Freshwater Biology (2003) 48, 1147–1160
Droughts and anti-droughts: the low flow hydrology
of Australian rivers
T. A. MCMAHON*,† AND B. L. FINLAYSON*,‡
*Cooperative Research Centre for Catchment Hydrology, and Centre for Environmental Applied Hydrology, The University
of Melbourne, Victoria, Australia
†Department of Civil and Environmental Engineering, The University of Melbourne, Victoria, Australia
‡School of Anthropology, Geography and Environmental Studies, The University of Melbourne, Victoria, Australia
SUMMARY
1. Droughts are not easily defined other than by culturally driven judgements about the
extent and nature of impact. Natural ecosystems are adapted to the magnitude and
frequency of dry periods and these are instrumental in controlling the long term
functioning of these systems.
2. In unregulated rivers, low flows are derived from water in long-term storage in the
catchment, commonly as shallow groundwater. Four types of low flow sequences are
evident for representative rivers from each of the seven flow regime zones in Australia and
an arid zone stream: perennial streams with low annual flow variability that have seasonal
low flows but do not cease to flow; perennial streams with high annual variability that
cease to flow in extreme years; ephemeral streams that regularly cease to flow in the dry
season; and arid zone streams with long and erratic periods of no flow.
3. Although Australian rivers record runs of consecutive years of low flows longer than
would be expected theoretically, the departures from the expected are not statistically
significant. Trends and quasi-cycles in sequences of low-flow years are observed over
decadal time scales.
4. Examples of the effects of river regulation on low flows in southern Australia indicate
that, while in detail the impacts of regulation vary, in general regulation mitigates the
severity of low flows.
5. It is our contention that the indigenous biota of Australian rivers are adapted to the
naturally occurring low flow conditions and that, while there is considerable scientific
interest in the effects of climate change on stream ecology, such studies have little practical
relevance for the management of indigenous biota in unregulated rivers.
6. The changes brought about by the regulation of rivers are much more rapid and
dramatic than those which might occur as a result of climate change and it is possible to
develop management procedures to mitigate them. In regulated rivers, the real problem
may be ‘anti-droughts’ – the removal of significant natural low-flow events from the flow
pattern.
Keywords: anti-drought, Australian rivers, drought, low flows, regulated rivers
Introduction
Correspondence: Brian L. Finlayson, School of Anthropology,
Geography and Environmental Studies, The University of
Melbourne, Victoria 3010, Australia.
E-mail: [email protected]
2003 Blackwell Publishing Ltd
Drought is a culturally loaded word in our society.
Objective definitions have proved elusive and mostly
they are defined in terms of their social, political
or economic impacts (Coughlan, 1985). Further,
1147
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T.A. McMahon and B.L. Finlayson
droughts are usually categorised as ‘meteorological’,
‘hydrological’ and ‘agricultural’ droughts. Keyantash
& Dracup (2002) have reviewed the many indices
which have been proposed for measuring the severity
of droughts in each of these three categories and point
out that precise quantification is ‘a difficult geophysical endeavour’ (p. 1167). Defining drought as a
phenomenon of the natural environment and attempting to measure severity is even more difficult. Keyantash & Dracup (2002) based their assessments on
two agricultural regions, where impact on agricultural
production provides the culturally relevant scaling
factor for defining a drought. In natural ecosystems,
individuals and communities must be capable of
surviving, in some way, the protracted dry periods
that are a natural feature of the system. It could be
argued that long dry spells (droughts?) in the natural
environment, while they may cause high mortality
rates among plant and animal species, are nevertheless one of the components which control the longterm functioning of these systems.
Any particular stream will have an expected pattern
of low flows, which depends on its seasonal hydrological regime type. Haines, Finlayson & McMahon
(1988) identified 15 regime classes globally. Ten of
these are found in Australia and seven form spatially
contiguous zones which can be used as the basis for
regionalisation of Australian streams (Fig. 1). The
nature of the low-flow periods experienced in these
streams includes regular periods of no flow or very
low flow that is a normal part of the regime (e.g. the
low winter flows of rivers in the Extreme Late
Fig. 1 Zones of seasonal regime types in Australia after Haines et al. (1988) and the locations and designations of stream gauging
stations whose records are used in this paper.
