The human role in changing river channels
K.J. Gregory ⁎
Department of Geography, University of Southampton, Southampton, S017 1BJ, UK
Received 15 August 2005; received in revised form 6 June 2006; accepted 6 June 2006
Available online 8 August 2006
Abstract
Direct consequences of the human role, where human activity affects river channels through engineering works including
channelization, dam construction, diversion and culverting, have been long recognised [Marsh, G.P., 1864. Man and Nature or Physical
Geography as Modified by Human Action. Charles Scribner, New York; Thomas Jr., W.L., (ed.) 1956. Man's Role in Changing the Face
of the Earth. Chicago, University of Chicago Press, Chicago.]. The less obvious indirect effects of point and reach changes occurring
downstream and throughout the basin, however, are much more recently appreciated, dating from key contributions by Strahler [Strahler,
A.N., 1956. The nature of induced erosion and aggradation. In W. L. Thomas (Ed.), Man's Role in Changing the Face of the Earth.
University of Chicago Press, Chicago, 621–638.], Wolman [Wolman, M.G., 1967. A cycle of sedimentation and erosion in urban river
channels. Geografiska Annaler 49A, 385–95.], Schumm [Schumm, S.A., 1969. River metamorphosis. Proceedings American Society of
Civil Engineers, Journal Hydraulics Division 95, 255–73.], and Graf [Graf, W.L., 1977. The rate law in fluvial geomorphology.
American Journal of Science, 277, 178–191.]. These are complemented by effects of alterations of land use, such as deforestation,
intensive agriculture and incidence of fire, with the most extreme effects produced by building activity and urbanisation.
Changing river channels are most evident in the channel cross-section where changes of size, shape and composition are now wellestablished,
with up to tenfold increases or decreases illustrated by results from more than 200 world studies. In addition the overall channel
planform, the network and the ecology have changed. Specific terms have become associated with changing river channels including
enlargement, shrinkage and metamorphosis. Although the scope of adjustment has been established, it has not always been possible to
predict what will happen in a particular location, because of complex response and contingency. The ways in which changes in cross-section
relate to reach and network changes are less clear, despite investigations showing the distribution of changes along segmented channels.
When considering the human role in relation to changing river channels, at least five challenges persist. First, because prediction of
the nature and amount of likely change at a particular location is not certain, and because the contrasting responses of humid and arid
systems needs to be considered, modelling is required to reduce uncertainty, as was first emphasised by Burkham [Burkham, D.E., 1981.
Uncertainties resulting from changes in river form. American Society Civil Engineers Proceedings, Journal Hydraulics Division 107,
593–610.]. Second, feedback effects incorporated within the relationship between changes at channel, reach and network scales can have
considerable implications, especially because changes now evident may have occurred, or have been initiated, under different
environmental conditions. Third, consideration of global climate change is imperative when considering channel sensitivity and
responses to threshold conditions. Fourth, channel design involving geomorphology should now be an integral part of restoration
procedures. This requires, fifthly, greater awareness of different cultures as a basis for understanding constraints imposed by legislative
frameworks. Better understanding of the ways in which the perception of the human role in changing river channels varies with culture as
well as varying over time should enhance application of design for river channel landscapes.
© 2006 Elsevier B.V. All rights reserved.
Keywords: River channel changes; River channel management; Human impact
Geomorphology 79 (2006) 172–191
www.elsevier.com/locate/geomorph
⁎ Tel.: +44 1428 724458.
E-mail address: k.j.gregory@ntlworld.com.
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doi:10.1016/j.geomorph.2006.06.018
1. Introduction
A paper published 50 years ago (Strahler, 1956) was
one of several key foundations for understanding of the
human role in changing river channels, so that 2006 is an
appropriate year to reflect upon the way in which our
understanding has grown and the significance of that
understanding has become appreciated. Six chronological
phases of river use (Downs and Gregory, 2004, Table 1.2,
p.9) catalogue the major ways in which human influence
has affected river channels over the last 5000 years
through dam construction, river diversions and engineering
since the first phase of the early hydraulic civilisations
affected river channels. The culmination of that phase,
and the five subsequent ones, produced river channels that
are extensively modified by human activity, although the
spatial pattern of modification varies across the world.
Such modifications, which include direct purposeful
changes (Brown, 1970) and indirect, less obvious changes
(Table 1), are now complemented by ways in which the
human role, in a third phase, is beginning to redress earlier
effects of human action; for example by dam removal
(Heinz Center, 2002; Graf, 2003) or by stream and river
restoration (Downs and Gregory, 2004; Sear and Darby,
in press).
Against this background, reticence to investigate the
human role in changing river channels is surprising
because studies did not really begin until 1955 (Lane,
1955; Strahler, 1956). Most emphasis was given to effects
on the hydrological regime rather than upon the channels
despite some important individual contributions such as
changes in downstream sediment transport because of
placer mining (Gilbert, 1917), modification of the
‘physiographical balance’ by interference with natural
conditions (Sonderegger, 1935) illustrated by effects on
the Rio Grande downstream of Elephant Butte dam, and
the importance of fluvial morphology in hydraulic
engineering (Lane, 1955), subsequently developed further
by Schumm (1977). Downstream of dams the effects of
scour were considered in engineering design (e.g. Komura
and Simons, 1967) but apart from occasional studies (e.g.
Sonderegger, 1935) little attention was given to the
channel further downstream. As the broader implications
of channel change became appreciated, at least four
fundamental influences emerged (Fig. 1). The spatial
relationship between gullying and induced aggradation
(Strahler, 1956) identified the network consequences of
channel change (Fig. 1A); the way in which changing land
use over time culminating in building activity and then
urbanisation (Wolman, 1967) was related to channel
condition (Fig. 1B) was formulated; the notion of river
metamorphosis and of thresholds (Schumm, 1969); and
the application of a rate law (Graf, 1977) involving
reaction–and relaxation–times as parts of the response
time pertinent to changing from one equilibrium condition
to another, are embraced in the diagram (Fig. 1C) showing
types of equilibrium (Gregory and Downs, in press). These
four contributions, complemented by important individual
papers (e.g. Leopold, 1973), provided important foundations
for understanding the human role in changing river
channels. By the time River Channel Changes (Gregory,
1977a) was published, a clear understanding existed of the
need to focus upon channel cross-section, channel
planform and network changes induced by human activity.
Since 1977, the impacts of human activities in
changing river channels have been further elaborated
so that the magnitude of potential change is now known
much more completely. Research benefiting from the
established foundations (Fig. 1), together with awareness
of the magnitude of various categories of human
influence (Table 2), provided the basis for nearly thirty
years of research, during which progress made was
founded upon important key contributions (Table 1).
