The ‘wet–dry’ in the wet–dry tropics drives river ecosystem structure and processes in northern Australia

Abstract

1. Northern Australia is characterised by a tropical wet–dry climate that regulates the distinctive character of river flow regimes across the region. There is marked hydrological seasonality, with most flow occurring over only a few months of the year during the wet season. Flow is also characterised by high variability between years, and in the degree of flow cessation, or intermittency, over the dry season.
2. At present, the relatively low human population density and demand for water in the region means that most rivers have largely unmodified flow regimes. These rivers therefore provide a good opportunity to understand the role of natural flow variability in river ecosystem structure and processes.
3. This review describes the major flow regime classes characterising northern Australian rivers, from perennial to seasonally intermittent to extremely intermittent, and how these regimes give rise to marked differences in the ecological character of these tropical rivers, particularly their floodplains.
4. We describe the key features of these flow regimes, namely the wet and dry seasons and the transitions between these seasons, and how they regulate the biophysical heterogeneity, primary productivity and movement of biota in Australia’s wet–dry tropical rivers.
5. We develop a conceptual model that predicts the likely hydrological and ecological consequences of future increases in water abstraction (e.g. for agriculture), and suggest how such impacts can be managed so that the distinctive ecological character of these rivers is maintained.

Full text

SPECIAL REVIEW
The ‘wet–dry’ in the wet–dry tropics drives river
ecosystem structure and processes in northern Australia
D.M.WARFE*,N.E.PETTIT
†
,P.M.DAVIES
†
,B.J.PUSEY
‡
,S.K.HAMILTON
§
,M.J.KENNARD
‡
,
S. A. TOWNSEND*, P. BAYLISS
–
,D.P.WARD
‡
,M.M.DOUGLAS*,M.A.BURFORD
‡
,M.FINN**,
S. E. BUNN
‡
ANDI.A.HALLIDAY
††
*Research Institute for the Environment and Livelihoods, Charles Darwin University, Darwin, NT, Australia
†
Centre for Excellence in Natural Resource Management, University of Western Australia, Albany, WA, Australia
‡
Australian Rivers Institute, Griffith University, Nathan, Qld, Australia
§
Kellogg Biological Station and Department of Zoology, Michigan State University, Hickory Corners, MI, U.S.A.
–
CSIRO Marine and Atmospheric Research, Cleveland, Qld, Australia
**CSIRO Sustainable Ecosystems, Winnellie, NT, Australia
††
Sustainable Fisheries Unit, Fisheries and Aquaculture, Department of Employment, Economic Development and
Innovation, Ecosciences Precinct, Brisbane, Qld, Australia
SUMMARY
1. Northern Australia is characterised by a tropical wet–dry climate that regulates the
distinctive character of river flow regimes across the region. There is marked hydrological
seasonality, with most flow occurring over only a few months of the year during the wet
season. Flow is also characterised by high variability between years, and in the degree of
flow cessation, or intermittency, over the dry season.
2. At present, the relatively low human population density and demand for water in the
region means that most rivers have largely unmodified flow regimes. These rivers
therefore provide a good opportunity to understand the role of natural flow variability in
river ecosystem structure and processes.
3. This review describes the major flow regime classes characterising northern Australian
rivers, from perennial to seasonally intermittent to extremely intermittent, and how these
regimes give rise to marked differences in the ecological character of these tropical rivers,
particularly their floodplains.
4. We describe the key features of these flow regimes, namely the wet and dry seasons and
the transitions between these seasons, and how they regulate the biophysical heteroge-
neity, primary productivity and movement of biota in Australia’s wet–dry tropical rivers.
5. We develop a conceptual model that predicts the likely hydrological and ecological
consequences of future increases in water abstraction (e.g. for agriculture), and suggest
how such impacts can be managed so that the distinctive ecological character of these
rivers is maintained.
Keywords: environmental flows, estuaries, flow regime, hydrological connectivity, tropical flood-
plains
Correspondence: D. M. Warfe, Tropical Rivers and Coastal Knowledge, Research Institute for the Environment and Livelihoods,
Charles Darwin University, Darwin, NT 0909, Australia. E-mail: Danielle.Warfe@cdu.edu.au
Freshwater Biology (2011) 56, 2169–2195 doi:10.1111/j.1365-2427.2011.02660.x
 2011 Blackwell Publishing Ltd
2169

Introduction
Natural flow variability is a key part of the physical
template of river systems, shaping biophysical heter-
ogeneity through space and time and leading to a
greater range of habitats (Tockner, Malard & Ward,
2000; Ward & Tockner, 2001; Thoms & Parsons, 2002).
Together, biophysical heterogeneity and temporal
variation in precipitation, flow and flooding operate
as selective pressures on biota and result in a broad
range of life history traits (Bunn & Arthington, 2002;
Lytle & Poff, 2004). Both mechanisms create greater
functional diversity, allowing a greater range of biota
to ‘share’ the same system and providing a greater
scope for ecosystem processes, thereby sustaining
high biodiversity and ecosystem resilience (Poff &
Allan, 1995; Poff et al., 1997; Ward et al., 2001; Naiman
et al., 2008; McCann & Rooney, 2009). However, if
flow variability is extreme, as in extended ‘supra-
seasonal’ droughts or extreme floods, it can have
negative effects on habitat availability, biological
populations and ecosystem processes (Lake, 2003;
Naiman et al., 2008). Maintaining or restoring natural
flow regimes and their natural variability is thus
fundamental to the ecology of river systems (Poff
et al., 1997; Puckridge et al., 1998; Naiman et al., 2008;
Reich et al., 2010).
Tropical rivers (including their floodplains and
estuaries) often have a characteristic and distinctive
seasonal pattern of flow, where most discharge occurs
during summer and results in marked wet and dry
seasons (Lewis, 2008). Rivers in the Australian wet–
dry tropics display this seasonality to a greater degree
than many other tropical regions, with most flow
occurring over a few months because of monsoonal
rainfall, and are also characterised by high interan-
nual variability in flows (Puckridge et al., 1998;
Petheram, McMahon & Peel, 2008). These character-
istics also vary across the region, reflecting the
inherent variability of flow regimes across the Aus-
tralian tropics (Kennard et al., 2010). Strong hydro-
logical seasonality has been proposed as a key factor
determining ecosystem structure and processes in
rivers generally, in both tropical (Junk, Bayley &
Sparks, 1989) and temperate regions (Tockner et al.,
2000), and specifically in the Australian wet–dry
tropics (Douglas, Bunn & Davies, 2005; Hamilton &
Gehrke, 2005). Indeed, the structure and function of
all rivers are regulated by the same fluvial processes,
particularly with regard to hydrological connectivity,
through longitudinal, lateral, vertical and temporal
dimensions (Ward, 1989; Tockner et al., 2000). The
pattern of these fluvial dynamics dictates when and
where hydrological connections occur and thus
defines the flow and flood regimes, and consequently
the ecological character, of different river systems
(Ward & Tockner, 2001; Boulton et al., 2008).
In contrast to the highly populated and⁄or hydro-
logically modified river catchments of some other
tropical regions (McClain, 2008), Australian tropical
rivers remain relatively intact. Northern-draining
Australian catchments cover approximately 17% of
the continent’s land area and generate over 60%
of Australia’s surface-water run-off, but support <2%
of the population (about one person per 2.5 km
2
)
(ABS, 2006; Woinarski et al., 2007). Consequently,
these rivers have undergone little development, drain
relatively undisturbed landscapes and have a rela-
tively unaltered hydrology (Douglas et al., 2005;
Woinarski et al., 2007). These rivers therefore provide
a good opportunity to understand the role of natural
flow variability in determining the ecological struc-
ture and function of tropical rivers in general, and this
improved understanding can inform the development
of regional water resources into the future. If the same
fluvial drivers operate in all river systems, and
Australian tropical rivers can be seen to represent
the relatively unimpacted end of a continuum of
hydrological alteration (Ward & Tockner, 2001;
Boulton et al., 2008), then understanding the role of
flow variability on the ecological structure and func-
tion of these systems provides a potential model for
river restoration and conservation efforts elsewhere.
This review describes the natural flow and flood
regimes of rivers in Australia’s wet–dry tropics,
compares them with rivers in other tropical regions
and highlights the key features of Australia’s wet–dry
flow regimes that result in their distinctive ecological
structure and function. We also describe how the
hydrological regime dictates periods of floodplain
inundation (from protracted to episodic), the variabil-
ity of which can also be a strong determinant of
aquatic ecosystem structure and processes. Evidence
for the importance of these flow features is predom-
inantly based on recent research across the region,
which has been synthesised in a series of workshops,
and hence we draw on a considerable amount of
hitherto unpublished research. We also draw on the
2170 D. M. Warfe et al.
 2011 Blackwell Publishing Ltd, Freshwater Biology, 56, 2169–2195

literature from other tropical systems to develop
hypotheses of flow–ecology relationships where re-
search from northern Australia is lacking. Finally, we
discuss how rivers in Australia’s wet–dry tropics may
be affected by water resource development and the
changing climate, suggesting approaches to support
the planning, use and management of these relatively
unaltered freshwater ecosystems.