2003 Blackwell Publishing Ltd, Freshwater Biology, 48, 1147–1160
Low flows in Australian rivers
Summer type) and flows that are lower than expected
for the time of year. These regime types occur across a
range of climatic moisture zones from permanently
humid to permanently arid. Consider, for example,
the range of climatic moisture status conditions found
across the zone shown on Fig. 1 as having an Extreme
Late Summer runoff regime. This zone includes rivers
that are perennial and rivers that flow only following
heavy rain, but they share the common characteristic
that the period of most flow is generally in late
summer.
The low flow behaviour of streams is further
complicated by position in the stream channel network. At the upper end of channel networks (the first
order streams of Strahler, 1952), channels may carry
flow only immediately following rainfall. The active
channel network extends upstream during wet periods in all regime and climate types. Thus the length
and severity of low flow periods, and frequency of
cease-to-flow, typically decrease with distance downstream in the channel network and, therefore, increasing catchment area. Bedrock type also mitigates the
nature of low-flow periods. Rivers draining catchments containing major aquifers may continue to flow
during long dry periods when neighbouring streams
on different bedrock have ceased to flow. Further, in
catchments containing aquifers, springs may keep
sections of the channel network flowing more or less
permanently as described by Lake (2003).
In addition to low-flow characteristics determined
by regular seasonal conditions and catchment characteristics, many rivers experience longer-term fluctuations as trends and quasi-cycles. There are
sequences of consecutive years when flows are significantly below (or above) the long-term average. These
result from a range of climatic mechanisms at the
global scale that affect Australia, including the El
Niño Southern Oscillation (ENSO) (Allan, Lindsay &
Parker, 1996); the Central Pacific sea surface temperature oscillations (Power et al., 1999); and those associated with the Southern Ocean, known as the
Antarctic circumpolar wave (White & Peterson,
1996). Relationships have also been observed between
the Indian Ocean sea surface temperatures and rainfall in Australia (Smith, 1994).
River regulation impacts on the nature and extent of
low flows in river systems. In Australia, as for the rest
of the world, the flows in many river systems are no
longer natural throughout the whole catchment
2003 Blackwell Publishing Ltd, Freshwater Biology, 48, 1147–1160
1149
(McCully, 1996; Smith, 1998). The construction of
storage dams and diversion weirs and pumping by
riparian water users introduce impacts on the patterns
of natural flows which vary according to the type of
regulation involved and the location in the river basin
(e.g. McMahon & Finlayson, 1995).
The impact of large storage reservoirs depends on
the use to which the water is put. Many irrigation
dams in Australia are located upstream of the irrigation area and the water is transferred downstream
along the natural river channel. This produces a major
change of seasonal flow regime but little change to
total flow, because water is released down the channel
when irrigation demand is high, which, in the major
irrigation districts of southern Australia, is usually the
period of natural low flow. Where the water is
diverted out of the system by inter-basin transfer or
for use in urban water supply, the impact on flow
downstream of the dam is a major reduction in the
amount of flow as well as a change in seasonal regime.
In both cases there is also a change to flow variability.
Where the dam is located in an area of high local
runoff, the impact of the dam will be mitigated
downstream by flows from unregulated tributaries.
Diversion weirs are commonly used on smaller
streams and typically can have a major effect on low
flows while the high flows pass over them. This is also
the case with riparian pumping schemes, which are
sometimes associated with a weir constructed to
create a pool from which to pump.
It is the case that most hydrological analysis
techniques have had their origin in engineering
hydrology and have been developed for engineering
uses such as the development of water resources or
prediction in hydrology. It is almost certainly the
case that had the development of such techniques
been carried out by biologists seeking to use them
to analyse biological problems, there would probably be a quite different suite available. Our
purpose here is to use the existing toolkit of
techniques to try to explain the low flow behaviour
of rivers in ways which may be relevant to river
ecology.
