use changes on
Holocene river evolution in Europe
Rivers across Europe changed profoundly at
the end of the last glaciation and continued to
change, although less dramatically, during the
mid–late Holocene, as climate changes
affected the hydrology, sedimentology and
vegetation cover of floodplains (Starkel, 1991;
Brown, 1997). Inherent differences in system
configuration, sediment availability and the
history of forcing events, among many factors,
induced spatial and temporal variability
in the transition from braided to meandering
or anastamosing channel planforms, but the
picture that has emerged indicates that many
European rivers had achieved a condition of
relative geomorphic stability by the midHolocene
(e.g., Kozarski and Rotnicki, 1977).
It is interesting to note that vegetationinduced
variation in bank strength may have
strongly influenced the transition in channel
planform (Millar, 2000).
Brown (1997) indicates that various river
systems in Britain have considerable thickness
of Lateglacial and post-Boreal sediments
(12 000–10 000 years ago), but relatively few
sediments from the early to mid-Holocene
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(Boreal and Early Atlantic; 9500–6500 years
ago). Inevitably, the distribution of geomorphic
processes and the resulting history of
sediment accumulation recorded in Holocene
alluvial sequences vary in different parts of
the landscape. In upland and piedmont zones,
where stream powers are higher, incision
dominated the post-glacial landscape. In contrast,
lowland zones were affected by rising
sea levels coupled with low floodplain gradients
and low stream power conditions, which
resulted in the accumulation of vertically
accreted alluvium. Changes to the pattern
and rate of overbank sedimentation can be
linked directly to vegetation development on
floodplains (Brown, 1997). Although alder
arrived in Britain before 9000 years ago, it
was around 2000 years later that it became a
major forest component with most lowland
floodplains covered by alder-dominated
woodland or alder–hazel woodland by around
6000 years ago (Brown, 1997). Over time,
changes to the flow and sediment regime of
these rivers transformed bedload-dominated
systems into more stable, suspended load systems,
commonly with anastamosed channel
networks (Brown, 2002). Vegetation and
woody debris likely played a critical role in this
stabilization phase (e.g., Brown and Keough,
1992; Brown and Quine, 1999).
Since the Neolithic period (around 5500
years ago), human activity has had a significant
impact on floodplain forest systems.The
stepped, diachronous nature of alder decline
in pollen diagrams in the mid-Holocene has
been attributed to changes in population density,
settlement history and land use control
(Brown, 1997). Although progressive changes
to forest cover are implied, and some
‘smoothing’ of sedimentary records has
undoubtedly occurred through reworking and
bioturbation, riparian vegetation clearance
probably occurred in a piecemeal manner. It is
inferred that intensive human land use was
initially localized and transient. Relatively low
rates of population growth in the midHolocene
were mirrored by gradual land use
changes, building out from fragmented areas
of forest clearance. Impacts extended over
millennia, rather than a few centuries. Major
episodes of woodland regeneration are also
likely to have occurred (Brown, 1997).
Similar patterns of riparian vegetation
changes have been inferred from southern
Poland, where the transition from fragmented
clearances set within a matrix of alder woodland
to large interconnected cleared areas
with woodlands isolated in the landscape
occurred around 6000 years ago (Godlowska
et al., 1987). Starkel (1995) inferred that
deforestation associated with the development
of arable cultivation and the formation
of meadows and willow scrub accelerated the
rate of lateral channel migration and induced
aggradation. Such assertions imply that channel
capacity was not markedly increased;
otherwise floodplain inundation rates and
concomitant aggradation would have been
diminished. Reported changes to these rivers
note adjustments to channel morphology and
migration rate, and increases in sediment flux,
rather than ‘catastrophic’ metamorphosis.
Profound adjustments to river morphology
seem to have occurred much later than this
initial phase of vegetation disturbance, associated
primarily with direct human modification
to river courses through channelization programmes
thousands of years later.
While there is good evidence from several
valleys for an increase in flooding and alluviation
during the Roman occupation of Britain
(around 2000–1600 years ago), along with
mid- to late-medieval alluviation in lowland
and piedmont valleys (around 800–500 years
ago), remarkably little change to channel
capacity, planform shape and lateral stability
was experienced along most lowland river
systems in Britain (Brown, 1997). The
impacts of channel instability associated with
simultaneous down-cutting of channels in
upper–middle catchments, following the
emplacement of channel embankments and
human-induced channel narrowing and
straightening (e.g., Gregory, 1977; Klimek,
1987; Lewin, 1987), were seemingly damped
down through catchments, and did not
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impose significant responses in lowland
reaches.