Landscape, climate and hydrology
The wet–dry tropics in northern Australia encom-
passes a region of approximately 1.3 million square
kilometres, stretching from Broome in the northwest
of Western Australia, eastwards to the northeast
Queensland coast (Woinarski et al., 2007; Fig. 1). The
region excludes the wet tropics of eastern Australia,
south of Cairns (Fig. 1), which experiences rainfall
throughout the year. It is an ancient, highly weathered
landscape, with soils largely leached of nutrients and
a lack of geological rejuvenation. The dominant
vegetation across the region is extensive grassy
woodland savanna, although other vegetation such
as Spinifex grasslands, heathlands and relict pockets of
tropical rainforest are also present (Woinarski et al.,
2007). While there are some ranges and escarpments
in the Kimberley, Arnhem Land and Cape York
Peninsula (Fig. 1), the region is generally of low
topographical relief (under 400 m altitude). A diver-
sity of freshwater and coastal marine ecosystems is
important in the region, including rivers, mangroves,
salt-marsh flats, floodplain wetland complexes,
ephemeral waterbodies and upland groundwater-fed
wetlands (Pusey & Kennard, 2009). Some of Austra-
lia’s largest and most diverse wetlands occur in
northern Australia, in Kakadu National Park and
Arafura Swamp (both east of Darwin, Northern
Territory), and the Southern Gulf region where
several large rivers merge during major floods and
vast areas are inundated (Pusey & Kennard, 2009).
Darwin is the largest urban centre in the Australian
wet–dry tropics with a population of about 80 000.
The remaining population, of which 40% is indige-
nous, lives in remote towns and communities scat-
tered across the region (Woinarski et al., 2007). The
dominant land use in northern Australia is cattle
grazing, which extends over about two-thirds of the
total land area, followed by traditional indigenous
land uses. Less than 2% of the land area is used for
production forestry, cropping, horticulture or mining,
all of which tend to be localised around permanent
water sources (Woinarski et al., 2007). Given the
relatively low demand for water, few rivers in
northern Australia are impounded: there are 27
storages >0.2 GL in the region, compared with 467
around the rest of the country (Pusey & Kennard,
2009). Prescribed seasonal burning is practised
throughout the region (Russell-Smith, Whitehead &
Cooke, 2009), but cattle grazing of native savanna
vegetation is potentially the major impact on catch-
ment processes through increased erosion, sedimen-
tation, nutrient export to aquatic ecosystems, and the
Timor Sea
Gulf of
Carpentaria
Fitzroy
Ord
Daly
Flinders
Mitchell
Coral
Sea
Western
Australia
Northern
Territory
Queensland
Broome
Darwin
Cairns
20°0′0′′S
10°0′0′′S
10°0′0′′S
20°0′0′′S
Daly
Mitchell
The Kimberley
Arnhem
Land
Cape
York
Peninsula
Fig. 1 Region comprising the wet–dry
tropics of northern Australia. Catchments
drain into the Timor Sea, the Gulf of
Carpentaria or the Coral Sea. Major
townships are identified (black circles), as
are catchments (labelled in white) men-
tioned in the text. The dotted lines are
lines of latitude.
Flow drivers of tropical Australian rivers 2171
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trampling of riparian and floodplain vegetation (Bro-
die & Mitchell, 2005; Pusey & Kennard, 2009).
Typically, more than 90% of annual rainfall in the
Australian wet–dry tropics occurs during the summer
wet season from November to April and is generated
by monsoonal lows and thunderstorms (Petheram
et al., 2008). Rainfall intensity (i.e. volume⁄time) dur-
ing these storms is among the highest recorded in the
world (Jackson, 1988). Climate diagrams illustrate that
while annual rainfall in Darwin is comparable with
locations in other tropical countries (such as Stung
Treng in Cambodia), it occurs as greater monthly
rainfall over a shorter wet season, and there is
comparatively less rainfall over the dry season
months (Fig. 2). Darwin’s rainfall is also greater than
average for Australia’s wet–dry tropics, which is
better represented by Cloncurry, Queensland (Fig. 2).
Rainfall rapidly declines from a maximum of
2000 mm year
)1
near Australia’s northern coast to
300–400 mm year
)1
at the inland boundary of the
region, averaging about 400 km south (Petheram
et al., 2008; Cresswell et al., 2009). Rainfall in the
wet–dry tropics is highly variable both spatially and
from year to year: over the period 1930–2006, the
range of annual flows was 2.5 times greater than the
mean (Cresswell et al. , 2009). High air temperatures
generate high evapotranspiration rates throughout the
year. These range from 700 to 900 mm year
)1
in the
Australia
12r 28' S / 130r 51' E / 30 m.
DARWIN
[61–31] +28.0rC 1490 mm
rC
34.4
19.4
50
40
30
20
10
0
500
300
100
80
60
40
20
0
J A S O N D J F M A M J
Australia
20r 40' S / 140r 30' E / 190 m.
CLONCURRY
[33–60] +25.5rC 457 mm
500
300
100
80
60
40
20
0
rC
37.8
10.6
50
40
30
20
10
0
J A S O N D J F M A M J
Venezuela
8r 7' N / 63r 32‘ W / 60 m.
CIUDAD BOLIVAR
[11–11] +28.2rC 973 mm
500
300
100
80
60
40
20
0
J F M A M J J A S O N D
rC
33.9
22.2
50
40
30
20
10
0
Cambodia
13r 31' N / 105r 58' E / 54 m.
STUNG TRENG
[27–42] +27.0rC 1841 mm
J F M A M J J A S O N D
500
300
100
80
60
40
20
0
rC
35.0
18.9
50
40
30
20
10
0
Fig. 2 Climate diagrams summarising
average monthly rainfall and temperature
in four tropical locations: Darwin, Aus-
tralia (top left); Cloncurry, Australia (top
right); Stung Treng, Cambodia (bottom
left); and Ciudad Bolivar, Venezuela
(bottom right). The solid line represents
rainfall on the right axis, the dashed line
represents temperature on the left axis,
and the shaded area represents the peri-
ods when evaporation exceeds rainfall.
Diagrams sourced from the Worldwide
Bioclimatic Classification System, 1996–
2009, S. Rivas-Martinez & S. Rivas-Saenz,
Phytosociological Research Center, Spain
(http://www.globalbioclimatics.org).
2172 D. M. Warfe et al.
 2011 Blackwell Publishing Ltd, Freshwater Biology, 56, 2169–2195

dry season from 1100 to 1200 mm year
)1
during the
warm wet season, and result in an annual water
deficit across the region except in the very wettest of
years (Cresswell et al., 2009; Fig. 2).
A characteristic of Australia’s tropical wet–dry
rivers is that their higher water temperatures coincide
with higher flows over the wet season, e.g. between 28
and 32 C in the Daly River, Northern Territory
(Townsend, Webster & Schult, 2011), but even higher
depending on timing and location. For example, water
temperatures up to 35 C have been measured in the
Daly River during the early wet season (Townsend
et al., 2011) and can reach 37 C in floodplain water-
holes of the Mitchell River, Queensland (Hamilton,
2010). The occurrence of warm water during the wet
season is similar to some South American floodplain
systems that are inundated during the warmer
months, e.g. the Pantanal wetland of Brazil, but
differs from elsewhere, e.g. the Okavango Delta in
Botswana and Andean tributaries to the Amazon and
Orinoco Rivers, where floodwaters coincide with
cooler water (Montoya et al., 2006; Hamilton et al.,
2007; Lewis, 2008; Hamilton, 2010). Higher water
temperatures during the high-water phase can lead to
greater primary productivity and microbial activity in
tropical systems (Winemiller, 2004; Hamilton, 2010).
For example, northern Australian floodplains that
have protracted inundation periods display marked
increases in macrophyte biomass during the wet
season (Finlayson, 1991; Pettit et al., 2011). However,
thermal optima are narrow for many tropical species
and, as they are living close to their maximum
tolerable limits, biotic diversity can decrease beyond
35 C (Deutsch et al., 2008; Hamilton, 2010). This
suggests that a modest increase in temperature could
produce unexpected changes in biotic composition
and ecosystem processes (Hamilton, 2010).
There are 60 major river catchments within the wet–
dry tropics of northern Australia that drain into the
Gulf of Carpentaria, the Timor Sea or the Coral Sea
(Fig. 1). The majority of rivers in these divisions are
relatively short and drain directly to the coast (Ham-
ilton & Gehrke, 2005; CSIRO, 2009). The geomorpho-
logical character of the region’s rivers varies greatly
but typically they are of low average gradient, have a
low density of streams per unit catchment area, and
have large lowland floodplains comprising up to a
third of the total catchment area (Stein et al., 2009). For
example, rivers draining into the Gulf of Carpentaria
can have floodplains of 20 000 km
2
that comprise over
35% of the total catchment (Pusey & Kennard, 2009).
Floodplain wetlands cover 25% of the entire region
(Pusey & Kennard, 2009) and represent the largest
area of unmodified wetlands in Australia (Woinarski
et al., 2007). The combined river and floodplain matrix
across northern Australia represents one of the last
free-flowing river networks in the world and is
consequently a globally significant asset (ATRG,
2004; Blanch, 2008).
The topographical separation of catchments, espe-
cially in the coastal low-relief terrain bordering the
Gulf of Carpentaria, is often limited and results in
connections between rivers during peak inundation in
the wet season. In contrast, rivers draining into the
Timor Sea are generally deeply incised and well
separated. Given that rainfall is greater nearer the
coast and declines inland, a substantial proportion of
run-off from northern rivers may originate in the
lowlands; floodplain wetlands can thus be inundated
by both overbank flows from upstream and local
sources. A key aspect of floodplains in northern
Australia is the variability in their period of inunda-
tion: some are flooded for months, whereas others are
flooded only episodically for days to weeks.
Most northern Australian rivers show a distinct and
predictable hydrologic seasonality reflecting the wet–
dry climate: high flows occur during the wet season
and low flows, often interrupted by lengthy periods of
zero flow, occur during the dry season (Kennard et al.,
2010). Rivers with intermittent flow are common
across northern Australia, although the degree of
intermittency varies depending on climate, latitude
and underlying geology. The most extreme intermit-
tent rivers, i.e. those with the longest periods of no
flow, occur near the southern boundary of the region
on the edge of the arid zone. Flow may cease for
extended periods of time (over 200 days per year),
with remaining water restricted to a series of discon-
nected in-channel and floodplain waterholes (Douglas
et al., 2005; Kennard et al., 2010). Perennial rivers, such
as the Daly River in the Northern Territory, are
uncommon across tropical Australia and supplied
mainly by groundwater during the dry season (Pethe-
ram et al., 2008).