In this paper, we explore the nature of low flows in
rivers using examples derived from Wullwye Creek,
New South Wales, a typical cool-temperate stream
with relatively low annual discharge variability by
Australian standards. Then we define and describe
low streamflow, characterise low-flow sequences, the
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T.A. McMahon and B.L. Finlayson
frequency of low-flow events, the length of runs of
low flows and the nature of trends and quasi-cycles
for a representative stream from each of the seven
flow regime zones, and the Todd River, Northern
Territory, which represents a stream in the most arid
part of Australia. We then examine the effects of river
regulation on low-flow periods, using the Snowy and
Lachlan Rivers as examples, and introduce the concept of ‘anti-drought’, i.e. the provision of low flows
that are larger and more persistent than those which
occur naturally.
Methods
Flow data for Wullwye Creek at Woolway (station
222007), catchment area 520 km2, southern New South
Wales (NSW; Fig. 1) were obtained from the NSW
Department of Land and Water Conservation. From
the 40 years of record, we have chosen 6 years of
continuous data, from 7 July 1978 to 21 April 1984, to
illustrate some characteristics of low flows. Streamflow hydrographs are characterised by two types of
flow, commonly referred to as quickflow and baseflow. Quickflow can be taken to represent the surface
runoff which occurs during and immediately following rainfall, while baseflow comes from the slow
drainage of the saturated storages in the catchment,
usually local groundwater. Low flows are mainly
baseflow, and the rate of baseflow is proportional to
the remaining volume of saturated storage that is
connected to the relevant stream reach. When flow is
supplied from these stores, points on the hydrograph
will plot as a straight line when discharge is plotted on
a log scale and time on a linear scale. As a general rule,
where the hydrograph begins to plot as a straight line,
surface runoff has ceased. Surface runoff is assumed to
have begun when the hydrograph rises. Recession
coefficients are calculated for a 73-day period during
the relatively wet summer of 1978/79 and for the
much drier summer of 1980/81 in Wullwye Creek.
Baseflow recession curves are described by:
Qt ¼ Q0 Kt ;
ð1Þ
where Qt, Q0 are the respective flows at time t and at
time 0 (when surface runoff ceases), K is called the
recession constant and t is time since surface runoff
ceased. For daily data, a typical value of K ¼ 0.95
(Pilgrim & Cordery, 1993).
For each of the seven seasonal flow regime types,
we have selected a ‘typical’ stream, for which a
reasonably long flow record is available (Table 1), and
we use these seven stream flow records to characterise
the pattern of low flows for each regime type in the
absence of significant flow diversion or regulation.
The Todd River is included as an example of streams
in the more arid parts of Australia which frequently
cease to flow. The analyses that follow are based on
monthly and annual flows. The mean annual runoff of
the eight catchments varied from 31 to 1040 mm
(Table 1). The range of variability of annual flows
observed for Australian streams is adequately represented by the coefficients of variation (CV) of annual
flows of these catchments (McMahon et al., 1992). The
lag one auto-correlation coefficient (r1) is a measure of
the persistence of the flow or, in other words, the
likelihood of successive years of low flows or successive years of high flows. Values shown in Table 1 are
typical of Australian streams (McMahon et al., 1992)
except for two large values, 0.27 and 0.35 for the Todd
and Thomson Rivers, respectively.
Table 1 Characteristics of stations used in this study. Regime refers to the pattern of flows through the year as defined by Haines et al.
(1988)
River
Station no.
Gauging station
Regime
Catchment
area (km2 )
Period of
record (years)
MAR
(mm)
CV
r1
Coranderrk Ck
Thomson
Styx
Barron
Ord
Broken
Queanbeyan
Todd
229115
225210
206001
110003
809302
404200
410701
060046
No. 2 Weir
The Narrows
Jeogla
Picnic Crossing
Coolibah Pocket
Goorambat
Googong
Wigley Gorge
Early spring
Moderate winter
Moderate late summer
Early autumn
Extreme late summer
Extreme winter
Moderate autumn
Extreme late summer
18.6
518
163
220
46100
1920
873
360
1909–99 (91)
1886–1970 (85)
1919–90 (72)
1926–99 (74)
Modelled (87)
1917–67 (51)
1913–74 (62)
1962–95 (34)
1040
510
700
640
83
120
130
31
0.28
0.42
0.57
0.57
0.71
1.00
1.06
1.50
0.06
0.35
0.17
0.04
0.01
0.10
0.07
0.27
MAR, mean annual runoff; CV, coefficient of variation of annual flow; r1, lag one serial autocorrelation.