Following sections show how conclusions reached about
Table 1
Some key contributions in interpreting human impact on river channels
Contribution Implementation Source
Types of channel
change
QsD≈QwS involving bed material load (Qs), particle diameter (D), water discharge
(Qw) and stream slope (S) used to indicate implications of six types of change
Lane (1955)
Induced aggradation Gullying or channel extension upstream complemented by aggradation downstream
(see Fig. 1A)
Strahler (1956)
Cycle of erosion in
urban channels
Changing channel morphology depicted during land use change from forest through
agricultural land to urbanisation (see Fig. 1B)
Wolman (1967)
River metamorphosis, thresholds and
complex response
Effects of changing discharge and sediment load on channel morphology indicated
by equalities such as: Qs
+
, Qw
++
≈S−
, d50+
, D+
, W+
, using equality to relate sediment
(Qs) and water discharge (Qw) to slope (S), median diameter of bed material (d50),
flow depth (D) and flow width (W)
Schumm (1969)
Rate law Change from one equilibrium to another through a response time and a reaction time
(see Fig. 1C)
Graf (1977)
173K.J. Gregory / Geomorphology 79 (2006) 172–191
Fig. 1. Foundations for studies of the human impact upon river channels. A is redrawn from Strahler (1956); B is adapted from Wolman (1967); and C
incorporates ideas from Graf (1977) and Schumm (1979). For explanation see Table 1 and text pp. 00–00.
174 K.J. Gregory / Geomorphology 79 (2006) 172–191
changing river channels can now be consolidated
(Section 2), providing a basis for identifying the
challenges which remain (Section 3), leading to
recommendations which can be considered (Section 4).
2. Changing river channels
2.1. Basic questions
During investigations of river channel changes, a
series of questions were formulated, not always
explicitly, to guide research in a wide variety of areas.
Identifying the questions which were asked is necessary
to consolidate knowledge of changes of river channels
(Gregory, 1987a). Such questions, listed on the left hand
side of Fig. 2, are affected by emphases in research, and
relate to who will be affected and will meet the costs.
Emphases in research (middle column, Fig. 2) include
contributions by engineers (e.g. Yearke, 1971; Parker
and Andres, 1976; Tsujimoto et al., 1980; Surian and
Rinaldi, 2003), geomorphologists (e.g. Williams, 1978;
Petts, 1980; Graf, 1980), ecologists, (e.g. Stanford and
Ward, 1979; Obrdlik et al., 1989) and geologists (e.g.
Booth, 1991) as well as by practising river managers
(e.g. Qiwei et al., 1982). The great range of types of
human action identified meant that separate studies were
undertaken including the influence of dams and
reservoirs, of channel impacts, and of land use change
leading to urbanisation; research was affected by current
debates which included emphasis upon the impact of
channelization in the US (e.g. Heuvelmans, 1974) and
debate about dams, including criticism of the work of
the Corps of Engineers (Morgan, 1970) leading to the
suggestion of a reverence for rivers (Leopold, 1977)
anticipating the progression from hard to soft engineering
(see Downs and Gregory, 2004). The types of
channel effect produced were most frequently considered
in terms of impact on the channel cross-section,
although some studies identified changes of channel
planform or, less frequently of the drainage network
(Gregory, 1977b). Investigations tended to focus upon
small or medium scale rivers whereas large rivers
present particular problems (Thorne, 2002). A focus on
relatively short time scales also occurred because the
potential contribution of palaeohydrological research
had yet to be demonstrated (Gregory, 1995). Additionally,
styles of adjustment due to human impact have to
be related to enhance understanding of the styles of
natural change, and be reflected in classifications of
channel typologies (Hooke, 1997).
Table 2
Examples of human impacts pertinent to change in river channels
Type of impact Examples Source
Dams At least 45,000 large dams (N15 m high or 5–15 m high
if reservoir volumeN3 million m3
)
World Commission on Dams (2000)
More than 400,000 km2
inundated behind the world's large dams McCully (1996)
Fragmentation of nearly all rivers in North America
due to presence of dams, with 80,000 damsN6 ft
(1.83 m) and including all structures may be 2.5
million in the USA
Dynesius and Nilsson, 1994; Heinz Center, 2002
More than 1500 large- and medium-sized dams in
India and 100 barrages on all major river systems
Gopal (2000)
Australia has 447 large dams and several million farm
dams which modify river flows
Schofield et al. (2000)
Channelization In the USA 26,550 km of major works gives a
channelized density of 0.003 km/km2
Leopold (1977)
In England and Wales 8504 km of major or capital
works gives a density of 0.06 km/km2
, and there is also
a further 35,500 km of river which is maintained
Brookes, Gregory and Dawson (1983)
Channel modification Average of b10% of length of Alpine rivers is in a
semi-natural condition ranging from 2.5% in Germany,
4.9% in Switzerland, 9% in Italy and 18% in France
Ward et al. (1999)
River diversions By the end of the 13th Century the 1780 km Beijing–
Hangzhou Grand canal built to link 5 river basins and
transfer water from Yangtze to North China Plain
Li, Liu and Mou (2000)
Water extraction Approximately 11% of freshwater runoff in USA and
Canada withdrawn for human use
Karr et al. (2000)
Water abstraction of about one fifth of total water
resources with 87% for irrigation agriculture in China
Li, Liu and Mou (2000)
175K.J. Gregory / Geomorphology 79 (2006) 172–191
Despite these factors a large number of investigations
has given answers to the questions listed in Fig. 2. The
question of what causes change has been answered by
development of an increasingly long list of human
impacts that can have effects upon channels. Table 3
illustrates the major types of influence, direct and
indirect, and gives examples of the magnitude of those
influences. This range of causes of channel change is
paralleled by a diversity of effects in terms of what will
change (Table 4), although the majority of changes
reported are either changes of channel capacity or of
channel width (Gregory, 1987a). How much change
occurs can be demonstrated from a range of studies
(Gregory, 1995); by defining channel change as the ratio
of channel capacity, width or depth after change to that
before, it is possible to show the range of changes which
have been reported (Table 4) although spatial interpolation
techniques have to be employed carefully (Ebisemiju,
1991). For certain types of change there have been
comprehensive summaries produced, including the
morphological and ecological effects downstream of
dams and reservoirs (Petts, 1984). The downstream
effects of 21 dams on alluvial rivers in the USA, mostly
in the semiarid western States, demonstrated considerable
variability in responses (Williams and Wolman,
1984). Channelization has also been the basis for a
comprehensive approach demonstrating the range of
ecological and morphological consequences (Brookes,
1988) and implications (Brookes and Shields, 1996).