An ecohydrological regionalisation of Australia’s
rivers has recently been developed to support the
generalisation of flow–ecology relationships and
responses to flow alteration (Kennard et al., 2010).
Flow drivers of tropical Australian rivers 2173
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The majority of rivers in the wet–dry tropics are
within three flow regime types (Table 1; Fig. 3):
stable summer baseflows (perennial rivers), predict-
able summer highly intermittent (intermittent rivers,
the most common type in this region) and extreme
harsh summer intermittent (extremely intermittent
rivers, with often ‘flashy’ and largely unpredictable
flows). Along with the considerable spatial variation
in flow regime (Kennard et al., 2010), and despite the
highly seasonal nature of flow, there can also be great
interannual and apparent decadal variability in wet
season discharge related to cyclonic events and El
Nin
˜
o-Southern Oscillation (ENSO), Interdecadal
Pacific Oscillation (IPO), and Indian Ocean Dipole
(IOD) climatic variation (Hamilton & Gehrke, 2005;
Shi et al., 2008; Hamilton, 2010; Kennard et al., 2010;
Fig. 3).
Key flow features underpinning ecosystem struc-
ture and processes
The characteristic hydrological seasonality has been
proposed as a strong driver of biotic assemblages and
ecological processes in rivers and floodplains of
Australia’s wet–dry tropics (Douglas et al., 2005;
Leigh & Sheldon, 2008). We hypothesise that four
key features of the flow regime underpin the structure
and function of these tropical Australian river sys-
tems: (i) peak wet season flows and their variability,
(ii) the drawdown period of flows and flood residence
times during the transition from the wet to the dry
season, (iii) the low and disconnected flows during
the dry season, and (iv) the initial flushing flows
during the transition from the dry to the wet season
(Fig. 4).
Wet season
Peak flows in the wet season, and their variability
from year to year, determine the structure of channels
and floodplains, regulate primary production on
floodplains and in riparian zones, and provide
hydrological connectivity for transporting nutrients,
sediments and organic matter. They also provide
opportunities for the movement and recruitment of
biota between reaches that are isolated during the dry
season. The wet season is the time in the annual
hydrograph when floodplains are reconnected with
their rivers, an important phase given the relatively
large proportion of catchment area occupied by
floodplains across the region. There is a continuum
in the inundation period, or flood residence time, of
floodplains across the region: some floodplains, such
as those in Kakadu National Park and the Daly River
(Fig. 1), generally flood each year and some areas can
be inundated for up to 6 months at a time (Pettit et al.,
2011). Others, such as the Fitzroy River (Western
Australia) and the Mitchell River (Queensland; Fig. 1),
Table 1 Summary of selected flow char-
acteristics representing ecologically rele-
vant flow regime components (magnitude,
frequency, duration, timing and rate of
change) for each flow regime class
describing rivers across Australia’s wet–
dry tropics, adapted from Kennard et al.
(2010) and Pusey et al., (2009). Relative
differences rather than actual values are
presented, and summer flows coincide
with the wet season
Flow characteristic
Stable
summer
baseflow
Predictable
summer highly
intermittent
Extreme harsh
summer
intermittent
Flow permanence Perennial Highly intermittent Extremely
intermittent
Seasonal timing Summer Summer Summer
Runoff magnitude High Moderate Low
Predictability High Moderate Low
Seasonality Moderate High High
Variability and skewness Low Moderate High
Number zero flows days Low Moderate High
1
st
percentile flood
Magnitude Moderate High High
Frequency Moderate Moderate Moderate
Duration High High Moderate
Rate of rise and fall Low Moderate High
25
th
percentile flow
Magnitude Moderate Low Low
Frequency Low Low Low
Duration High High High
Reversals Moderate Low Low
2174 D. M. Warfe et al.
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have floodplains that may only be inundated for days
to weeks, even in years of high rainfall (Fig. 5). On
floodplains that experience short periods of inunda-
tion lasting from days to weeks, there has been little
development of aquatic plant communities adapted to
long periods of inundation, and aquatic primary
production is largely limited to permanent water-
holes. Primary production of terrestrially-adapted
plant communities can be initially suppressed and
then increase following the recession of floodwaters
Daily runoff (ML day
–1
km
–2
)
0.01
0.1
1
10
50
0.01
0.1
1
10
50
1997/98
1
10
50
1975 1980 1985 1990 1995 2000
1975 1980 1985 1990 1995 2000
1975 1980 1985 1990 1995 2000
0.01
0.1
1
10
25
0.01
0.1
1
10
25
0.01
0.1
1
10
25
Sum
Aut
Win
Spr
1993/94
28/2/1994
7/3/1994
14/3/1994
Sum
Aut
Win
Spr
22/1/1998
29/1/1998
5/2/1998
1997/98
Sum
Aut
Win
Spr
22/1/1998
29/1/1998
5/2/1998
0.1
1
10
100
150
0.01
0.1
1
10
100
150
0.05
0.1
1
10
100
150
Class 3 – Stable summer baseflow
(G8140063 – Douglas River, CA = 834)
Class 10 – Predictable summer highly intermittent
(G9030089 – Waterhouse River, CA = 2753)
Class 12 – Extreme harsh summer intermittent
(G9152081 – Julia Creek, CA = 1081)
Fig. 3 Example hydrographs of daily run-off (ML day
)1
km
2
) for a typical stream gauge in each flow regime class (note different
y-axis scales for each class). Variation in run-off is shown for three scales of temporal resolution: the long-term record, a year and a
3-week period encompassing the flow event with the highest peak magnitude. The number, name and upstream catchment area (km
2
)
of each stream gauge are given in parentheses.
Flow drivers of tropical Australian rivers 2175
 2011 Blackwell Publishing Ltd, Freshwater Biology, 56, 2169–2195

when soils are recharged with nutrients and moisture.
In these latter systems, there is only a relatively short
period available for nutrient transfer and use, aquatic–
terrestrial fluxes in food resources and the active
movement of organisms across the floodplain (Doug-
las et al. , 2005). These floodplains clearly have a
different ecology to those inundated for long periods,
but we are unable to find any literature on equivalent
systems elsewhere that bears on the observations we
report here. The ecology of these systems, particularly
the ability of biota to take advantage of short inun-
dation periods, represents a significant knowledge
gap for northern Australia. Consequently, much of the
ensuing discussion refers to the more studied systems
in the Northern Territory that are characterised by
relatively long inundation periods.
Rapid pulses of wet season discharge can have high
erosional power through the bedrock-constrained
valleys, headwaters and steep gorges that are features
of Australia’s northern escarpment country, resulting
in little material entrapment in these reaches (Brodie
& Mitchell, 2005). A large proportion of the sediment
in Australian rivers, including those in the northern
wet–dry tropics, is derived from channel and gully
erosion (Prosser et al., 2001; Wasson et al., 2010).
Although the escarpments are generally restricted to
Arnhem Land in the Northern Territory and the
Kimberley Plateau in Western Australia (Fig. 1), and
most of the region lies below 400 m altitude, erosional
forces can still be high in low-gradient, sand-bed
rivers (Brooks et al., 2009). For example, since 1990,
more frequent overbank flows have contributed to
increased riverbank erosion and slumping, channel-
widening and sedimentation in the Daly River,
Northern Territory (Wasson et al., 2010). On flood-
plains that are inundated for long periods, rainfall and
overbank flows redistribute and deposit sediments
according to floodplain topography, and as water
velocity dissipates over the extent of the floodplain,
the inundation extent being related to flow magnitude
(Steiger & Gurnell, 2002; Naiman, De
´
camps & McC-
lain, 2005; Wasson et al., 2010). On floodplains with
short inundation periods, floods are rapid and epi-
sodic, occurring as ‘sheet flow’ over an essentially
terrestrial system, but can still exert significant
erosional force and move a considerable amount of
sediment (D. P. Ward, unpubl. data). Variability in
annual rainfall and flood peaks, combined with
landscape topography, can thus dictate the structural
heterogeneity of both channels and floodplains, and
consequently the presence and persistence of aquatic
refugia in channels and floodplain waterholes
May
Month 2005–2006
0
500
1000
1500
2000
2500
3000
3500
4000
August
September
October
November
December
January
February
March
April
June
July
Discharge (m
3
s
–1
)
Dry season:
Low flows and
disconnection
Dry to wet:
Storm events
and flush flows
Wet season:
Peak flows and
interannual
variability
Wet to dry:
Drawdown and
disconnection
Fig. 4 Sample hydrograph from the Daly
River (Northern Territory), over 1 year
from August 2005 to July 2006, illustrating
key flow features of rivers across the
wet–dry tropics of northern Australia.
0
10
20
30
40
50
60
70
80
90
100
3/01/2008
22/02/2008
12/04/2008
1/06/2008
21/07/2008
9/09/2008
29/10/2008
18/12/2008
Date
Area of maximum flood extent (%)
Fig. 5 Percentage of floodplain area on the Daly (black) and
Mitchell (grey) River floodplains, inundated by floodwaters
during 2008 (D. P. Ward, unpubl. data).
2176 D. M. Warfe et al.
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through the dry season (Richards, Brasington &
Hughes, 2002; Bunn et al., 2006).