2003 Blackwell Publishing Ltd, Freshwater Biology, 48, 1147–1160
Low flows in Australian rivers
Low-flow events can be classified by the average
recurrence interval (return period) of low-flow
sequences of consecutive days, months or years. For
low-flow events of <12 months duration, the analysis
is relatively straightforward as the sequence can be
treated in the same way as the annual or partial flood
series (Gordon, McMahon & Finlayson, 1992). For the
major low-flow events of longer than 12 months, the
fundamental issue is the requirement for the assumption of independence of an n-consecutive-years event
(say 3 years) drawn from a record length of N years
(say 60 years) of annual streamflow data. In the
example of the 3-year event, there are N/n (60/
3 ¼ 20) largely independent sequences but N ) n + 1
(60 ) 3 + 1 ¼ 58) overlapping and highly correlated
sequences. When we choose a specific n-years event
from N years of historical data, we are essentially
sampling the n-years event from the overlapping
sequence. Thus the analysis must take this high
correlation into account and a procedure to do this
has been developed by Srikanthan & McMahon
(1986). The steps in the procedure have not been
listed here, but note that the simplest technique
assumes that the annual flows can be represented by
either a Normal or a Gamma distribution.
Taking annual flow below the median as the
measure of low flow, we calculated the deviations
from the median for the seven representative Australian streams. From these data we could compile
frequency distributions of lengths of runs below the
median. We then superimposed theoretically derived
frequency distributions of expected runs below the
median (as a function of the length of data) in order to
compare the extent to which the distribution of runs
of low flows in these representative Australian
streams differ from that which would be predicted
theoretically. The method follows the derivation by
Yevjevich (1982) but is modified to take into account
the finite length of the data record and assumes the
annual flows are independent variables, an assumption which holds true for five of the analysed streams.
Flow data from the Snowy River in eastern Victoria
and the Lachlan River in south-central NSW, each of
which is regulated by a large storage reservoir, are
used to demonstrate the nature and magnitude of the
impacts of regulation on low-flow behaviour. The
Snowy River is a coastal river in southeastern
Australia with a catchment area of 13 400 km2 and a
natural mean annual flow of 2020 · 106 m3. Flow from
2003 Blackwell Publishing Ltd, Freshwater Biology, 48, 1147–1160
1151
the upper 14% of the catchment is diverted into the
Murray and the Murrumbidgee Rivers, inland flowing streams from which the water is extracted for
irrigation farming. At the diversion point, the Jindabyne Dam, only 1% of the original flow is left in the
river for local riparian users (Finlayson, Gippel &
Brizga, 1994).
The Lachlan River, more than 900 km in length,
drains a catchment of 84 700 km2 and provides
irrigation for 500–1000 km2 annually. The Lachlan
system consists essentially of a linear stream controlled by one major reservoir (Wyangala Dam with a
storage capacity of 1220 · 106 m3), a very small dam
and two off-river storages. The annual flows are
characteristic of highly variable western flowing NSW
streams. For this system, irrigation water is taken
directly from the river or from regulated effluent
streams. Wyangala and the other storages were
constructed to provide more reliable supplies.
We compare the characteristics of low flows before
the dams were constructed with the flows as regulated by the dams. The analyses differ from those used
in our regional survey above. First, we compare
annual flow totals and compare daily flow duration
curves before and after the dams were completed.
Results
Nature of low streamflow
Any record of streamflow will exhibit consecutive
periods of low flow of various lengths. For example,
the early years of the 40-year record of flows in
Wullwye Creek at Woolway contained many fewer
extended periods of low flow than the latter period
(Fig. 2a). This inconsistency through time is an
important feature of Australian rivers. The flow during
the selected 7-year period (1978–84) at Wullwye Creek
was characterised by a high-flow period in winter and
low-flow period in summer, but, while the interannual
pattern was consistent, the magnitude of the flows
was not. For example, the mean daily winter (July–
September) flow of 0.006 mm day)1 for the year 1982
was 1% of that for 1978, while the mean daily summer
flow of 0.0001 mm day)1 for 1982/83 was 0.2% of
the mean daily summer flow for 1978/79. These yearto-year extremes illustrate the range of hydrologic
conditions that aquatic flora and fauna encounter even
in a system of relatively low variability.