Although many answers to these initial questions have
been provided, it has been less easy to decide how long
change will take or how it occurs. Downstream of dams
most degradation occurred during the first decade or two
after dam closure but bed degradation varied from
negligible to about 7.5 m in the cross-sections studied
(Williams and Wolman, 1984). It is not easy to determine
when change will start and finish, because this may be
triggered by the incidence of hydrological events of a
certain size; or where it will occur, because the incidence
of change depends upon the characteristics of the
channel (Phillips, 2003); or why it will change hazards
and risks and may increase costs.
2.2. Approaches to the investigation of channel change
During the course of numerous individual investigations
of river channel change, it was appreciated that a
more comprehensive view was required to succeed the
Fig. 2. Questions posed in relation to human impact upon river channels together with emphases in research and methods of moving towards a more
holistic approach. Developed from a diagram in Gregory (1987a).
176 K.J. Gregory / Geomorphology 79 (2006) 172–191
Table 3
What causes change and what may change (see Fig. 2)
What causes change What may change
Processes:
hydrology H,
sediment S
River channel
Channel Adjustment locally evident Adjustment spatially extensive
Channel cross-section/point
Dam construction H− S− then S+ Scour below dam Downstream may have reduced channel
capacity but variations possible due
to sediment trapped
Weirs H− S− then S+ Scour below weir
Diversion of flow, including
mill leats, HEP
H− Aggradation after flow diverted, scour
possible close to where drainage returned
to channel
Not applicable
Abstraction of flow H− Not applicable
Return flow, drains, outfalls H+ Not applicable
Bridge Crossings H+ Scour around bridge piers Not applicable
Culverts under roads and crossings H+ Scour at downstream end Not applicable
Channel reach
Desnagging and clearing S+ Sediment transport increase and may have
accelerated erosion
Not applicable
Grazing S+ Localised bank and channel erosion
may occur
Not applicable
Embankments and levee construction H+ May have apparent effects downstream
Channelization H+ S+ Effects downstream may arise from increased
flow velocities. Knickpoint recession may
occur upstream from channelization
: Bank protection and stabilization H+ S−
: Resectioning, dredging H+–S−
: Channel straightening, cutoffs H+
Clearance of riparian vegetation/
tree clearance
S+ Localised bank and channel erosion
may occur
Not applicable
Beaver removal H+, S+ Localised channel adjustment possible Not applicable
Sediment removal, mining gravel
extraction
S+ Localised channel adjustment Sediment transfer downstream or scour may
affect channel morphology
Sediment addition, mining spoil S+ Localised aggradation possible May affect aggradation downstream and
upstream as well
Boat waves, bank erosion S+ Can induce localised bank erosion Not applicable
Invasion by exotic vegetation species S− Stabilise sections of channel and banks May reduce sediment transfer downstream
Afforestation S− but may
increase from
tracks
Progressive decrease of channel capacity possible but depends upon forest
drainage system
Conservation measures H− S− Progressive decrease of channel capacity possible but depends upon management of
drainage network
Restoration and allied techniques H− S− Designed channel characteristics Reduced erosion, and deposition, downstream
Drainage basin
Network
Drainage schemes H+ Channel adjustment possible where flow
extracted
Not applicable
Agricultural drains H+ Localised effects possible where drainage
reaches channel
Not applicable
Irrigation networks H− Channel adjustment possible where flow
extracted
If many locations of flow extraction can have
noticeable effects
Ditches H− Channel adjustment possible as result of
flow extracted
Not applicable
Stormwater drains H+ Scour around outfalls If numerous outfalls can then increased peak
(continued on next page)
177K.J. Gregory / Geomorphology 79 (2006) 172–191
emphasis placed on one type of change, for example
those downstream of dams. Such a comprehensive or
holistic view was obtained in at least five ways (Fig. 2,
right hand column). Some studies arose as a consequence
of investigation of hydrological process changes and in
such cases it was inevitable that different types of
channel adjustment were included. Thus, in the Platte
watershed of southwestern Wisconsin, three- to five-fold
increases in flood magnitude arose from land use change
to agricultural uses (Knox, 1977) with channel metamorphosis
including headwater and tributary channels
that are relatively wide and shallow when compared to
pre-settlement forms, whereas main channel forms are
narrow and deep compared to pre-settlement forms
(Knox, 2003). Channel changes must be considered
bearing in mind that river metamorphosis can occur in
certain areas without major environmental changes
(Brizga and Finlayson, 1990), especially when significant
changes occur in storm events.
A conceptual model was also a means of achieving a
more holistic approach: river metamorphosis, a complete
change of morphology could be instigated if
changes in discharge and sediment load were of
sufficient magnitude (Schumm, 1969, 1973). Schumm
(1973, 1977) employed empirical equations as a
framework for discussing natural and man-induced
changes of river morphology, an approach not dissimilar
from the approach of Lane (1955) with six categories of
river channel change (Table 1). This method provided an
indication of the major ways in which channel
morphology might change (see Table 1) but could not
be proscriptive because of uncertainty associated with a
particular environment. Most long term models of
channel adjustment, however, have been one-dimensional
and neglected some aspects of adjustment, such
as channel widening and downstream aggradation
(Doyle and Harbor, 2003).
A third way of providing a more comprehensive
understanding was to consider the drainage basin context
by focusing on the interrelationship of all channel changes
within a single basin. Consideration of channel metamorphosis
throughout the basin of Dumaresq Creek, NSW
Australia could be related to land use change since the
early nineteenth century, to dam construction at the end of
the nineteenth century, and to urbanisation in the second
half of the twentieth century, including a significant effect
instigated when rainwater tanks were succeeded by piped
water supply (Gregory, 1977b).
Table 3 (continued)
What causes change What may change
Processes:
hydrology H,
sediment S
River channel
Channel Adjustment locally evident Adjustment spatially extensive
flows induce erosion downstream
Spatial
Deforestation H+ S+ Gully development may occur with knickpoint
recession upstream. Downstream may have
channel change and often increased capacities
but depending upon sediment availability
Grazing S+ Local effects by degradation of
channel banks
Fire, burning H+ S+ Channel change after fire events
Agriculture, ploughing H+ S+ Localised effects often where
tributaries join main streams
Land use, conservation measures H− S− Progressive decrease of channel capacity possible but depends upon management of
drainage network
Afforestation S− but may
increase from
tracks
Progressive decrease of channel capacity possible but depends upon forest
drainage system
Building construction S+ Channel affected by large sediment load
locally
Temporary drainage complements
drainage network
Urbanisation H+ S− Scour may occur at stormwater outfalls Stormwater drains augment drainage network
Discharge is shown as H with increased flows designated as H+, and decreased flow as H−. Similarly, sediment transport increased is shown as S+
and decreased sediment transport as S−.