Wet season peak flows can also structure riparian
vegetation communities and produce distinct lateral
zonation largely controlled by bank height, sediment
deposition and fluvial disturbance (Pettit, Froend &
Davies, 2001; Lamontagne et al., 2005; Petty & Doug-
las, 2010). Attenuation of floods and flood variability,
as has occurred in the lower Ord River (Western
Australia; Fig. 1) because of the dual impoundments
of Lake Kununurra and Lake Argyle, can lead to
reduced structural heterogeneity in the riparian veg-
etation community and narrowing of the riparian
zone (Pettit et al., 2001). Wet season floods also
restructure channels through the provision and trans-
port of wood and other organic matter that contribute
to substratum heterogeneity as well as habitat and
food resources for instream biota (Pusey & Arthing-
ton, 2003). Recent research in the Daly River indicates
that the annual fluvial disturbance of riparian zones
can lead to high turnover of wood deposited in
channels; up to 50% of instream wood can be
translocated over a single wet season, suggesting that
the instream habitat for biota can be highly dynamic
from year to year with interannual flood variability
(N.P. Pettit & D.M. Warfe, unpubl. data). Fish com-
munities associated with wood patches in the Cina-
ruco River (Venezuela) reassemble after wet season
floods in a non-random manner, according to specific
habitat patches regardless of the variability of those
patches (Arrington & Winemiller, 2006). Such fidelity
to habitat patches remains to be investigated in
northern Australian rivers, but early research suggests
that instream wood plays a significant role in struc-
turing fish assemblages (N.E. Pettit, D.M. Warfe, M.J.
Kennard & B.J. Pusey, unpubl. data).
Wet season flows are characterised by low sediment
loads and nutrient concentrations because of the
highly weathered, ancient geological nature of Aus-
tralia’s tropical soils (Moliere et al., 2004; Brodie &
Mitchell, 2005), and rivers across Australia’s wet–dry
tropics are predominantly heterotrophic and nutrient-
limited (Webster et al., 2005; Ganf & Rea, 2007). In
contrast, while typically still nutrient-limited (Burford
et al., 2011), estuaries appear to be more autotrophic
owing to the higher aquatic productivity of mangrove
forests (Alongi, Clough & Robertson, 2005; Burford
et al., 2008a). In Darwin Harbour, nutrients are pre-
dominantly provided by tidal rather than tributary
inputs and primary production is dominated not only
by mangroves but also by benthic algae and phyto-
plankton, albeit to a lesser degree (Burford et al.,
2008a). In the central Gulf of Carpentaria, wet season
nitrogen inputs via river flows do not appear to
contribute much to primary productivity, which seems
instead to be supported by cyanobacterial nitrogen
fixation (Burford, Rothlisberg & Revill, 2009). Never-
theless, riverine nutrient inputs during the wet season,
while low, could still potentially contribute to primary
production closer to the coast (Burford et al., 2011).
Wet season hydrology appears to be a main driver
of productivity on floodplains, and, consequently,
rates of primary production are variable across the
riverine landscape (Davies, Bunn & Hamilton, 2008).
Floodplains with extended periods of inundation can
support substantial aquatic primary production dur-
ing the wet season (Pettit et al., 2011). These floodplain
dynamics are consistent with the Flood Pulse Con-
cept, which proposes that seasonal inundation and
subsequent drainage are the primary drivers of
ecological processes in large floodplain rivers (Junk
et al., 1989; Winemiller, 1996; Tockner et al., 2000).
Estimates of carbon production on the Magela Creek
floodplain in Kakadu National Park (east of Darwin,
Northern Territory) show that primary production
shifts from being predominantly algal-based and
restricted to refugial waterholes during the dry season
to extensive macrophyte production during the wet
season (Pettit et al., 2011). However, despite dominat-
ing wet season primary production and biomass on
the floodplain, macrophytes do not appear to contrib-
ute directly to the aquatic food web. Rather, they
provide a large surface area for the attachment of
epiphytic algae, which support most of the secondary
production on the floodplain (Davies et al., 2008; Pettit
et al., 2011). Similar observations have been made on
the Orinoco floodplain in South America (Hamilton,
Lewis & Sippel, 1992; Lewis et al., 2001). There is also
evidence that algae produced on the floodplain
during the wet season, even on floodplains with short
inundation periods, may subsidise food webs in
upstream reaches via the movement of fish consumers
(T.D. Jardine, B.J. Pusey, S.K. Hamilton, N.E. Pettit &
S.E. Bunn, unpubl. data). Such subsidies may be
important for upstream food webs, as primary pro-
duction in rivers during the wet season is generally
low because of the scouring effect of high flows
(Townsend & Padovan, 2005).
Flow drivers of tropical Australian rivers 2177
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Extensive macrophyte growth can also provide
important habitat for floodplain fauna. Magpie geese
(Anseranas semipalmata Latham) are widespread and
abundant across tropical Australian floodplains, par-
ticularly in Kakadu National Park (Bayliss & Yeo-
mans, 1990; Morton, Brennan & Armstrong, 1990). In
years of early monsoonal rainfall, there is subsequent
high growth of wild rice (Oryza spp.) and water
chestnut [Eleocharis dulcis (Burm.f.) Trin. ex Hensch],
the latter providing nesting sites during the wet
season and abundant food for adults and fledglings
during the drawdown phase, and the former provid-
ing food for newly hatched goslings (Bayliss &
Yeomans, 1990; P. Bayliss, unpubl. data). Hence, there
is a general positive relationship between wet season
rainfall, peak floods and both nesting success and dry
season survival of magpie geese, particularly in years
of early rainfall (Whitehead & Saalfeld, 2000). Magpie
goose populations also appear to exhibit decadal
trends that are tightly coupled with decadal trends in
rainfall and river flows. On average, wetter years are
followed by drier years over an approximate 20-year
period that is mirrored in magpie goose numbers
across the ‘Top End’ of the Northern Territory
(Bayliss, Bartolo & van Dam, 2008).
Peak flows towards the end of the wet season
often extend the persistence of aquatic habitats and
are positively correlated with the abundance of
plotosid and ariid catfish (Madsen & Shine, 2000).
The abundance of catfish is in turn positively
correlated with the body condition and yearling
abundance of their main predator, the aquatic
filesnake (Acrochordus arafurae McDowell) (Madsen
& Shine, 2000). Conversely, the extended inundation
period afforded by late season flooding reduces the
available habitat, and thus the abundance, of the
dusky rat (Rattus colletti Thomas), which results in
poorer body condition and reduced reproduction in
its main predator, the water python (Liasis fuscus
Peters) (Madsen et al., 2006). These unusually strong
relationships illustrate that variability in both the
magnitude and timing of annual peak flows can have
far-reaching effects on the composition of tropical
floodplain communities, well into the following dry
season. These relationships are unlikely to be as
strong or prevalent on floodplains of short inunda-
tion periods.
An important consequence of wet season flows is
that they provide both lateral and longitudinal con-
nectivity throughout the entire drainage system and
the opportunity for aquatic invertebrates, fish and
reptiles to move between reaches to spawn (Douglas
et al., 2005). Preliminary evidence from the Daly River
catchment suggests that the emergence of aquatic
insects peaks during the wet season, as does the input
of terrestrial arthropods into streams (E. A. Garcia &
M. M. Douglas, Charles Darwin University, unpubl.
data). The same pattern has been observed in tropical
rivers in Hong Kong, where the lateral flux of aquatic
and terrestrial insects across the riparian zone also
peaks during the wet season and provides an impor-
tant link between aquatic and terrestrial food webs
(Chan, Zhang & Dudgeon, 2007, 2008). The emergence
of mature macroinvertebrates during the wet season
has been suggested to be an evolutionary response to
flood-induced mortality of large larvae, as well as
providing available habitat for new recruits (Dud-
geon, 2000; Jacobsen et al., 2008).
Of the 90 fish species recorded from freshwaters of
the Daly River, one-third moves between freshwater
and estuarine reaches to spawn, and one-third area
vagrant estuarine species, such as bull sharks (Car-
charhinus leucas Mu
¨
ller & Henle), and can be found
hundreds of kilometres upstream (B.J. Pusey & M.J.
Kennard, unpubl. data). Many of the remaining
species move between different freshwater reaches
during the wet season for spawning. Many of these
fish species are widespread across northern Australia
but, because most rivers are intermittent, their move-
ments generally only occur during wet season flows
when river, floodplain and estuarine reaches are
connected. In the Magela Creek system (northeast of
the Daly River in Kakadu National Park), sooty
grunter (Hephaestus fuliginosus Macleay) move down-
stream from escarpment refugia during the wet
season to spawn (Bishop, Pidgeon & Walden, 1995).
Plotosid catfish move upstream into tributaries (Pu-
sey, Kennard & Arthington, 2004), and juveniles of a
number of species such as freshwater sole (Leptachirus
triramis Randall) and barramundi (Lates calcarifer
Bloch) also use wet season flows to move upstream,
potentially escaping predation pressure in more open
downstream reaches (Pusey et al., 2004; Staunton-
Smith et al., 2004). Like barramundi, the giant fresh-
water prawn, Macrobrachium rosenbergii (De Man), is
catadromous (Short, 2004) and local anecdotal evi-
dence and observations from the Daly River catch-
ment indicate that the juveniles move upstream
2178 D. M. Warfe et al.
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en masse during the late wet season, constituting a
‘sushi train’ along the littoral margins of the main
channel (B. J. Pusey, N. E. Pettit & D. M. Warfe, pers.
obs.).
Peak flows also have effects beyond the freshwater
reaches of tropical Australian rivers and floodplains,
clearly illustrating the importance of longitudinal
connectivity through these systems. Analysis of com-
mercial penaeid prawn and finfish fisheries in north-
ern coastal waters suggests that fisheries catches are
good in years of high wet season inflows (Loneragan
& Bunn, 1999; Robins et al., 2005). The commercial
catch of both banana prawns (Penaeus merguiensis
Fabricius) and barramundi (Lates calcarifer) has been
shown to be positively related to years of high
freshwater inflows (Vance, Staples & Kerr, 1985;
Bayliss et al., 2008; Balston, 2009). Furthermore, the
recruitment of king threadfin salmon (Polydactylus
macrochir Gu
¨
nther) and the recruitment and growth of
barramundi are also positively related to wet season
peak flows, which likely increase hydrological con-
nectivity to estuarine habitats and therefore provide
greater access to estuarine nursery areas (Staunton-
Smith et al., 2004; Robins et al., 2006; Halliday et al.,
2008).