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T.A. McMahon and B.L. Finlayson
Fig. 2 Daily streamflows for Wullwye Creek at Woolway (222007), southeastern Australia, from (a) 25 March 1949 to 21 June 1999,
(b) 7 July 1978 to 21 April 1984, (c) 9 November 1978 to 20 January 1979 (linear scale), (d) 9 November 1978 to 20 January 1979
(logarithmic scale), and (e) 1 November 1980 to 19 January 1981 (logarithmic scale). Periods of missing data in panel (a) are indicated
by bars below the zero line.
The separation of surface and baseflow is important
in understanding the source of water for the stream.
Over the 73 days of flow in Wullwye Creek during the
wet summer of 1978/79, 58 days (79%) were baseflow
(A-B¢, B-C¢, C-D¢ and D1 onwards), yielding 66% of
the total flow (Fig. 2c). Because the immediate source
of this water is different to the source of surface
runoff, the water quality of the flows may also be very
different.
From eqn (1), K values (recession constant) for A, B
and C in Fig. 2d are 0.92, 0.92 and 0.89, respectively. In
the case of D1, D2 and D3, where there are three
different recessions in sequence, the values are 0.82,
0.90 and 0.96, respectively. Given the slopes of
recessions A, B and C, it is probable that the recession
beginning at D1 is a result of interflow rather than
groundwater discharge, where interflow is defined by
Mosley & McKerchar (1993) as ‘…rapid subsurface
flow through pipes, macropores and seepage zone in
the soil’, and Pilgrim & Cordery (1993) suggest that
for daily data K for interflow ranges from 0.8 to 0.9.
An 80-day sequence of daily flows during the much
drier summer of 1980/81 included five minor stream
rises (Fig. 2e). The recession coefficients were 0.77,
0.72, 0.63, 0.60 and 0.56. These were much steeper
than the D2 and D3 values observed in the summer of
1978/79. Furthermore, the slopes became progressively steeper over the 80 days. In mid-November it
2003 Blackwell Publishing Ltd, Freshwater Biology, 48, 1147–1160
Low flows in Australian rivers
1153
took 9 days for the discharge to reduce by an order of
magnitude but by mid-January it took only 4 days.
From a hydrologic point of view, this indicates that
the main sources of groundwater during the recessions in the summer of 1978/79 were dry, so that the
recessions in 1980/81 were only from localised
groundwater (probably bank storage) and as the
flows were so low, evaporation from the stream,
together with transpiration from riparian vegetation,
was greater than the groundwater discharge (point
potential evaporation in the area typically is about
6 mm day)1; Wang et al., 2001).
The recessions shown in Fig. 2e were for extremely
severe conditions and illustrate the reducing effectiveness of rainfall as the catchment dries out. Over this
80-day period, the average discharge from the
520 km2 catchment was only 0.08 m3 s)1 (equivalent
to 0.4 mm month)1 runoff) which for an average flow
velocity of, say, 0.5 m s)1 and an average stream width
of 10 m, the depth of flow was only about 16 mm.
Under these conditions, small falls in discharge will
make relatively large reductions in wetted channel
area and thus habitat availability for aquatic organisms. At higher flows, quite large recessions in baseflow will have much less impact on habitat availability,
because they mainly reduce water depth and these
differences are mitigated by channel shape (Fig. 2c).
Characterising low flow sequences
Figure 3 shows time series of annual streamflows of
three of the streams from the seven chosen to
represent the regime zones of Australia. Coranderrk
Creek has a low annual CV and the Queanbeyan River
a high CV, and the Todd River represents the arid
zone. Differences in streamflow variability are clearly
apparent and other steams in Australia grade between
these extreme cases.