Starkel (1995) noted that forest clearance
on floodplains and an increase in soil erosion
in southern Poland during the late Roman and
Medieval times caused a tendency towards
aggradation, and, in some reaches, the formation
of braided channels. Pronounced
increases in the yield of fine-grained sediments
are inferred for the late pre-historic to
medieval period around 500 years ago (e.g.,
Bork, 1989), with local variability considered
to record differing phases of agricultural
expansion and agricultural intensity (Brown,
1997). However, these geomorphic adjustments
occurred thousands of years after the
initiation of human impacts on riparian vegetation
cover. Subsistence practices of
previous millennia in Old World settings were
replaced by feudal land tenure practices that
did not promote extensive, synchronous
clearance of catchments and river corridors,
which would have led to broad-scale landscape
transformation.
By the end of the medieval period, deforestation
of floodplains for pasture and arable
cultivation was almost complete across most
of Britain and northwest Europe, the few large
exceptions generally being forests preserved
for hunting (Brown, 1997). Montgomery and
Piégay (2003) refer to manipulation of riparian
vegetation cover along European streams
during this period as ‘riparian gardening’,
characterized by successive clearing campaigns,
tree selection favouring root network
growth and efforts to prevent bank erosion.
Although initial endeavours at river clearing
and engineering date back to the Roman
era (e.g., Herget, 2000), systematic river
channelization only began in earnest in the
1600s. Since then, lowland rivers across
Europe have been extensively modified by
programmes that set out to ‘control’ natural
variability, primarily in the interests of navigation
and flood control (Petts, 1989; Brown,
1997). Virtually all lowland reaches were
‘homogenized’ for human purposes. Seemingly,
it was in this interval of direct human
intervention to river courses, rather than the
indirect responses associated with riparian
forest clearance over preceding millennia,
that profound river metamorphosis occurred.
Societal changes accompanied this phase of
landscape adjustment. For example, the
industrial revolution and associated drift of
the rural poor into cities brought about
changes to the intensity of land use, impacting
on sediment yield. Examples of the
impacts of such changes in land use intensity
have been demonstrated through appraisals
of the impacts of collectivization on soil
erosion across many parts of eastern Europe
(e.g., Hanusˇin, 2000). Rivers have continued
to adjust to ongoing changes in their boundary
conditions, whether intentional or
otherwise.
It is interesting to note that impacts on
European riparian forests from the midHolocene
onwards were manifest primarily
through localized removal of riparian
vegetation, subsequently leading to wholesale
clearance of floodplains, without necessarily
impacting directly on the loading of woody
debris until the commencement of channelization
programmes. Modelling work
reported by Brooks (1999a, b) indicates that it
is changes to the wood loading in some rivers
that triggers profound adjustments in morphology,
rather than alterations to floodplain
forest cover or bank vegetation per se. This
may, in part, explain why adjustments to
European rivers were relatively small prior to
the implementation of channelization and
desnagging programmes. It was not until
loadings of woody debris were extensively
reduced for navigation purposes, and to clear
channels to enhance flow conveyance associated
with perceived flood inundation
problems, that river metamorphosis occurred.
As noted by Montgomery and Piégay
(2003), it is difficult to discern the role of
wood in temperate fluvial systems of Europe
because so little wood has been retained and
there are no historical documents that
demonstrate the presence or abundance of
wood in pristine European rivers. Today, the
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British Isles has no river systems with primary
(natural) cover on floodplains, very few
without channels that have been channelized
and highly engineered, and none without
some management of flow regimes through
reservoirs, weirs or land drainage. As a consequence,
most ‘lowland British floodplains
generally exhibit a greatly reduced set of
floodplain processes and relatively low rates
of channel erosion and floodplain change,
having in effect entombed themselves’
(Brown and Quine, 1999: 12). However, as
outlined below, analysis of the remaining
reaches of river with intact riparian vegetation
and associated loads of woody debris in
humid-temperate settings of the New World
may not provide ideal analogues with which
to appraise the history of geomorphic river
responses to human disturbance.