178 K.J. Gregory / Geomorphology 79 (2006) 172–191
More extensive than a single basin approach are
regional audits of causes and consequences of channel
change, as have been particularly successful in Australia.
Because human impacts on river systems constitute a
major area of study in modern fluvial geomorphology,
catchment alterations, including deforestation, grazing,
cropping, urbanisation, and changes in conservation
practices, can cause changes in the delivery of water and
sediment to the channel and hence channel morphological
adjustments occur (Warner, 1984). Subsequently
human impacts on channels in New South Wales were
seen (Warner and Bird, 1988) as direct (channel
regulation, channelization, vegetation management,
urban drainage improvements) or non-deliberate inadvertent
(aggregate extraction and mining disturbance,
flood mitigation, additions of water discharge, loss of
discharge), and the impacts were analysed spatially by
Rutherfurd (2000).
Although such comprehensive studies consider
channel adjustments over longer periods of time, a
long term perspective has the advantage that humaninduced
channel adjustment can be placed in the context
of long term natural/secular change over periods as long
as 4000 years (Gregory, 1995). In lowland temperate
environments relatively stable multiple channel systems
(anastomosing) were more common in the late Holocene
(10,000–2000 years B. P.) than today (Brown, 1995).
The natural state of lowland rivers in much of northwest
Europe was multi—rather than single thread, braided
anastomosing or anabranching (Brown, 2002). A further
benefit of a long term perspective is use of a geomorphological
hazards approach to provide a basis for understanding
adjustments of the river channel; types and
degrees of channel adjustments can be analysed according
to the degree of risk or geomorphological hazard,
characterised by Schumm (1988) as either abrupt change
Table 4
How much change occurs of river channels?
Cause of change Range of
channel
change
ratios
Number
of
studies
used
Results Major reasons for variations
River regulation
by reservoir and dam construction,
weirs, navigation
0.09–3.0 77 73% of
studies
show
reduction
Response varies with place and river characteristics, depending on factors
including geomorphological context, type of regulation, sediment transport
post-impoundment, sediment availability, and specific stream power. Bench
development may occur where channels contract, planform may change and
sinuosity decrease. Reservoirs affect tributary channels through higher base
levels
Urban development 0.15–
N10.0
46 72% of
studies
show
increase
Channel capacities and widths may increase with associated changes of pool
riffle spacing. Bed lowering may occur, entrenchment and gullying may be
related to boundary disturbance and road crossings; and instability affected
by erodibility, riparian conditions
Channelization and reach changes
including river training, mineral
extraction, dam removal, removal
of woody debris
0.23–
N12.0
65 52% of
studies
show
increase
Braided patterns often now less common due to changed flood discharges
and sediment supply; after channelization increases in width may be
followed by aggradation; stability at bridges, roads and culverts depends
upon erodibility, riparian and hydraulic conditions; mining effects may
persist due to sediment released as slugs; unstable reaches may be associated
with tributary confluences; Large Woody Debris (LWD) can occupy 2% of
stream bed but account for 50% of flow resistance; more data required on
effects of dam removal
Catchment land use changes 0.05–
15.0
66 61% of
studies
show
increase
Changes may include channel pattern, and multiple often changed to single
thread channels; changes affected by proximity to thresholds, nature of
coupling through different parts of the catchment, because sediment moved
may be stored and not coupled to rest of basin, and some catchments more
resistant to change than others; streams may become entrenched; some
basins show disproportionate geomorphological change for small
hydrological change due to hypersensitivity; incidence of floods may
trigger changes
Water transfers 0.95–
N
9.0
11 82% of
studies
show
increase
Increases where water transferred in; if water extracted response may be
similar to dams
Channel change ratio equal to the capacity, width or depth after change divided by that before. Developed from Brookes and Gregory (1988), Gregory
(1995, 1987b), Downs and Gregory (2004) with additional data.
179K.J. Gregory / Geomorphology 79 (2006) 172–191
that produces a catastrophic event, progressive change
that leads to abrupt change, or progressive change that
has slow but progressive results. Thus, the adjustment
process can be placed in the long term context of the
complex of risks and hazards that exists in the fluvial
system, reflecting relative stability of the river channel
(Downs and Gregory, 2004). For example, by characterising
channel segments in a system according to the
associated stream channel hazards it is possible to indicate
the most significant hazards that require management and
thence to suggest management options, as demonstrated
for semiarid systems in Fountain Hills, Arizona (Chin and
Gregory, 2005).
2.3. Prediction for channel management
The outcome of numerous studies of river channel
adjustment has been that the magnitude of river channel
change, described by terms including enlargement,
shrinkage and metamorphosis, has become broadly
known, but it has not always been possible to predict
what will happen in a particular location (e.g. Table 4),
because of complex response and contingency. Complex
response (Schumm, 1979) is the variety of changes which
may occur after crossing a geomorphic threshold, whereas
historical contingency means that the state of systems or
environment is at least partially dependent on one or more
previous states or events in the past (Phillips, 2001). The
state of a given feature, being uniquely dependent upon
local history or a specific past event, signifies inheritance
and conditionality; therefore reasons for uncertainty
include scale, location, convergence, divergence, singularity,
sensitivity and complexity (Schumm, 1985). Cases
of hypersensitivity (Brown and Quine, 1999), where a
large amount of change results from a small hydrological
change, and of undersensitivity (see Fig. 1C) relate to the
proximity at particular locations to threshold conditions.
This lack of consistent channel response and the incidence
of multiple modes of channel adjustment are demonstrated
where channel scour occurs for about 60 km
downstream of Livingston dam in Texas (Phillips et al.,
2005).
A more holistic view of changing channels can be
developed by focusing upon the relationship between
channel cross-section, planform and channel extent; one
way of achieving this has been by examining the spatial
pattern of channel adjustments. In the Monk's Brook,
Hampshire, UK, the distribution of channel change
downstream of urbanisation was mapped by using sections
of channel of similar character (Gregory et al., 1992). Once
the complete drainage network was delimited, it was
subdivided into segments with similar characteristics by
general procedures for stream reconnaissance (Gregory and
Chin, 2002). This involves either auditing the characteristics
of the channels (Kellerhals et al., 1976; Thorne,
1998), so that at a subsequent second stage the audited
information can be used to determine categories, or by
delimiting the extent of a particular channel type (e.g.