Wet to dry season transition
The transition from the wet to the dry season is the
period when rainfall ceases and flows steadily
decrease, where floodwaters recede on long-inunda-
tion floodplains, and intermittent rivers start to
become hydrologically disconnected. This is also a
key time ecologically: aquatic plant biomass is at its
peak on river floodplains that have been inundated
for some time, aquatic biota respond to receding
waters by moving into refugial reaches to wait out the
coming dry season, and waterbirds congregate in
large numbers as aquatic resources become more
concentrated in diminishing aquatic habitats.
Floodplains with long inundation periods are char-
acterised by extensive macrophyte growth and diver-
sity (Davies et al., 2008; Pettit et al., 2011); biomass
generally peaks in the late wet season when floodwa-
ters begin to recede (Finlayson, 1991). A similar
pattern has been observed on floodplains with short
inundation periods, but macrophyte growth in these
systems is largely restricted to floodplain waterholes
and terrestrial grasses, such as Dicanthium spp.,
dominate the floodplain instead (N. E. Pettit, pers.
obs.). The biomass of attached algae, namely epiphytic
diatoms, is also greatest in the late wet season, before
declining as the water recedes and causes the macro-
phytes on which they grow to senesce and the
available aquatic habitat to contract (Pettit et al.,
2011). Aquatic macroinvertebrates associated with
macrophytes on the floodplain reflect this pattern of
plant production, peaking in abundance during the
late wet season and transition into the dry season
(Marchant, 1982; Outridge, 1988; Douglas & O’Con-
nor, 2003). A conceptual model, developed from data
from South American and African floodplains, sug-
gests there is a peak in microbial activity on the
floodplain during the late wet season as senescent
macrophyte material is consumed (Winemiller, 1996).
Microbial dynamics on northern Australian flood-
plains represent a major knowledge gap for northern
Australia, but it appears that fire, rather than micro-
bial activity, is a major consumer of plant material
during the wet to dry season transition (Pettit et al.,
2011).
The transition between the wet and the dry season
appears to be a key time for the large-scale move-
ments of organisms. Unlike spawning movements
during the wet season, movements during this tran-
sition period are more likely to be associated with
finding refuge as aquatic habitats begin to disconnect
and contract. For example, saltwater crocodiles (Croc-
odylus porosus Schneider) that have moved onto the
floodplains during the wet season return to channel
reaches as floodwaters recede (Jenkins & Forbes,
1985). Melanotaeniid rainbowfish and ambassids,
among other species, moved from floodplain water-
holes to upstream refugial areas towards the end of
the wet season in the Magela Creek system in Kakadu
National Park (Bishop et al., 1995). Recent research on
fish movement in tributaries of the Daly River shows
large numbers of melanotaeniid rainbowfish and
plotosid catfish moving downstream during the tran-
sition from the wet to the dry season, particularly in
intermittent rivers that were in the process of becom-
ing disconnected (D.M. Warfe & N.E. Pettit, unpubl.
data). Fish assemblage structure in the remaining
waterholes of rainforest streams in Queensland has
been shown to be influenced by the magnitude of the
preceding wet season, which in effect ‘sets up’ the
assemblage that will persist through the dry season
(Perna & Pearson, 2008). Evidence from streams in
Flow drivers of tropical Australian rivers 2179
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Australia’s wet tropics indicates that, while macro-
invertebrate structure is not affected by season
(Cheshire, Boyero & Pearson, 2005), the rate of
macroinvertebrate colonisation peaks during the late
wet season flows as early instars disperse to suitable
habitats (Benson & Pearson, 1987). We are analysing
data on benthic macroinvertebrate assemblages to
determine whether these patterns hold in Australia’s
wet–dry tropical rivers.
There are also increased aggregations of waterbirds
on the floodplain during the transition from the wet to
the dry season, as floodwaters contract to waterholes
and aquatic resources become more concentrated.
Darters, cormorants, pelicans and grebes that feed on
aquatic invertebrates and fish become more abundant
around waterholes (Franklin, 2008), and assemblages
can shift as water depths decrease (Chatto, 2000).
Magpie geese move to areas of high resource avail-
ability during this period (Traill, Bradshaw & Brook,
2010), occurring in their highest densities where a
range of macrophyte species required for nesting and
feeding occur (Bayliss & Yeomans, 1990), and can
represent the largest proportion of waterbird biomass
on the floodplain (Pettit et al., 2011).
Dry season
The dry season is a period of limited resources as
aquatic habitats become disconnected and contract
across most of the region. Isolated waterholes on
floodplains and in intermittent rivers become critical
for sustaining aquatic biota and play an important
refugial role during the dry season (Bunn et al., 2006).
There appears to be considerable variation in water
quality and primary production between isolated
waterholes, both on the floodplain and along river
channels (Butler, 2008). Some waterholes are naturally
turbid, and the dominant primary production sup-
porting their aquatic food webs is the narrow band of
benthic algae in the littoral zone (Bunn, Davies &
Winning, 2003) or potentially the phytoplankton in
the water column (Robertson et al., 1999). Other
waterholes can support a very high biomass of
macrophytes and benthic algae and have clear, deep
water (Finlayson, 1991; Butler, 2008; Davies et al.,
2008). In the latter case, it appears that the epiphytic
algae attached to these macrophytes support the
aquatic food web (Hamilton et al., 1992; Douglas et al.,
2005; Pettit et al., 2011). However, on floodplains with
only short inundation periods, terrestrial animals such
as wallabies, horses and cattle have been observed
consuming macrophytes when terrestrial vegetation
becomes scarce at the end of the dry season (S. K.
Hamilton, pers. obs.).
Reflecting the isolated nature of refugial waterholes,
the species richness of dry season fish assemblages in
intermittent rivers and reaches in the Daly catchment
tends to be lower than in perennial reaches (B.J. Pusey
& M.J. Kennard, unpubl. data). Macroinvertebrate
communities differ between intermittent and peren-
nial reaches, with assemblages comprising more lentic
and lotic taxa, respectively (Humphrey, Hanley &
Camilleri, 2008; Leigh & Sheldon, 2009). Trophic
diversity within aquatic food webs can narrow
(D.M. Warfe, N.E. Pettit, E.A. Garcia & M.M. Douglas,
unpubl. data), and the diets of resident fish can also
narrow and become poorer in quality (Balcombe et al.,
2005) as consumers are forced to become more
dependent on local food resources (T.D. Jardine,
D.M. Warfe & N.E. Pettit, unpubl. data). Narrowing
of fish diets owing to limited resource availability
during the dry season also appears to be a common
feature in Central and South American rivers (Winem-
iller, Agostinho & Caramaschi, 2008). In perennial
rainforest rivers in Queensland, feeding links between
fish and their food sources do not vary greatly
between the wet and the dry season, supporting the
hypothesis that intermittency can lead to more limited
resources and more striking seasonal changes in food
webs (Rayner et al., 2010).
As the dry season progresses, the available habitat
contracts, increasing the potential for predation and
competition, as has been shown in both Neotropical
floodplain rivers (Winemiller, 1996; Rodriguez &
Lewis, 1997) and Australian arid-zone rivers
(Arthington et al., 2005). In shallow waterholes
(<3 m), habitat reduction can be accompanied by a
deterioration in water quality that can contribute to
fish kills when flow resumes (Townsend, Boland &
Wrigley, 1992). It is possible that fish mortality may
fuel algal and bacterial growth towards the end of the
dry season, as has been shown in Australian arid-zone
rivers (Burford et al., 2008b). Furthermore, late dry
season flowering of riparian species such as Melaleuca
leucadendra L. (Pettit, 2000) can attract insects and
flying foxes (Pteropus alecto Temminck) (Vardon et al.,
2001), potentially contributing riparian subsidies to
depleted waterholes and supporting juveniles of
2180 D. M. Warfe et al.
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species that breed at the end of the dry season, for
example barred grunter (Amniataba percoides Gu
¨
nther)
(Pusey et al., 2004; N. E. Pettit & D. M. Warfe, pers.
obs.).
In the few perennial rivers within northern Australia,
dry season baseflows are maintained by groundwater
discharge and their typically oligotrophic nature (e.g.
Townsend & Padovan, 2005) reflects the low nutrient
concentrations in the supporting aquifers. For exam-
ple, in the Daly River, as groundwater contributes a
larger proportion of flow over the course of the dry
season, nitrate concentration and turbidity decrease,
and the potential euphotic zone exceeds the river
depth (Townsend & Padovan, 2005; Townsend et al.,
2011). The clear water and low nutrient concentrations
common to these perennial rivers can make them
vulnerable to algal blooms as a result of nutrient
enrichment (Ganf & Rea, 2007). In the Daly River and
its major tributaries, photosynthesis increases over the
dry season because of the accumulation of primary
producer biomass rather than higher incident radia-
tion (Webster et al., 2005; Townsend et al., 2011).
While photosynthesis has been shown to be light-
limited in these generally open-canopy rivers, the
accumulation of primary producer biomass is proba-
bly nutrient-limited within the hydraulic and other
physical constraints of the river (Webster et al., 2005;
Townsend et al., 2008). Phytoplankton is likely to
briefly contribute a significant proportion to primary
production in the early dry season, before the
subsequent growth and accumulation of benthic algae
and submerged macrophytes dominate primary pro-
duction (Townsend & Padovan, 2005; Townsend et al.,
2011). As the dry season progresses, producer bio-
mass generally increases before being scoured out
during the first flows of the early wet season (Town-
send & Padovan, 2005), a pattern also apparent in the
low-nutrient wet–dry rivers of the Neotropics (Cotner
et al., 2006).