By examining the flows of the streams in the seven
climatic zones in Australia (Fig. 1), we have identified
four general types of low-flow sequences that may be
of particular relevance to stream ecology. The first is
for perennial streams that have low annual variability
and, given the climatic zones in which they are
located, even under extreme low-flow conditions
would not be expected to cease to flow. Three streams
fall into this category: Corranderrk Creek (CV ¼ 0.28),
Thomson River (CV ¼ 0.42; Fig. 4a), and Barron River
(CV ¼ 0.57).
2003 Blackwell Publishing Ltd, Freshwater Biology, 48, 1147–1160
Fig. 3 Annual streamflow series for (a) Coranderrk Creek at No.
2 Weir, (b) Queanbeyan River at Googong, and (c) Todd River at
Wigley Gorge.
The second type is illustrated by the Queanbeyan
River (CV ¼ 1.06), which is a perennial stream but in
extreme years cease-to-flow can occur (Fig. 4b). The
Styx River (CV ¼ 0.57) and the Broken River
(CV ¼ 1.0) are also representatives of this type.
The third type is represented by the Ord River
(CV ¼ 0.71; Fig. 4c), which is an ephemeral stream
where cease-to-flow occurs during the winter each
year. Note also that while flow occurs every summer,
there are substantial differences between successive
summers.
The fourth type of flow sequence, illustrated by the
Todd River (CV ¼ 1.5; Fig. 4d), is a highly variable
ephemeral arid-zone stream. While the Todd has an
Extreme Late Summer regime, there is much more
year-to-year variability in the regime than is the case
for the Ord (Fig. 4c). While most of the large flow
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T.A. McMahon and B.L. Finlayson
Fig. 4 Monthly streamflows for the (a) Thomson River at The Narrows, (b) Queanbeyan River at Googong, (c) Ord River at Coolibah
Pocket, and (d) Todd River at Wigley Gorge.
events in the Todd occur in late summer, there are
some years with no summer flows at all (e.g. 1969/70;
1979/80) and some years when the major flow events
do not occur in late summer (e.g. 1986). This
behaviour is typical of streams that have their headwaters within the semiarid and arid zones of inland
Australia though in terms of water resources they
have only minor local significance.
Frequency of low-flow events
The computed rank 1 (i.e. the lowest in the record)
1-, 2-, 3- and 5-year low-flow sequences of five
Australian rivers are given in Table 2. Note that for
the Ord and Styx, neither the Normal nor the Gamma
distribution was applicable and the average recurrence intervals could not be computed using this
method. These results can be used as the basis for
making probabilistic statements about the likelihood
of occurrence of events of these magnitudes and
enables observed events to be scaled within the
record. For example, the lowest 1-year flow at
Coranderrk is an extreme event when compared with,
say, the lowest 1-year flow for the Barron. Clearly this
Table 2 Average recurrence interval of lowest n-years flows.
(Could not be computed for the Styx and the Ord)
Recurrence interval of the
n-years low flow
River
Station no.
1
2
3
5
Corranderrk Ck
Thomson
Barron
Broken
Queanbeyan
229115
225210
110003
404200
410701
370
99
14
79
19
250
270
60
370
58
470
67
140
300
110
110
140
170
75
150
analysis is heavily dependent on the appropriateness
of the probability distribution being used but it does
provide a consistent procedure which can be used in
ecological studies to assess the ‘importance’ of
observed low-flow events to the system.
Length of runs of low flow
Another relevant property of low flows is the lengths
of runs of low-flow years, defined here simply as
years with flows less than the mean, although other
definitions are possible (Fig. 5a,b). Note that the
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Low flows in Australian rivers
1155
Fig. 5 Above and below median streamflows for (a) Coranderrk Creek, and (b) Queanbeyan River, and theoretical and actual
frequency distributions of runs below the median for (c) Coranderrk Creek and (d) Queanbeyan River.
magnitude of deviations below the mean for the
Queanbeyan River are small compared with Coranderrk Creek, indicating a much more skewed distribution of annual flows in the case of the latter.
If this behaviour can be described using a theoretical model, then it is possible to predict the likelihood
of occurrence of runs of low flows of any given length.
This approach is commonly used in water resources
analysis but clearly also has ecological significance.