In summary, many river courses in
northern Europe have been subjected to
multiple forms of disturbance over millennia,
such that their contemporary character and
behaviour reflect the cumulative nature of
responses to these impacts. Despite the
profound nature of human-induced
disturbances to vegetation cover, and associated
increases in the fine-grained sediment
yield, the record of systematic river metamorphosis
over the past 5000 years is quite
equivocal, other than those changes associated
with channelization over the past 400
years or so. This picture contrasts starkly
with landscapes in parts of the New World in
which human-induced disturbances to
riparian vegetation cover brought about
fundamental changes to river character and
behaviour within a matter of decades of European
settlement.
VI River responses to land use changes
in the New World
The history of land use practices in the New
World differs markedly from that experienced
in the Old World. This reflects, in part, differences
in social ‘development’ associated
with the process of colonization of the New
World.The very underpinnings of agricultural
expansion in the New World were totally
different to the aim and intent of practices
that had been experienced in the Old World.
Large-scale, agricultural ‘development’,
applied with virtual total disregard for preexisting
cultural practices, was driven by
state-run (or sponsored) capitalist exploitation
for the benefit of the mother lands in the
Old World. Despoilation of other parts of the
New World, while sparing the homeland, was
encouraged by colonial powers, as it brought
benefits of new products and resources
without the need to consider human or
environmental costs.
In addition to fundamental differences in
social and political context, clearance of riparian
vegetation and removal of woody debris in
New World societies was facilitated by far
more efficient tools than were available at the
time of Old World woodland clearance and
land use changes (see Crosby, 1986; Lines,
1991; Flannery, 1994; Diamond, 1997). This is
not to say that changes to riparian vegetation
cover were brought about by sophisticated
and well-orchestrated plans, using wellconsidered
and clearly-targeted practices.
Rather, as graphically illustrated in Figure 1, vast
tracts of land were cleared in seemingly haphazard
acts of bastardry in the nineteenth and
twentieth centuries using axes, bullocks, fire
and ring-barking practices, with no consideration
given to the consequences of these actions.
Riparian zone clearance was an early
priority of pioneer settlers, as these were the
most fertile and well-watered lands. Initial
desnagging efforts minimized the loading of
woody debris in rivers to improve navigation;
later programmes were applied for flood
mitigation purposes or even to remove
perceived barriers to fish migration (Triska,
1984; Gregory et al., 1993; Maser and Sedell,
1994; Gippel et al., 1994). New World
agricultural systems were more extensive,
widespread and synchronous than initial
endeavours in the Old World, employing
technologically advanced practices. These
combined activities brought about rapid
landscape changes.
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In contrast to the clearance of European
forests over millennia, the vast forests of
North America vanished over centuries or
even decades, with forest stands near rivers
usually the first to be logged (Montgomery
and Piégay, 2003). By the late nineteenth
century, efforts to clear wood from rivers had
extended to the west coast of the USA
(Sedell and Frogatt, 1984; Collins et al.,
2002). While large rivers were being cleared
of snags and jams, smaller tributary streams
were cleared through practices such as splash
damming, salvage of in-stream wood or
stream cleaning (Triska, 1984; Harmon et al.,
1986; Maser and Sedell, 1994).
The recent history of newly colonized
areas, combined with some documentary
records, provides compelling evidence of the
changes to river forms and processes. In
many instances profound landscape changes
were observed within individual lifetimes.
Historical evidence derived from analysis of
old documents such as explorers notes, maps,
surveyors plans, paintings and photographs
provides an intriguing, but fragmentary, set of
insights into the nature and rate of these
38 Responses of Old/New World rivers to vegetation/woody debris clearance
Figure 1 The social underpinnings of expansionist agricultural exploits in the New
World differed from the relatively fragmented and piecemeal practices that developed
over previous millennia in the Old World. This picture, taken a few years after
European settlement of the northeastern Cape of North Island, New Zealand,
graphically demonstrates the rape and pillage of landscapes that accompanied pioneer
settlements. Haphazard felling of trees was accompanied instantaneously by stocking
by non-native animals, impacting dramatically on erosion rates on hillslopes and along
river corridors. This photograph, entitled ‘Sheep crossing Mata River at Puketoro
Station, East Coast, NZ’, taken by Frederick Hargreaves (Reference Number
304. 6–20), is used by permission of the Tairawhiti Museum, Gisborne
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geomorphic changes (e.g., Johns et al., 1998;
Davis and Finlayson, 1999). In many
instances, catastrophic landscape disturbance
in the New World brought about river metamorphosis
within the first generation after
colonization. For example, Knox (1972, 1977,
1987, 1989) identified significant incision,
channel straightening and channel widening in
the period following deforestation, mining and
cultivation practices associated with the sudden
replacement of woodlands by arable and
pastoral agriculture, following European settlement
of Wisconsin, North America.