Table 5
Examples of classification of stream channels useful in relation to the adjustment of river channels (developed from Gregory and Chin, 2002)
Basis of classification Example of specific studies which are based on: Source
Comprehensive, descriptive Channel characteristics: Rosgen, 1994; Thorne,
1998
Specific characteristics: often based on
downstream sequence
Frissell et al., 1986;
Bisson and
Montgomery, 1996
Quantitative reconnaissance: Gordon et al. (1992)
Adjustment Stability: Simon, 1989; Johnson et
al., 1999; Bledsoe and
Watson, 2001
Comparison of stability indicators:
including distinguishing between
stable and degrading sites
Doyle et al. (2000)
Thresholds: Olsen et al. (1998)
Potential River recovery potential: Fryirs and Brierley
(2000)
Restabilizing potential: developed for stream
channels in urban watersheds
Henshaw and Booth
(2000)
Disturbance Susceptibility to disturbance: designed for UK
streams
NRA (1990)
Combined Stability and stream classification: Myers and Swanson
(1996)
180 K.J. Gregory / Geomorphology 79 (2006) 172–191
Gregory et al., 1992). Segments of stream channels may
then be classified in at least 5 ways (Table 5): comprehensive
methods that describe the characteristics of
segments; those that focus upon adjustment, often
expressed in terms of stable or unstable segments or
proximity to thresholds; approaches based upon potential
for recovery (Fryirs and Brierley, 2000) or rehabilitation
(Henshaw and Booth, 2000); those that consider degrees of
disturbance giving geomorphological sensitivity (e.g.
NRA, 1990); and those that combine some of these
attributes. Such methods (Table 5) can be combined with
stream classification (Myers and Swanson, 1996), often
requiring subsequent survey andresearchto follow up rapid
methods of assessment (Gregory, 2002b) to decide which
management strategies might be used most effectively.
Rapidmethodsthat havebeensuggestedincludequalitative
reconnaissance surveys (Gordon et al., 1992), network
characterisation for channel segments related to catchmentbased
plans for stormwater management (Gregory, 2002b),
and a geomorphic approach to the identification of river
recovery potential (Fryirs and Brierley, 2000).
Once segments have been identified, they can be
described in terms of hazards affecting the stream channels.
Hazards have usually been associated with natural events
(Alexander, 1992) and geomorphic hazards, generally
regarded as landscape changes that affect human systems
(Gares et al., 1994), have been identified in relation to the
fluvial system (Schumm, 1988, 1994). Although not
specifically applied to urban fluvial systems it is the
hazards associated with urban stream channels that have
necessitated a range of management responses in the
context of proposals for the restoration of stream channels.
Hazard, in the sense used by Schumm (1994), is a potential
danger or risk and is comparable to geomorphic hazards
(Schumm, 1988) where landform changes, natural or
otherwise, may adversely affect the geomorphic stability of
a place. The assessment of flood hazard for land use
planning has been described in Arizona (Rhoads, 1986) but
hazards have not previously been associated with adjusting
channel segments in an urban area as exemplified in
Fountain Hills, Arizona (Chin and Gregory, 2005).
More complete answers to the questions posed in Fig. 2
can be provided for ‘what’ and ‘how’ questions, but less
readily for the ‘when,’ ‘where’ and ‘why’ questions.
Answers to questions about the human role in changing
river channels in specific places are still required to
understand why change occurred and also to assist in the
management of river channels. As river channel management
has progressed from hard to soft engineering
methods and from a design with nature to a management
with nature approach (Downs and Gregory, 2004,
especially Table 11.1) what has been described as a
“paradigm lock” can occur because scientists do not grasp
what managers require, and managers and stakeholders do
not appreciate the scientific alternatives available (Bonell
and Askew, 2000; Endreny, 2001; Gregory, 2004a).
Knowledge of changing river channels can assist in
relaxing the paradigm lock existing between the managers/stakeholders
and the researchers analysing change
in river channels. Challenges, therefore, remain for further
assessments of changes in river channels to be related to
the management of river channels.
3. Challenges for understanding river channel
change and applying the results
3.1. Prediction of channel change
Applying knowledge of change in river channels to
management requires a greater appreciation of the answers
to the questions of when, where and why (Fig. 2); in
securing that appreciation at least five challenges have to
be addressed. The first challenge is the prediction of the
nature and amount of change likely at a particular location.
This is not easily ascertained as can be shown by a number
of examples. Along the Kansas River some reaches,
inactive during the late 1800s and early 1900s, became
active after 1960, but for other reaches the reverse was
true, and overall only 53% of the river had been actively
eroding in the past 125 years (Burke, 1984). Degrees of
instability along a channel were demonstrated by Graf
(1984), who subsequently showed that locational probability
could indicate the most probable previous location
and configuration of the channel for the Salt River Arizona
(Graf, 2000). Arid and humid systems exhibit contrasting
responses, albeit perhaps at the end of a spectrum of
change. This may be because the effects of sediment
storage and infrequent large events lead to episodic
reaction in semiarid environments as shown for channels
in New South Wales (Nanson and Erskine, 1988). Rivers
in drylands are characterised by extreme variability of
flow with long periods of little or noflow interspersed with
occasional and sometimes extreme floods (Tooth and
Nanson, 2000) and, thus, exhibit both equilibrium and
nonequilibrium conditions. This is highlightedin Fountain
Hills, Arizona where the relationship between channel
capacity and drainage area for semiarid channels contrasts
with such relationships for humid areas (Chin and
Gregory, 2001, 2005). In some semiarid areas the
occasional large event can erode channels, so that channel
capacities are greatly increased, whereas the equivalent
large events in humid areas simply exceed the channel
capacity and induce flow over the flood plain. Given the
wide range of responses in different environments it is,
181K.J. Gregory / Geomorphology 79 (2006) 172–191
therefore, necessary to reduce uncertainty, as first
emphasised by Burkham (1981) in the context of the
location of channel change. One way of achieving this is
by an adaptive modelling process (Wilcock et al., 2002).
3.2. Feedback effects
A second challenge is that feedback effects, incorporated
within the relationship between changes at channel,
reach and network scales, can have considerable
implications, especially because channel changes now
evident may have occurred, or have been initiated, under
different environmental conditions. For example, the
incidence of large channel forming events (e.g. Erskine,
1999; Rutherfurd, 2000) and geomorphically effective
floods (Newson and Macklin, 1990) in relation to
vertical instability in river channels can trigger a
feedback mechanism that requires an understanding on
how the Quaternary climatic history and lithology
establish particular thresholds. The incidence of channel
changes may not depend solely upon the magnitude of
hydrological events, because coupling throughout the
system operates at time scales ranging from the
individual event with a return period of decades to
centuries for downstream coupling (Harvey, 2002).