While bottom-up controls probably limit primary
producer biomass, in common with the Neotropics
(Cotner et al., 2006; Roelke et al., 2006), there is
evidence that benthic algae in Australia’s wet–dry
tropics are also partly regulated by grazing pressure
from macroconsumers such as freshwater prawns and
catfish, particularly during the dry season when there
is no disturbance from floods (Douglas et al., 2005;
M.M. Douglas, unpubl. data). Such top-down control
of algal resources has been observed in other tropical
regions, e.g. Costa Rican streams (Pringle & Hama-
zaki, 1997), Andean streams (Flecker & Taylor, 2004)
and the Cinaruco River in Venezuela (Winemiller
et al., 2006), suggesting that it is a characteristic
pattern of tropical streams and rivers (Douglas et al.,
2005). There are suggestions that the meiofauna may
also regulate benthic algae in tropical sand-bed rivers
(e.g. Winemiller et al., 2006), but their importance in
northern Australian rivers remains to be investigated
(Humphreys, 2008).
Perennial rivers also provide important habitats
within the dry season landscape for flow-dependent
aquatic species. Juveniles of numerous fish species,
such as sooty grunter (Hephaestus fuliginosus) and
Butler’s grunter (Syncomistes butleri Vari), are pre-
dominantly restricted to riffle areas where they can
escape predation pressure from larger fish, but also
where there is more food (Pusey et al., 2004). Peren-
nial reaches support distinctive macroinvertebrate
communities consisting of groups such as baetid
mayflies, hydropsychid caddisflies and hydrophilid
beetles that prefer flowing water (Humphrey et al.,
2008; D.M. Warfe & N.E. Pettit, unpubl. data).
Together with microalgae, these macroinvertebrates
are a source of benthic food that supports higher
trophic levels (Douglas et al., 2005; Townsend &
Padovan, 2009).
Other stream channel features can also become
important as they become exposed or accessible
during the dry season. Periods of low flow expose
channel sediments that can become important sites for
the germination and establishment of riparian seed-
lings (Pettit et al., 2001). The pig-nosed turtle (Carett-
ochelys insculpta Ramsay) is restricted to perennial
rivers, such as the Daly River, which are deep enough
to allow extensive movement (up to 14 km) by
females as they search for suitable egg-laying sites
(Georges et al., 2003). The females target fluvially
reworked sand banks in the inside of bends, at
tributary mouths or behind large boulders and wood
aggregations, and lay their eggs at the end of the dry
season after water temperatures have begun to rise
(Georges et al., 2003).
Dry to wet season transition
The major feature of the transition from the dry to the
wet season is the occurrence of storm run-off events
and the hydrological reconnection of isolated stream
Flow drivers of tropical Australian rivers 2181
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reaches. The first flow events of the wet season are
often of poor water quality as they flush organic
material and suspended particulates through the
system and into the estuary, sometimes creating
hypoxic conditions and causing fish kills in rivers
and floodplain waterholes (Townsend et al., 1992;
Townsend & Edwards, 2003; Butler, 2008). Tropical
waters generally have a higher oxygen demand, and
much lower oxygen saturation concentrations, than
temperate waters for a given organic loading, and
thus they are potentially vulnerable to anthropogenic
organic loading (Lewis, 2008). Surface run-off events
during the transition from the dry to the wet season
can also bring a pulse of organic material, nutrients
and sediments from the surrounding catchment
(Schult et al., 2007); consequently, the impacts of fire
and cattle grazing on aquatic systems are likely to be
most concentrated at this time (Townsend & Douglas,
2000). Similarly, mobilisation of trace metals and
reduced substances from floodplain soils may cause
fish kills (Hart & McKelvie, 1986). This is especially
likely in areas where acid sulphate soils occur on
former marine sediments or as a result of past mining
activities, e.g. in the Finniss River, south of Darwin
(Taylor, 2007).
Run-off events during the early wet season were an
important factor regulating mechanical breakdown,
and consequent biotic breakdown, of leaf litter in an
Amazonian floodplain stream (Rueda-Delgado, Want-
zen & Tolosa, 2006). In general, decomposition rates of
litter in tropical streams are strongly influenced by
water temperature, turbidity, pH, salinity, dissolved
organic carbon, nutrients and oxygen (Wantzen et al.,
2008). Early evidence from streams in the Daly River
catchment also suggests that flow regime, leaf species
and the physical character of streams play a stronger
role in leaf litter breakdown than microbial activity (T.
Davies, N.E. Pettit & P.F. Grierson, unpubl. data).
However, knowledge of microbial processes in Aus-
tralian wet–dry tropical rivers is scant.
The reconnection of isolated refugia within rivers is
an important time in the annual hydrograph for the
movement of biota, as flows commencing after the
initial poor-quality flush provide an opportunity for
fauna to move into more favourable reaches and
throughout the river system. Reaches in rivers and on
floodplains that act as refugia during the dry season
represent important sources of recolonising biota
(Outridge, 1988; Perna & Pearson, 2008). Macroinver-
tebrates can rapidly colonise re-wetted reaches from
upstream perennial reaches and floodplain water-
holes via the dispersal of aerial adult stages, as well as
from the hyporheic zone where microcrustaceans,
oligochaetes and dipterans appear to aestivate over
the dry season (Paltridge et al., 1997). The early wet
season is a key time for saltwater crocodiles, Crocody-
lus porosus, to disperse throughout the river system
and into floodplain waterholes (Jenkins & Forbes,
1985); their nesting effort is positively correlated with
high rainfall and relatively cool weather over this
period (Webb, 1991). Juvenile barramundi move from
estuarine habitats to upstream floodplains and tribu-
taries (Pusey et al., 2004). Males can spend between 3
and 5 years in freshwater riverine habitats before
returning to estuaries for spawning, and there is also
evidence for considerable movement throughout riv-
erine habitats during this time (Griffin, 1987; Pusey
et al., 2004). Recent research on fish movement in
tributaries of the Daly River showed that rainbowfish
(Melanotaenia australis Castelnau) and hardyheads
(Craterocephalus stercusmuscarum Gu
¨
nther) moved up-
stream after flushing flows and were markedly more
abundant where flow had ceased during the dry
season rather than at perennially flowing sites (D.M.
Warfe & N.E. Pettit, unpubl. data).
Consequences of altered flow regimes
The major implications of the distinct seasonal flow
regime in Australia’s wet–dry tropics are that the
biota within these systems must be able to take
advantage of short periods of resource-rich condi-
tions during the wet season, and be able to with-
stand relatively long periods of resource-poor
conditions during the dry season (McMahon &
Finlayson, 2003; Lytle & Poff, 2004; Douglas et al.,
2005). The fact that these processes occur annually,
and that the river and floodplain biota demonstrate
seasonally triggered traits such as fish spawning,
reptile nesting and vegetation fruiting, suggests that
these ecosystems are effectively ‘locked’ into the
strong seasonal pattern and support an aquatic biota
that is adapted to cope with, and benefit from, the
marked hydrological seasonality (Junk et al., 1989;
Bishop & Forbes, 1991; Davis et al., 2010; Pettit et al.,
2011). Recent research has found spatially concor-
dant distributions of vegetation, fish and invertebrate
assemblages across the region, which are primarily
2182 D. M. Warfe et al.
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driven by environmental gradients, flow regime
being a common structuring driver of all three
assemblage types (D.M. Warfe & N.E. Pettit, unpubl.
data). The corollary to this is that alterations and
disruptions to the natural flow regime are likely to
have far-reaching consequences for aquatic biodiver-
sity and ecosystem processes in Australia’s wet–dry
tropics (Bunn & Arthington, 2002).
Importantly for Australia’s tropical rivers, their
flow regimes (in most cases) are relatively unmodi-
fied: <3% of the total water extracted in Australia
occurs in the north (approximately 2000 GL), about
10–20% of which is taken by groundwater pumping
(ABS, 2006; Cresswell et al., 2009). There are 27
impoundments with a capacity >0.2 GL across north-
ern Australia – compared with 467 in the rest of the
country – most of which are in Queensland and serve
for urban water supply and irrigation (Pusey &
Kennard, 2009). The largest impoundment in the
region, the Ord River Dam (Lake Argyle) in the
Kimberley region of Western Australia, clearly illus-
trates the impacts of such storages on wet-dry tropical
flow regimes. The lower Ord River was once a highly
variable intermittent system. It is now a perennial
system with water being released during the dry
season months to meet irrigation and hydroelectric
demand, and all but the largest floods are attenuated
by the dam (Doupe
´
& Pettit, 2002). This has resulted
in sedimentation and narrowing of the river channel
below the dam (Cluett, 2005), and riparian vegetation
assemblages becoming narrower and more homogen-
ised (Pettit et al., 2001). It has also led to reductions in
banana prawn populations because of lower salinity
levels from perennial freshwater inflows (Kenyon
et al., 2004), and lower primary production in the
estuary (Burford et al., 2011).
Water planning has occurred in a few catchments
across the region but, on the whole, rivers are
unregulated and water is allocated on a licence-by-
licence basis by state government agencies. The
Northern Territory Government has a policy of
capping water allocations to retain 80% of natural
flows in rivers and aquifers in the wet–dry region of
its jurisdiction. The policy recognises that unimpeded
flow regimes provide a range of ecosystem goods and
services to the region. For example, the productivity
of commercial fisheries in the tropics, worth AU$220
million annually (Robins et al. , 2005), and the recrea-
tional fishing industry, worth up to AU$26 million to
the economy of the Northern Territory (NTG, 2009),
rely on naturally flowing rivers. Similarly, saltwater
crocodile harvesting and magpie goose hunting are
worth AU$40 million and AU$2 million per annum
(respectively) to the Northern Territory economy and
are heavily reliant on floodplains retaining their
natural inundation regimes (Delaney, Fukuda &
Saalfeld, 2009; Leach, Delaney & Fukuda, 2009). These
intact freshwater systems sustain nature-based tour-
ism, one of northern Australia’s major industries
valued at AU$1.5 billion annually (Woinarski et al.,
2007; Clark et al., 2009). Furthermore, freshwater
ecosystems are culturally and socio-economically
important to local indigenous communities (Jackson,
Storrs & Morrison, 2005); current research suggests
that the economic value of the use of freshwater flora
and fauna in some communities may comprise up to
20% of the median household income (M. Finn,
unpubl. data).