Various studies in the Pacific Northwest of
the USA and British Columbia, Canada have
shown how rivers in old growth forest settings
that retain their full loading of woody
debris and riparian vegetation tend to be
characterized by single thread meandering
channels or anastomosing channels (Abbe
and Montgomery, 1996, 2003; Kellerhals and
Miles, 1996; Millar, 1998, 2000; Collins et al.,
2002). Upon removal of riparian vegetation
and woody debris, these channels are rapidly
transformed into broad, shallow braided
rivers (Kellerhals and Miles, 1996; Millar,
2000).
Despite marked differences in terms of
landscape setting, whether viewed in terms
of relief, tectonic setting, glacial history or
sediment delivery relationships, a remarkably
similar set of geomorphic adjustments has
been reported following clearance of humidtemperate
riparian forests in southeastern
Australia. Typically, alluvial or semi-alluvial
rivers along the east coast, the eastern tablelands
and in the upper reaches of westerly
draining rivers have become deeper, wider,
straighter and more homogenous in the
period since European settlement (e.g.,
Pickup, 1976; Eyles, 1977a, b; Henry, 1977;
Erskine and Bell, 1982; Erskine, 1986; Prosser,
1991; Prosser et al., 1994; Rutherfurd, 1996,
2000; Warner, 1997; Brierley and Murn,
1997; Brooks and Brierley, 1997, 2000; Fryirs
and Brierley, 1998, 2001; Page and Carden,
1998; Wasson et al., 1998; Brooks et al.,
2003).
In many instances changes to river
geomorphology in southeastern Australia
were initiated along the lowland course of the
river – the reach first subjected to European
disturbance (Brooks and Brierley, 1997,
2000). For example, contributors to the
Hunter Royal Commission in 1870 (Moriarty,
1869) described the channel capacity of the
Hunter River, north of Sydney, as doubling in
size during their lifetime. In a similar vein, in
1803 Governor King called for vegetation
removal along the lower course of the
Hawkesbury-Nepean River, near Sydney, to
be discontinued (Lloyd, 1988). This implies
that within 15 years of European settlement
of Australia, conversion of the original riparian
forest community to one dominated by
pasture grass had significantly reduced bank
shear strength, as well as altered in-channel
resistance characteristics, bringing about
dramatic transformations in river morphology.
The fundamental changes to channel
boundary conditions caused by the initial
riparian vegetation disturbance set in place a
sequence of events still being experienced
today. Irreversible geomorphological changes
have ensued, at least over timescales of
centuries (e.g., Fryirs and Brierley, 2001;
Brooks et al., 2003; Brooks and Brierley,
2004). For example, within a few decades of
direct human disturbance to the riparian
vegetation cover along the Cann River in
East Gippsland, Victoria, coupled with a
desnagging programme, channel capacity
increased by 700%, channel depth increased
by 360%, there was a 240% increase in
channel slope, and there was a 150-fold
increase in the rate of lateral channel
migration (Brooks et al., 2003). Numerous
thresholds have been crossed as a result of
historical channel changes, particularly the
relationship between average length of
woody debris pieces and channel width and
exceedance of critical bank height once
incision was initiated. Profound changes to
patterns and rates of sediment flux
have ensued, with marked differences
in downstream coupling and associated
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residence times for sediment storage from
catchment to catchment.
The critical issue for the argument
presented here is that parallel forms of
geomorphic river response to disturbance of
riparian vegetation cover have been observed
in New World settings of North America and
Australia, despite self-evident differences in
their geomorphological settings. In this
manuscript we contend that human disturbance
to riparian vegetation has been a more
forceful agent of change over the last few
centuries, effectively over-riding other
environmental controls. However, the longterm
consequences of these changes may
vary markedly along river courses in North
America relative to Australian rivers, as the
potential for geomorphic recovery in these
different settings is starkly different,
reflecting critical differences in sediment
storage along river courses and the rate of
sediment generation and transfer.