More specifically, the identification of the connectivity
Fig. 3. Contemporary perspectives relevant to human impact on river channels. A1 shows a view of the drainage basin developed from Schumm
(1977) and related to sediment transfer (Church, 2002) that displays patterns of variation on A2. B1 demonstrates a conceptual view of changing
channels (Fryirs and Brierley, 2000) where the vertical line represents change from an intact to a degraded condition, whereas two alternative
pathways are shown to a restored condition or to a created one. Steps in measuring river condition for B1 are shown in B2 (after Fryirs and Brierley,
2000).
182 K.J. Gregory / Geomorphology 79 (2006) 172–191
of coarse sediments in down river channel systems is
important for understanding the mechanisms and
propagation of channel change (Hooke, 2003). More
extensively, buffers, barriers and blankets may account
for delays in the adjustment process and have to be seen
as creating critical threshold conditions (Fryirs et al., in
press). One further feedback effect is the ability of a
changed channel to accommodate discharges. Downstream
of an urban area a channel may increase in
capacity but this increased capacity in turn can
accommodate larger flows and so may reduce the incidence
of flooding, a consequence of enlarged channels
that results from incision in west central Wisconsin
(Faulkner, 1998). Furthermore, if increased delivery of
sediment is involved, consequent aggradation and
reduced capacities may lower the ability of a channel
to accommodate flood discharges, as suggested for the
Kungai Selang in Malaysia (Brookes and Gregory,
1988).
3.3. Global change
Third, consideration of global climate change is
imperative when considering the sensitivity and response
of channels to threshold conditions. Although
comparatively few studies have explored the ways in
which channel change may be affected by future global
climate change, studies of past periods have shown how
alterations of the incidence of hydrological events have
disrupted the pattern of coupling in the fluvial system
(Gregory et al., 2006a,b,c). Relatively little of the
international global change science initiative has made
appropriate use of experience gained from past
environmental changes (Baker, 1995; Alverson et al.,
2003; Oldfield, 2003) although sustainability in either
weaker or stronger variants (Williams and Millington,
2004) can be informed by a longer term understanding
of environmental change. Despite progress made in
palaeohydrology (Gregory, 1983) and other palaeoenvironmental
research, investigations of climate change and
associated global change have not fully explored specific
impacts on the hydrological and channel systems,
although these effectively demonstrate the major impacts
of global change. An important consequence of change in
hydrological and channel systems is that the pattern of
thresholds may alter so that the catchment picture with its
various manifestations (Fig. 3A) may vary quite significantly
(Church, 2002). As basin changes affect land
cover in a river basin, then, a key question is how such
changes either amplify or reduce the climate signal
recorded in fluvial sediments (Macklin et al., 2006). The
paradigm lock (Endreny, 2001) disparity described above
requires links to be forged between the reductionist
research of subdisciplines and practice. For example, in
the context of research on dam removal, it has been
argued (Graf, 2003) that adaptive science needs to
identify significant questions, seek to answer them, and
then, in consultation with managers, redefine the questions.
Such adaptive science could benefit understanding
of other aspects of river channel change.
3.4. Incorporating geomorphological design
Fourth, to achieve an effective link between research
and practice, channel design should be an integral part of
restoration procedures. The concept of designing with
nature has roots traceable in Europe to the mid 19th
Century (Petts et al., 2000); it was originally adopted by
landscape architects (e.g. McHarg, 1969, 1992) and
utilised by ecologists as a method whereby ecology could
inform the planning process. It progressed to the appreciation
that, although a general ecological foundation was
influential, more input from hydrology, geomorphology,
and other branches of physical geography and environmental
sciences was needed (McHarg, 1996). As river
landscapes rather than the channel alone are increasingly
visualised as the basis for restoration and softer management
techniques, this effectively requires concern with
design aspects of the entire river landscape. Geomorphologists,
physical geographers or environmental scientists,
involved in such design procedures, do not require an
eclectic super-scientist approach, but rather participation
in a multidisciplinary team as one member who has the
advantage of knowledge of the evolution of river channel
systems and river landscapes (Gregory, 2004b). Such
members can facilitate the improved application of
hydrology, geomorphology and biology to effective management
of rivers (Calow and Petts, 1992), possibly
including the concept of patchworks (Bravard and
Gilvear, 1996) or the use of channel types (Table 5).
Such knowledge of the evolution of systems is necessary
for implementing the idea of working with nature which
was advocated for river regulation (Winkley, 1972). This
could remedy the absence of geomorphology in design
considerations, as highlighted by McHarg (1996). It has
been argued that the growth of geomorphology, as a
practical profession, requires geomorphologists to devote
effort to developing and refining a design science to
support the profession (Rhoads and Thorn, 1996 p.135).
Impressive examples now exist of ways in which geomorphologists
have contributed to the design of restoration
projects (e.g. Brookes et al., 2004). Involvement of geomorphological
approaches is even more vital as awareness
of the need for an alternative to hard engineering
183K.J. Gregory / Geomorphology 79 (2006) 172–191
produced a general movement towards a softer management
approach which included restoring the landscape to
a more ‘natural’ character. A design science could provide
the basis for a professionalization of geomorphology that
codifies a body of information, tools and skills for
licensing or certification of programs (Rhoads and Thorn,
1996), and defines alternative strategies for management
at the reach and watershed scales (Brookes, 1995).
A suite of methods for incorporating geomorphological
information into existing practices of river management
embodies the basic notion that geomorphology has
contributions to make across the broad sector of river
management (Sear and Newson, 2003; Sear and Arnell,
2006) and some specific design areas have already been
identified (Newson, 1995). Geomorphological techniques
could be developed to complement the way in which a
method for re-establishing biological components of a
river corridor landscape as proposed for the Salt River,
Arizona, based on ecosystem modelling (Cook, 1991).