Given the sparse human population, low-intensity
land use and relatively low consumptive demand for
water, Australia’s tropical rivers and estuaries are,
hydrologically, the least impacted in the country
(NLWRA, 2002; Stein, Stein & Nix, 2002) and repre-
sent some of the most unaltered systems in the world
(Woinarski et al., 2007). They also represent a poten-
tially great water resource, and consequently, there is
persistent interest in developing Australia’s northern
rivers to help support the country’s food production,
particularly in the wake of extensive droughts in the
Murray-Darling catchment such as the ‘Millenium
Drought’ over 2000–08 (sensu Lake, Likens & Ryder,
2010). However, recent research on sustainable yields
across the region indicates that the water balance is
effectively ‘closed’ (Cresswell et al., 2009). Despite
large wet season rainfall, the landscape is annually
water-limited, and the low topographical relief, lack
of suitable dam sites and high evapotranspiration
rates do not support the viability of water storages
(Petheram et al., 2008; CSIRO, 2009).
Owing to the above constraints, large-scale devel-
opment of Australia’s wet–dry tropical rivers seems
unlikely at this time, yet there is still interest in
smaller-scale development. Surface and groundwater
abstraction, the construction of small off-stream sto-
rages and barriers such as tributary impoundments
and road crossings may produce substantial and
cumulative alterations in natural flow regimes. The
ecological consequences of such alterations relate to
Flow drivers of tropical Australian rivers 2183
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hydrological connectivity and material transport, the
availability of habitat, and opportunities for the
movement and recruitment of aquatic biota, and
could be extensive given the dependence of these
ecosystems on existing flow regimes. Based on our
review of how flow regimes and their key features
structure riverine, floodplain and estuarine ecosys-
tems in the Australian wet–dry tropics, we have
summarised the predicted hydrological and ecological
responses to small-scale, but cumulative, water devel-
opment in the region (Fig. 6).
A major consequence is likely to be reduced dry
season baseflows in perennial rivers (Fig. 6). A
reduction in baseflow has the potential to reduce the
availability of critical flow-sensitive habitats (e.g.
riffles) for turtles, juvenile fish and benthic biota,
consequently reducing aquatic primary production
and faunal recruitment, and negatively impacting the
food resources for larger species such as freshwater
crocodiles, barramundi and black bream (Douglas
et al., 2005; Webster et al., 2005; Pusey & Kennard,
2009; Townsend & Padovan, 2009; Chan et al., 2010;
Fig. 6). In extreme circumstances, reductions in base-
flow during the dry season could disconnect reaches
and shift the flow regime class from perennial to
intermittent (Pusey & Kennard, 2009); such shifts
between flow regime classes are likely to be accom-
panied by major ecological change (Poff et al. , 2010).
Similarly, water extraction in intermittent rivers can
increase the period of hydrological disconnection
between refugial reaches to more extreme intermit-
tency, hampering the movement and migration of
aquatic biota during seasonal transitions (Fig. 6) and
potentially increasing the risk of localised disturbance
to disconnected populations and genetic ‘bottlenecks’
(Pusey & Kennard, 2009).
Water extraction is likely to affect surface and
groundwater interactions, potentially decreasing
groundwater levels and affecting riparian vegetation
communities reliant on groundwater (O’Grady et al.,
2006; Pusey & Kennard, 2009; Fig. 6). Groundwater
can be important in maintaining the persistence of
freshwater refugia over the dry season, so extraction
may reduce the size and persistence of these refugia,
thereby reducing the amount of already-limited
aquatic habitat available across the landscape towards
the end of the dry season (Pusey & Kennard, 2009;
Fig. 6). Also, groundwater extraction may disturb the
subterranean and groundwater ecosystems that are
thought to play important filtering and water purifi-
cation roles (Humphreys, 2008; Pusey & Kennard,
2009).
There is also the potential for water development,
particularly the construction of instream storages and
barriers, to reduce flood peaks during the wet season
and therefore reduce the extent and duration of
floodplain inundation, as well as altering the timing
of flood peaks (Fig. 6). Alterations to flood dynamics
can disrupt cues for spawning, nesting or hatching for
many species such as barramundi (Bayliss et al., 2008),
magpie geese (Delaney et al., 2009) and pig-nosed
turtles (Georges et al., 2003), respectively, negatively
affecting population recruitment. Reductions in flood-
plain inundation can also limit the opportunities for
biota to move on and off the floodplain during the
transition phase to the dry season, potentially impact-
ing the ability of fauna to reach freshwater refugia
(Pusey & Kennard, 2009; Fig. 6).
Climate change predictions are imprecise for north-
ern Australia, but there is an increased likelihood of
increased temperatures and evapotranspiration, and
also of extreme storm, cyclone and drought events
(CSIRO and BoM, 2007; Cresswell et al., 2009). Mod-
elling flows under climate change scenarios suggests
that river levels are likely to be lower more often, but
even more so under water development scenarios
(McJannett et al., 2009). While this is likely to reduce
the availability of critical habitats for aquatic biota,
specific ecological responses are difficult to quantify
because of limited knowledge around environmental
flow thresholds. Given the low altitude of most of the
wetlands and floodplains throughout tropical Austra-
lia, particularly in the Northern Territory, one of the
major projected impacts is saltwater intrusion attrib-
uted to rising sea levels and storm surge, and
consequent losses of coastal freshwater habitats,
biodiversity and wetland-dependent populations of
waterbirds (Traill et al.
, 2009; Hamilton, 2010). Altered
rainfall and storm events are likely to alter flood
regimes, which dictate channel structure (Wasson
et al., 2010) and floodplain inundation events that
determine floodplain structure, extent and seasonality
(Hamilton, 2010). Redistributions of both habitats and
biota may also occur as populations expand or
contract at the limits of their climatic range, and those
species with restricted distributions or limited dis-
persal capacity are particularly at risk (Woinarski
et al., 2007). The consequences of such species
2184 D. M. Warfe et al.
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Stressor
Ecological
response
Hydrological
response
Driver
Dry season
Dry to wet
transition
Wet season
Wet to dry
transition
Water extraction
↑ Hydrological
disconnection
↓ Flush
magnitude
Early
waterhole
disconnection
↓ GW
connectivity and
recharge
↓ P eak flow
magnitude
Altered
metabolic
processing
↓ Sediment
nutrient loads
↓ Channel
structural
heterogeneity
↓ Rip veg
structural
heterogeneity
↓ Rip veg
width
↓ Wood
provision and
turnover
↓ Instream
habitat
↓ Opportunity
for biotic
movement
↓ Aquatic
biodiversity
↓ Sediment
nutrient loads
↓ Aquatic 1°
production
↑
Encroachment
terrestrial veg
↓ Faunal
recruitment
Delayed
waterhole
reconnection
↓ Flushing of
poor WQ
↓ S ediment ,
OM and nutrient
loads
↓ Waterhole
size, no. and
persistence
↓ Opportunity
for biotic
movement
↓ Opportunity
for biotic
movement
Delayed
recolonisation
aquatic
habitats
↓ Faunal
recruitment
Water extractionWater extractionWater extraction
↑ Hydrological
disconnection
during dry
↓ Waterhole
size, no. and
persistence
↓ Baseflows in
perennial
rivers
↓ Instream
habitat
↓ Aquatic 1°
production
↓ Faunal
recruitment and
diversity
↑
Encroachment
terrestrial veg
↓ FW inflows
to coasts
↑ Saltwater
intrusion
↓ Floodplain
inundation
↓ Floodplain
structural
heterogeneity
↓ Waterhole
connection
↓ Aquatic 1°
production
↑
Encroachment
terrestrial veg
↓ Waterbird
and faunal
recruitment
Altered peak
flow timing
Disruption life
history cues
↓ Waterbird
and faunal
recruitment
↓ FW inflows
to coasts
↓ Fisheries
catch and
recruitment
Flow regime
shifts to
intermittency
↓ Instream
habitat
↓ Aquatic
biodiversity
Altered rip veg
composition
and
and
Fig. 6 Conceptual ecological model based on the information reviewed in this paper, illustrating the predicted hydrological and eco-
logical responses of water extraction for each of the key flow features comprising flow regimes in the wet–dry tropics of Australia. Arrows
within boxes indicate an increase or decrease in that particular component. Boxes are defined by their shape in the lower right corner.
Flow drivers of tropical Australian rivers 2185
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redistributions are unknown, but are likely to cascade
to other biota through food web interactions (Pusey &
Kennard, 2009), potentially leading to increased spe-
cies invasions, increased disease and the formation of
novel communities (Traill et al., 2009).
There are major knowledge gaps around how the
effects of climate change might interact with increas-
ing water use, and also how these effects will interact
with land-use impacts such as land clearing and cattle
grazing, weeds and feral animals, as well as cropping
and nutrient inputs. In particular, more information is
required on environmental flow requirements, hydro-
logical connectivity across riverine landscapes, mech-
anisms for tolerating low- or no-flow periods, and
potential flow thresholds to predict more effectively
the consequences of water use on these ecosystems.