VII Why did rivers in the Old and
New World respond differently to the
clearance of riparian vegetation and
removal of woody debris?
Our capacity to interpret the geomorphic
responses of river systems to the clearance of
riparian vegetation is constrained, in large
part, by the lack of undisturbed reference
sites and the incomplete preservation of alluvial
sequences. Even across much of the New
World, few river systems retain an intact
riparian forest and associated loading of
woody debris. Given this shortcoming, our
insights into the nature, patterns and rate of
geomorphic adjustment to human disturbance
are often inferential, and the link
between cause and effect must be implied
rather than directly proven.Theoretical models
that incorporate both bank vegetation and
woody debris as components of bank
strength and hydraulic roughness (e.g., Millar
and Quick, 1993, 1998; Shields and Gippel,
1995; Huang and Nanson, 1998) show good
agreement with empirical observations of
river change when the bank vegetation is
removed or modified and when woody debris
is removed, and as such provide a rational
means of corroborating the field evidence,
such as it is.
We contend that prior to human disturbance,
geomorphic adjustments in systems
with an intact riparian forest were localized
and were mediated by resisting components
induced by the vegetation cover and loading
of woody debris, such that river systems
were able to re-equilibrate whenever they
were subjected to ‘natural’ disturbance
events (Brooks and Brierley, 2002). The
inherent roughness of these riparian landscapes
was such that thresholds for
geomorphic change were virtually unattainable
and floods were unable to bring about
river metamorphosis. Pronounced geomorphic
change could only occur if the landscape
was ‘sensitized’ through changes to these
resisting elements. This required a very
unlikely set of circumstances, such as concatenations
in which a series of large floods
immediately followed a major fire. Detailed
analyses of floodplain deposits along alluvial
reaches of the Cann and Thurra Rivers in
East Gippsland, Victoria, indicate that such
scenarios have not occurred in this part of
southeastern Australia over at least the last
20 000 years (Brooks and Brierley, 2002;
Brooks et al., 2003). Even if such unlikely circumstances
should eventuate, these systems
had sufficient potential for geomorphic recovery,
given within-system roughness, the
availability of seed sources and the fact that
disturbance was not system-wide (in contrast
with subsequent systematic catchment-wide
clearance of hillslope and riparian vegetation).
In other words, we postulate that the inherent
resilience of these valley floors ensured
that they were subjected to progressive
aggradation, but such behaviour was no
longer sustainable once core resisting components
of river structure were removed
(Montgomery et al., 1996; Brooks and Brierley,
2002; Brooks et al., 2003). Post-disturbance,
these systems are now characterized by
progressive degradation (cf. Erskine, 1999).
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Undoubtedly there are some examples of
rivers in steep, sandy settings where thresholds
of stability are exceeded in the absence
of direct channel or riparian zone disturbance
(e.g., Cohen and Brierley, 2000). However,
examples such as this are rare, and represent
one extreme of the population of river styles
and responses to natural disturbances.
The near-instantaneous reduction to the
buffering capacity of riverscapes in New
World settings following the removal of vegetative
roughness elements effectively lowered
fundamental threshold conditions that determine
bed level stability and critical bank
height (e.g., Thorne and Furbish, 1995;
Brooks et al., 2003), such that rivers became
highly sensitive to change. As a consequence,
flood events that brought about minor perturbations
under intact vegetation conditions
were much more geomorphologically effective
under altered boundary conditions.
Exceedance of threshold conditions brought
about fundamental shifts in river character
and behaviour. Along many river courses the
primary initial response was channel incision,
followed by lateral expansion (Schumm et al.,
1984). In this scenario, the progressively
enlarging channel is increasingly able to concentrate
flow energy at flood stage, further
enhancing erosional activity, and the channel
becames decoupled from its floodplain (cf.
Elliot et al., 1999).
The short lag time between disturbance
and metamorphosis, typically measured in
terms of a few decades, ensured that once
critical trigger events were experienced it was
exceedingly difficult for systems to recover.