The impact and legacy of freshwater mills on English
Table 6
Elements of design for river channel landscapes (developed from Gregory, 2004a and tables in Gregory, 2003; Downs and Gregory, 2004)
Major design stage Requirement involved
Preliminary stage The basis for an approach
• Importance of place — construct strategy with awareness of the spatial environmental context
• Implications of scale — Use catchment-scale integrated basin planning with a holistic approach to channel and flood
management
• Situation in time — refer to the temporal position in the sequence of channel development, with any detectable phases in
the palaeohydrology or sediment budget record (Table 1, Fig. 1)
• Cultural context — cultural differences between countries and regions may require differential responses to river channel
management challenges
• Political framework — including requirements for legal implementation ensuring that institutional organization and
structures are sufficiently flexible
Environmental assessment
• Collect historical data on floods, flood hazard and flood mitigation measures, channel behaviour and channel
adjustments
• Consider the period of records used as the basis for earlier channel management decisions
• Review causes of possible change (Table 3) and potential effects (Table 4)
• Take into account high spatial and temporal variability of floods and flood impacts, and their feedback effects
• Select appropriate time scale, augmenting the continuous record as necessary
Outline planning
• Use integrated, basin wide planning, and a holistic approach for channel management
• Use any detectable phases in the palaeohydrology or sediment budget record to set the management into a temporal
pattern
Implementation Reviewing alternatives
• Utilise a basin framework to identify homogeneous reaches requiring similar management activity, reaches of channel
that are unstable/sensitive, as a result of mitigation or management measures or impact of human activity, including those that
may become sensitive in the future (Table 5)
• Set the pattern of sensitive reaches in a dynamic basin context by taking account of changes in sediment history
including phases of storage and exhaustion and past river channel adjustments
• Use environmental condition of reaches to select approaches and to identify assessment techniques — based on
principles of preservation and natural recovery, restoring flow and sediment transport, prompted recovery, morphological
reconstruction, and instability management
• Identify hazards created by erosion and sedimentation together with those of flood discharges, with structures designed
for high sediment loads
• Adopt non-structural and do nothing approaches wherever possible, using sustainable procedures that have least
damaging environmental impacts
• Work with nature and not against it, emulating nature in river designs using knowledge of past and present to determine
what is ‘natural’; restore environmental (habitat) heterogeneity but let the river do the work
• When restoring channels give careful consideration to:
Is restoration feasible for the particular channel?
Is restoration to be to a more natural state or to some specific prior condition and if the latter what is the basis for the
decision?
Does the restored state present the most stable channel which will avoid impacts downstream or upstream?
Consider ‘natural’ in any area as a social construct which must be negotiated with the local community giving
opportunity for education of that community in relation to palaeohydrology
Implementation Reviewing alternatives
184 K.J. Gregory / Geomorphology 79 (2006) 172–191
rivers needs to be considered in restoration (Downward
and Skinner, 2005) exemplifying how knowledge of
channel change can be usefully employed. This study
shows that where mill structures have been maintained,
geomorphological stability can be sustained, but where
maintenance has not occurred, then failure of mill
structures can lead to extensive channel instability. More
extensively, the removal of dams (Graf, 2003; Heinz
Center, 2002) introduces new aspects of channel design
and restoration. Furthermore, the method of Channel
Migration Zoning used in the US to guide planners and
river managers (Rapp and Abbe, 2003), is similar to
locational probability maps (Graf, 2000). Other examples
using a geomorphological approach are exemplified by
river recovery potential (Fryirs and Brierley, 2000),
visualised in the context of the assessment of geomorphic
river condition (Fig. 3B). This enables the geomorphologically
interpreted pattern of recovery to be applied to a
specific catchment as the basis for proposing the most
expedient restoration course for particular reaches. This
has also been developed in the light of barriers, buffers,
and blankets which are discontinuity phenomena acting as
constraints upon river recovery potential after disturbance
events of different magnitude and frequency demonstrate
(dis)connectivity in a catchment (Fryirs et al., in press).
A more geomorphological contribution to the design
of river landscapes, incorporating knowledge of the
adjustments of river channels, could follow ‘principles’ of
river channel management (Gregory, 2003). By collapsing
the comparatively few explicit statements of
principles available to guide river managers, it is possible
to suggest how these principles (Downs and Gregory,
2004) might be modified in the light of research on river
channel change to yield basic principles (Table 6) that can
be considered in river channel design (Gregory, 2004b).
3.5. The cultural dimension
Fifth, greater awareness is needed of the impact of
different cultures as a basis for understanding constraints
imposed by legislative frameworks as well as by public
attitudes. Culture is not automatically considered by
geomorphologists although a more cultural physical
geography has been reviewed (Gregory, 2000). Just as a
more society-oriented climatology or cultural climatology
can be envisioned (Thornes and McGregor, 2003) so
we can now contemplate a cultural geomorphology. This
was implicitly embodied in impressive contributions by
Yi Fu Tuan, including views of physical environment in
Topophilia (Tuan, 1974) and personal experience of
Space and Place (Tuan, 1977). Is it now timely for a
‘cultural turn’ that affected human sciences (Jackson,
2003) including human geography (Johnston, 1997) to
be embraced by geomorphology? Implications of
different conceptions of nature for progress towards a
more integrated geography (Urban and Rhoads, 2003)
raise the possibility of links with human geography and
social scientists which, from a human geography viewpoint,
have appeared as entanglements of nature and
culture (Harrison et al., 2004). A trend in this direction
Table 6 (continued)
Major design stage Requirement involved
• Ensure that the scheme implemented is as sustainable as possible and capable of adaptive modification
• Rationalize risk to support decision-making and assess the risks involved
• Management with Stakeholders— including formulation of shared visions, and stakeholder education
• Set priorities in relation to competing claims, statutory obligations
• Employ a detailed appraisal process, consult widely, considering all the environmental issues at the range of
appropriate scales alongside the engineering and economic objectives
Effecting the design Catchment scale approach to design with nature (see Gregory, 2004b; Downs and Gregory, 2004) including:
1. catchment and corridor policies
2. methods for improving network connectivity
3. in-stream measures
4. channel reconstruction
5. methods for reinforcing the channel perimeter
Post project
consideration stage
Keep areas under review by Adaptive ecosystem management including
• Post-project appraisal so that knowledge about impacts of river management and significance of river channel change
continues to grow
• Incorporating Future Conditions — including managing natural recovery and created environments and developing
improved predicted models
• Coping with Uncertainties — requiring adaptive management and education of river managers
• Ensure continuing proactive involvement of the range of management bodies
Table 6 (continued)
185K.J. Gregory / Geomorphology 79 (2006) 172–191
does not detract from the existing prospectus of investigations
of form, process and change in geomorphology,
but rather follows from them. For example, it has been
contended that ‘where cities once exploited, abused and
then ignored rivers in their midst, they are now coming to
recognise, restore and appreciate them’ (Bolling, 1994,
p. 207).Thus, with knowledge of processes and changes
of river channels it seems expedient to consider cultural
reactions and perceptions.
A general definition of culture as a particular form or
stage of civilisation includes the pattern of human
knowledge, belief, and behaviour embracing language,
ideas, beliefs, customs, codes, institutions, tools, techniques
and works of art, and so should be reflected in
legislation and in ethical values. Thus, what is called
nature in one culture at one time may be viewed very
differently in a culture affected by different political,
historical, and social factors (Palmer, 2003, p. 33).