This information is also necessary for identifying key
species and processes for monitoring, and to assist
with the identification of ecosystems or areas of high
conservation value. Hamilton & Gehrke (2005) out-
lined the key knowledge requirements for Australia’s
tropical rivers: the sustainable availability of water for
human use; hydrological, biogeochemical and ecolog-
ical linkages at landscape scales; an understanding
and valuation of ecosystem processes and services;
and managing climate change. We note that, despite
the advances in knowledge of Australia’s northern
rivers achieved over the past 5 years and documented
here, the knowledge gaps identified by Hamilton &
Gehrke (2005) are still relevant.
Principles for water management in northern
Australia
There have been numerous papers outlining key
principles of providing environmental flows to pre-
serve the world’s freshwater resources (Poff et al.,
1997; Bunn & Arthington, 2002; Pinay, Clement &
Naiman, 2002; Arthington et al., 2006), and many
more describing methods for implementing them
(Cottingham, Thoms & Quinn, 2002; Arthington &
Pusey, 2003; Hughes & Rood, 2003; Tharme, 2003;
Acreman & Dunbar, 2004). Ultimately, these methods
all rely on a conceptual understanding of the rela-
tionships between flow variation and ecological
responses that can be used to define environmental
flow allocations, but in many cases, this knowledge
remains scant (Naiman et al., 2008). Historically, many
environmental flow studies have focussed on meeting
the needs of specific taxa or populations, the assump-
tion being that maintaining assets of high environ-
mental ‘value’ will maintain ecological health. This
often assumes direct and linear relationships between
patterns of species distribution and ecological pro-
cesses (Anderson et al., 2006). Our understanding of
the links between ecological pattern and process is
limited at best; nonlinear relationships are common
and patterns are often a product of processes occur-
ring at multiple and interacting scales (Walker, Shel-
don & Puckridge, 1995; Harding et al., 1998; Bendix &
Hupp, 2000; Bunn & Davies, 2000; Tockner et al.,
2000). It would seem reasonable to focus the goals of
environmental flows towards maintaining ecosystem
resilience and integrity; the biotic processes leading to
high biodiversity should then follow (Ward, Tockner
& Schiemer, 1999; Bunn & Davies, 2000; Ward et al.,
2001).
A recent advance in environmental flows method-
ologies is the ‘Ecological Limits of Hydrologic Alter-
ation’, or ELOHA framework (Arthington et al., 2006;
Poff et al., 2010), which incorporates the essential
scientific requirements for setting environmental
flows. It has been developed in acknowledgement of
the fact that the increasing rate of water development
is outstripping the ability to assess environmental flow
requirements and that better links between science
and management are needed. The ELOHA framework
develops flow–ecology response relationships within
flow regime types, which can then be empirically
tested in an adaptive manner. It thus lends itself to
application at a regional scale and across a range of
river types, and can also address different levels and
types of flow alteration such as regulated versus
unregulated rivers (Naiman et al., 2008).
In regions of limited data on flow–ecology relation-
ships, the ELOHA framework maximises the ability to
extract useful information from across the region, and
we propose it as the most suitable framework to
derive environmental flows for rivers in Australia’s
wet–dry tropics. An ecohydrological classification,
incorporating the first and second steps in the ELOHA
framework, has already been developed for Austra-
lian rivers and identifies three ecologically relevant
flow classes in the wet-dry tropics, their range of
variability, and their extent of hydrological develop-
ment (Kennard et al., 2010). The third step in the
ELOHA framework requires the deviation between
current and natural flows to be calculated for each
2186 D. M. Warfe et al.
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flow class (Poff et al., 2010). While the availability of
gauged data across northern Australia is problematic
because of sparse spatial coverage and inadequate
time series, low water use and predominantly unim-
peded flow regimes mean that most rivers have an
essentially natural flow regime with low hydrological
disturbance (Stein et al., 2002). The present paper
characterises the ecological structure and function of
those tropical flow classes and is therefore a step
towards developing the flow–ecology relationships
specific to each flow class, the final step in the ELOHA
framework.
While the ELOHA framework, along with most
methods and case studies of environmental flows,
recommends retaining the natural flow regime as far
as possible to maintain naturally functioning aquatic
ecosystems (Poff et al. , 1997), there are also other
management approaches that could be used in com-
bination to guide water resource management in the
region. One approach is to designate ‘representative’
rivers for protection. We do not recommend an
approach favouring the ‘top 20%’ of rivers, for
example, as this requires a difficult decision on which
rivers should be considered in the ‘top 20%’ and on
what basis, particularly given the often high levels of
uncertainty surrounding the choice of decision-sup-
port tools (Borchers, 2005; Burgman, Lindenmayer &
Elith, 2005). Rather, we advocate using existing
ecohydrological classification systems (e.g. Stein et al.,
2009; Kennard et al., 2010) to ensure that rivers
representing each hydrological and geomorphic
‘class’ are included in any conservation management
scheme (Fitzsimons & Robertson, 2005).
Another approach is to recognise the variability
of flow regimes in the wet–dry tropics and the range
of biotic responses it engenders, and retain a mosaic of
hydrological units or habitats in the lateral, longitu-
dinal, vertical and temporal dimensions over which
flow variability operates. For example, floodplains
could be managed to sustain a range of waterhole
types, from permanent and deep through to intermit-
tent but inundated annually, across a diversity of
systems with varying flow and flood regimes. Such a
multi-scalar approach explicitly incorporates the role
of hydrological connectivity and its role in mediating
the transfer of materials and aquatic organisms
between hydrological ‘elements’ (Pringle, 2001). It
would also support a shifting habitat mosaic that
underpins aquatic and terrestrial biodiversity, food
web stability and ecosystem processes (McCann &
Rooney, 2009; Larned et al., 2010), and thus the
distinctive ecological character of river ecosystems in
Australia’s wet–dry tropics.
Conclusions
Australian tropical rivers are driven by fluvial
dynamics that are distinct from rivers elsewhere and
result from a particularly strong wet–dry climatic
seasonality with high interannual variability. Many of
these river systems have intermittent flow and limited
permanent freshwater refugia during the dry season,
in spite of extensive flooding during wet season flows.
This review has described the critical features of flow
regimes that maintain ecosystem processes in rivers of
Australia’s wet–dry tropics. Peak wet season flows are
responsible for connecting river channels, floodplains
and estuaries across their spatial extent and play an
important role in material transfer and regulating
biotic production and recruitment. Transition periods
between the wet and the dry seasons, and vice versa,
are key times for biota to move between different
parts of the riverscape for spawning and nesting, or
finding more favourable habitats and easing pressure
on resources. And the dry season is a time where
resources can become scarce and aquatic refugia
become critical for sustaining aquatic biota through
to the following wet season. This marked seasonal
cycle has led to the development of life history
strategies and ecosystem processes that allow the
biota to cope with, and benefit from, such ‘boom and
bust’ conditions similar to those observed in more
arid-zone rivers (Bunn et al., 2006).
Hydrological seasonality describes flow variability
on an annual scale, but variability at larger temporal
scales is equally important, regulating the degree of
‘boom’ and ‘bust’ from year to year. For example, the
2008–09 wet season saw massive flooding in the
southern Gulf rivers in Queensland and fairly average
flooding across the rest of the region during the mid-
wet season, whereas in the 2009–10 wet season, most
rainfall was delayed to almost the beginning of the
dry season and numerous floodplains experienced
only limited inundation, if any at all (e.g. the Daly and
Fitzroy rivers, respectively). In contrast, the 2010–11
wet season has seen near-record flooding in these
same rivers. Such interannual variability generates
spatial heterogeneity that can favour different biota
Flow drivers of tropical Australian rivers 2187
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from year to year (e.g. Madsen & Shine, 2000). It
thereby sustains a wider range of biota over the long
term, which in turn supports the ecosystem stability
and resilience required to persist through ‘extreme’
conditions (McCann, 2000; McCann & Rooney, 2009).
However, to manage variability for ecosystem resil-
ience, it is critical to understand the range of
variability experienced within each flow class in the
wet–dry tropics so that the full range of natural
variability is experienced, but not exceeded, and
ecological thresholds are not crossed, potentially
altering these ecosystems irrevocably.
Rivers in Australia’s wet–dry tropics are notable
for their relatively unaltered hydrological nature and
undeveloped catchments, so the fluvial dynamics we
observe are largely natural and this sets them apart
as a model for understanding flow–ecology relation-
ships. The conceptual model summarises the key role
of seasonal hydrology and hypothesises the hydro-
logical responses and ecological consequences of
water development in Australian wet–dry tropical
rivers. Given that, on a global scale, these rivers are
some of the few remaining naturally functioning
examples, their ecological and socio-economic value
should not be considered just in terms of local
communities, but also in the context of national and
international settings. This review has outlined some
of the potential ecological consequences of water
development in the region and suggested approaches
to managing these rivers sustainably. There is an
ecohydrological classification of tropical Australian
rivers, and the development of flow–ecology rela-
tionships is underway, as is a burgeoning under-
standing of how these relationships might vary with
flow type and hydrological variability. Thus, there is
an excellent opportunity to implement the ELOHA
framework across the wet–dry tropics in northern
Australia, to test the hypotheses that underpin our
conceptual model (before major water resource
development proceeds), and to adaptively manage
the region’s water resources into the future.
Acknowledgments
The authors thank two anonymous reviewers and
Prof. Alan Hildrew for their considered suggestions
that greatly improved the manuscript. This paper
stems from research on tropical rivers, floodplains
and estuaries conducted by the authors as part of the
Tropical Rivers and Coastal Knowledge (TRaCK)
research hub. TRaCK received major funding for its
research through the Australian Government’s Com-
monwealth Environment Research Facilities initiative,
the Australian Government’s Raising National Water
Standards Programme, the Fisheries Research and
Development Corporation and the Queensland Gov-
ernment’s Smart State Innovation Fund.
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