The systematic nature of riparian vegetation
clearance, along with the fact that vegetation
regrowth was seldom possible (whether associated
with stocking rates or subsequent land
management practice), almost entirely
negated the opportunity for future recruitment
of woody debris and associated
geomorphic recovery mechanisms. In some
instances recovery was further inhibited
by sediment exhaustion. In these situations,
geomorphic changes experienced over a
remarkably short interval induced river
responses that are effectively irreversible over
timeframes of centuries or even millennia
(Fryirs and Brierley, 2001; Brooks and Brierley,
2004). Once threshold conditions were
breached, a completely different set of riverforming
processes was established under
altered channel and catchment boundary
conditions.
The implication presented here postulates
that piecemeal disturbance of riparian
vegetation cover along rivers in the Old
World enabled progressive adjustments
and/or recovery to occur over millennia, and
fundamental threshold conditions that
prompt river metamorphosis were not
breached prior to the implementation of
channelization programmes. Inevitably, evidence
for such responses may have been
swamped or ameliorated by climate-induced
signals.
Ultimately, environmental changes and
anthropogenic activities act concurrently in
determining the nature, timing and pattern of
river changes. The relative influence of these
factors is likely to vary in each catchment,
given their unique configuration, pattern of
linkages and history of landscape-forming
events. Unfortunately, unravelling these histories
of geomorphic river adjustment to
human disturbance will always remain
conjectural, with generalities masked by
system-to-system variability and nonexistent
or incomplete historical records. In addition,
much of the evidence for geomorphic changes
has been subsequently completely eroded,
buried or built upon.
Despite these limitations, the geomorphic
evidence presented in this paper does not
indicate that river systems in the Old World
‘fell apart’ following disturbance to riparian
vegetation cover in a way that has been
evidenced in various parts of the New World.
If profound river metamorphosis did occur
following clearance of riparian vegetation,
substantive evidence would surely have
been found to support such dramatic river
changes, even if it is fragmentary. Viewed in
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this manner, we contend that progressive
geomorphic readjustments were experienced,
in which fundamental internal
threshold conditions were not breached.
Although threshold conditions for river metamorphosis
were lowered following piecemeal
and fragmentary removal of riparian vegetation
and woody debris in the Old World, river
systems retained sufficient resistance such
that geomorphic changes were moderated,
and rivers were able to re-equilibrate following
disturbance events. As the impacts of
human disturbance were not catchmentwide
at the same time, the geomorphic
effectiveness of flood events was moderated
relative to the situation faced in New World
landscapes. Inevitably, local river adjustments
occurred, especially in upper catchment
reaches, and overbank sedimentation rates
may have increased along middle–lower
reaches. However, wholesale and systematic
changes to river character and behaviour
were not experienced in the same way that
occurred in New World settings.
VIII Conculsion and implications
It has long been recognized that vegetation
cover is the key control on river character and
behaviour that can be modified by human
activity. One of the key factors in interpreting
river responses to human disturbance lies in
understanding the interplay between riparian
vegetation, its close associate woody debris,
and all other variables within the fluvial system,
such as sediment calibre and supply rate,
flow dynamics, bank material composition
and valley morphology. This involves assessing
the control exerted by riparian vegetation
and woody debris on various feedback
linkages that influence channel capacity,
hydraulic roughness, channel slope, sinuosity,
sediment transport rates, bank strength
and floodplain evolution. Variations in vegetation
character associated with vegetation
structure, root networks, etc., can affect
the relationships between these variables.
Human-induced changes to riparian vegetation
cover and the associated loading of woody
debris tend to sensitize rivers, enhancing
the capacity for forms and processes to shift
outside their ‘natural’ range of variability.
In some landscape settings the role of humandisturbance
has brought about geomorphic
changes that have not been manifest in these
systems for many thousands of years (e.g.,
Brooks and Brierley, 1997; Brooks et al.,
2003).
Regardless of environmental setting, contemporary
river morphodynamics across
most of the globe have adjusted to conditions
in which riparian vegetation and woody debris
are either absent or highly altered. We contend
that the differing rate and extent of
riparian vegetation clearance and removal of
woody debris in the Old World compared
with the New World settings induced profound
differences in the nature and extent of
disturbance response. Near continuous land
use changes over thousands of years in Old
World settings induced progressive adjustments
in river morphology, culminating in a
phase of pronounced human-induced metamorphosis
following channelization over the
last 500 years or so. In contrast, river
metamorphosis in New World settings
occurred within a few decades of European
settlement, reflecting the breaching of
fundamental geomorphic thresholds following
wholesale destruction of catchment-wide
riparian vegetation cover in near instantaneous
geological time.