Variations from one culture to another may arise in part
as a result of different environments, including rivers, so
it is to be expected that, at a particular time and place,
culture will have an effect upon the way in which river
management is perceived. One must understand public
attitudes to rivers in the past and present to fully
appreciate the nature of river management in different
cultures. Moreover, it is also increasingly important to
acknowledge cultural differences in relation to future
plans. It is therefore not realistic to prescribe a universal
approach to managing rivers but only one which reflects
the influence of the particular cultural overlay.
Therefore, geomorphologists can now raise awareness
of such cultural distinctions and consider them when
constructing recommendations. Just as it is necessary to
understand the character of past cultures that, for example,
have led to the five periods of engineering impacts on the
Rhine River channels and flood plain (Herget et al., 2005),
so it is important to include such awareness in relation to
present and future management. Provisionally some of the
different strands that need to be considered are suggested
in Table 7, and provide a filter to be applied when
contemplating the aspects of channel design outlined in
Table 6. Holism has been thought of as requiring a more
basin-based and temporal view (e.g. Gregory and Downs,
in press) and it also requires awareness of legislative
frameworks, ethical approaches and public attitudes that
are integral to culture.
Although benefits should arise from greater understanding
of how the perception of the human role in
changing river channels varies with space as well as
over time, this is an area where relatively little research
and explicit review has occurred. The cultural associations
of river landscapes (Table 7) have been explored
Table 7
Cultural issues for consideration prior to implementing river channel management, for example employing the context suggested in Table 6
Cultural strand Illustrative implications/questions
Knowledge of river channels and their perception Assumptions about rivers and their mechanics
Attitude to rivers: water, sediment, biota, channel, floodplain, riverscape
Are rivers revered? Associated with religion, mythology, customs, beliefs?
Language of rivers
How are rivers portrayed in literature, visual arts, the media?
Presentation in terms of threats, risks?
Valuation of rivers Past and present valuation of rivers, commercially and aesthetically
Economic value, expenditure on maintenance and management
Pollution control
Risk tolerant and allowing for continuing change
Attitude to the management of river channels Non interference or structural solutions or non-structural options
Upstream and downstream effects and spatial context considered?
Understanding that rivers have a history
Individual decision makers or public involvement and shared vision?
Embrace post-project appraisal and adaptive management
Expression of river aesthetics in relation to environment Leave undisturbed
Interpretation of ‘design with nature’
Restore to ‘garden’ character or to perceived natural condition
Involvement of ethical considerations as unwritten codes to
guide action
Human ecology viewpoint?
Shallow or deep ecology perspective?
Conservation and sustainability pre-eminent?
Legislation for rivers Codes of practice and laws; institutions with responsibility for rivers
Local, regional, national or international controls upon decision-making or some
amalgam of these?
How political systems react to, and reflect the above issues
186 K.J. Gregory / Geomorphology 79 (2006) 172–191
(Penning-Rowsell and Burgess, 1997) leading to the
conclusion that the river landscape will increase in
importance as the emphasis in river and catchment
management moves away from reliance on a land use
planning or an engineering approach towards enhancing
what the general public finds important and valuable in
their local river scene. As public opinion is increasingly
considered in river management (e.g. House and
Fordham, 1997), public perception of rivers can be
influential. Some specific examples are known of
differences between geomorphological knowledge and
local perception that may condition decision-making
(e.g. Brizga and Finlayson, 1994; Finlayson and Brizga,
1995). How well the dynamics of the catchment
hydrosystem are understood and explained the perception
by individuals may affect all forms of local decisionmaking
(Downs and Gregory, 2004). Along the Missouri
River in Montana, for example, landowners believe that
the operation of the Fort Peck dam has initiated bank
erosion, whereas geomorphological evaluation indicates
that bed degradation and bank erosion have declined
since construction of the dam (Darby and Thorne, 2000).
On the Herbert River, Queensland, the widespread
perception of an aggrading river since European
settlement is not supported by historical accounts, by
gauging station data since 1940, nor by cross-sections
compared since 1968 (Ladson and Tilleard, 1999).
Anecdotal observation suggests that in some countries
river channels are treated very sensitively, in some they
are landscaped and almost cosmetically managed, and in
some they are ignored and serve as dumps for refuse.
Arguably, hard and soft engineering are aspects of the
cultural perception of what is acceptable and desirable,
and differences also occur in reactions to adjustment and
to management in different cultures. Such cultural effects
are superimposed upon the differences in river channel
change that exist between major world areas. Thus it has
been questioned whether humid temperate rivers in old
and new worlds responded differently to the clearance of
riparian vegetation and woody debris: dramatic river
metamorphosis in southeastern Australia arose because
geomorphic thresholds were breached in a way that
differed from that experienced in Old World landscapes
(Brierley et al., 2005). In Fountain Hills Arizona a
preliminary questionnaire of local perceptions revealed
that the majority favoured maintaining channels with an
appearance perceived to be near-natural, although views
were quite divergent. Such views should be considered
when deciding how the channels should be maintained,
managed, and restored (Chin and Gregory, 2005).
Cultural tradition should be included when linking
human actions, the changing river health, and plans for
stream rehabilitation (Booth et al., 2004). Attitudes to
wood in rivers can vary considerably (Gregory and Davis,
1993; Gregory, 2002a) and the cultural setting (Piegay
et al., 2005) can influence how management of wood
relates to changes of the river channel.
4. Conclusion
The human role in changing river channels has been
exercised for more than 4000 years. Only since 1956 has
this topic been addressed in widespread explicit scientific
investigations. In that half century, answers have been
obtained for a number of questions, including what
changes occur in rivers in response to human impacts and
how these changes occur, but other questions, especially
concerning when, where, and why the changes occur have
proved more difficult to answer. The extrinsic and
intrinsic human roles affecting channels have been
thought of as the human role in changing river channels
but they have now been complemented by a more
deliberate human role. This more direct impact embraces
sustainable design of channels, which is conceived to
acknowledge local cultural attitudes (Table 7) to channels,
to channel change, and to channel management. To guide
progress, and also to encapsulate the achievements of the
past fifty years, it is now possible to provisionally suggest
how knowledge of the human role in changing river
channels might positively constrain the management of
river channels (Table 6). It is only by refinement of such
statements (Table 6) that we can progress the applications
of research, seeking to reduce the paradigm lock between
research and practice because the human role in changing
river channels is pertinent to both.
Acknowledgement
The helpful comments by W. Andrew Marcus on the
draft text are gratefully acknowledged and the support of
a NGS grant.
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