Catchments comprise complex, interactive
landscapes. Their unique configuration
and history fashion catchment-specific
patterns and rates of biophysical fluxes and
associated responses to disturbance.
Whether induced by environmental (climatic)
changes, bushfire activity or as a consequence
of human activity, changes to riparian
vegetation cover may induce catchment-wide
changes to river structure and function, with
profound implications for the nature and
rate of biogeochemical fluxes (e.g., Brierley
et al., 1999).
Given the complexity of biophysical linkages
and the cumulative nature of disturbance
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impacts, along with profound variability in the
inherent sensitivity of individual river systems
to adjustment, it is often difficult to isolate
cause–effect relationships that can directly link
changes in river morphology to discrete underlying
factors. Within any given catchment, not
all landforms have responded to the last external
influence in the same way, resulting in
considerable complexity in patterns and/or
rates of landscape responses to disturbance
events. As impacts following disturbance are
conveyed through a system, consequences
may be manifest for some time, possibly up to
thousands of years. Patterns, rates and consequences
of geomorphic river responses to
disturbance are catchment-specific.
Some landscapes are more resistant to
change than others (Brunsden and Thornes,
1979). Different rivers respond to changes in
discharge, sediment load or vegetation cover
in different ways over different timeframes.
For example, river sensitivity to change
following disturbance to riparian vegetation
is likely to vary markedly dependent on
substrate texture. In sand-bed rivers with
noncohesive banks, riparian vegetation has a
proportionally higher effect on total bank
strength than the same vegetation would
have if bank materials comprise cohesive
sediments. Hence, channel response to vegetation
removal will be more rapid and
proportionally greater in the channel with
noncohesive bank sediments relative to the
channel with cohesive banks. A similar situation
applies for removal of woody debris;
indeed, system sensitivity to change associated
with bed-level instability may be even
more pronounced.
In systems with large buffering capacity
and/or with large thresholds to overcome,
there may be considerable time lags between
perturbation and morphological response.
The nature and level of perturbation may
be such that some landscapes are capable
of withstanding external disturbance, while
others rapidly undergo metamorphosis.
Profound variability in the manner and rate of
river responses to disturbance in differing
landscape settings, and associated potential
for geomorphic recovery, reflect, in part, sediment
availability. The resistance of a river
system to disturbing forces, and the associated
proximity to thresholds, dictate the
ability of a system to recover after disturbance
(Downs and Gregory, 1993). In relative
terms, the capacity of river systems to
recover following disturbance may be
enhanced in transport-limited landscapes,
where sediments are readily available to be
reworked, compared with supply-limited
landscapes, where the timeframe for recovery
may be prolonged once sediments are
evacuated from a reach. The latter scenario
may be evidenced along many river courses in
Australia (e.g., Fryirs and Brierley, 2001;
Brooks and Brierley, 2004). Antecedent controls
on sediment availability can have a
distinct spatial element, shaping landscape
responses to subsequent disturbance events.
Variability in sediment availability, supply
rates and associated reworking are key determinants
of differences in river response to
human disturbance evident in Australian
rivers relative to their New World counterparts
in North America or Old World rivers in
northern Europe. The availability of seed
sources preserved in remnant pockets of
native vegetation, or alternatively the expansion
of exotic vegetation along river courses,
are further considerations that determine the
potential for geomorphic recovery.
Acknowledgements
This study marks the culmination of many
years of work in analysis of river responses to
human disturbance in southeastern Australia,
supported by Ph.D. scholarships awarded to
Andrew Brooks and Kirstie Fryirs by Land
and Water Australia and Macquarie University,
respectively, and associated research
grants provided by these organizations over
the past 10 years. The dating evidence upon
which many of the interpretations reported
here have been based was supported by various
AINSE grants. We gratefully acknowledge
these differing forms of support. In addition,
G.J. Brierley et al. 43
at Masarykova Univerzita on March 12, 2016ppg.sagepub.comDownloaded from
we thank several colleagues for sharing their
insights into river character and behaviour in
many fruitful discussions, especially Tim
Cohen and John Jansen. Comments on an
earlier version of this manuscript by Tony
Brown helped clarify various points.
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