For the rivers used in the present study the maximum
run lengths are considerably above theoretical expectations from an independent series, but the differences
are not statistically significant at the 5% level:
Coranderrk has 2 · 4 years of consecutive low
flows, Thomson 1 · 7 years, Styx 2 · 6 years, Ord
1 · 6 years, Broken 1 · 8 years and Queanbeyan
1 · 8 years. Such long periods of low flow are more
likely to influence the structure of aquatic communities and their physical habitat. The longer periods
would be seen by Erskine & Warner (1988) as being
part of their drought- (and flood-) dominated
regimes, which they argue play an important role in
2003 Blackwell Publishing Ltd, Freshwater Biology, 48, 1147–1160
determining channel form and behaviour, at least in
the coastal streams of NSW, Australia.
Trends and quasi-cycles
Coranderrk Creek and the Queanbeyan River, rivers
with the lowest and highest CV, respectively, in our
seven station sample, are used to illustrate the quasicycles in these records. Again, as we move from the
least to most variable streams, the quasi-cycles
become stronger, with extended periods up to several
decades of low and/or high flows. Some of the cycles
are particularly dramatic. The Queanbeyan River
(Fig. 6b) exhibited only small variations during the
period from 1920 to the early 1940s and this was
followed by a dramatic shift to larger annual flows
such that the period from 1947 to 1966 yielded more
than double the mean annual flow of the previous
decades. Such major shifts over decadal time scales
are noteworthy features in Australian river flow
records and more likely to be of particular relevance
to the ecological functioning of these systems.
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T.A. McMahon and B.L. Finlayson
Fig. 6 Moving average plots of annual streamflow in (a)
Coranderrk Creek and (b) Queanbeyan River.
Although we have not formally analysed these
records for trend here, slight trends are apparent in
the case of both the Queanbeyan River and Coranderrk Creek although in opposite directions (Fig. 6).
Interestingly, the record for Wullwye Creek, which is
not regulated and was chosen more or less randomly
just to illustrate baseflow recessions earlier in this
paper, shows a dramatic downwards trend over the
period of record (Fig. 2a). Further discussion of this
type of behaviour in Australian rivers can be found in
Finlayson & McMahon (1991).
Regulated river systems
The Lachlan River at Cowra is regulated by the
Wyangala Dam and as the river is used as a conduit to
deliver irrigation water, there is no significant effect
on total flow but the seasonal regime has changed
somewhat (Fig. 7). Consistent with this use of the
river, the summer flows are higher post-regulation
and the winter flows are lower as the dam is being
refilled for the next irrigation season (Fig. 7b).
While Fig. 7 suggests that the effect of regulation
is marginal, the daily flow duration curves suggest otherwise (Fig. 8). There is evident a dramatic
Fig. 7 Annual streamflows (a) and mean monthly streamflows
(b) for the Lachlan River at Cowra.
difference in the flow duration curves before and after
regulation for the Lachlan River at Cowra and
Booligal, with the most significant differences being
at the low-flow end of the curves. At Cowra, for
example, the 95% low-flow post-regulation is three
orders of magnitude larger than the natural flow. At
Booligal prior to regulation, the river ceased to flow
for 25% of the time, whereas in the present regulated
regime the river rarely ceases to flow. More details are
given in Panta et al. (1999).
In the case of the Snowy scheme, water is diverted
out of the catchment. When the scheme was designed,
a small river valve was built into the Jindabyne Dam
large enough only to provide stock and domestic
water to riparian landholders living just downstream
of the dam. The first gauging station is 25 km
downstream of the Jindabyne Dam at Dalgety. Here
mean annual flow before the dam was 1187 · 106 m3
and after the dam this was reduced to 68 · 106 m3.
Daily flow duration curves for the Snowy at Dalgety
for the predam period (1950–67) and the postdam
period (1968-99) show that low flows have been
2003 Blackwell Publishing Ltd, Freshwater Biology, 48, 1147–1160
Low flows in Australian rivers
1157
indicator of baseflow contribution in natural streams
and in regulated rivers is an indication of the
persistence of low flow. For the Snowy at Dalgety,
this ratio has tripled in value following regulation,
from 0.22 to 0.66.
Discussion and concluding remarks
Fig. 8 Daily flow duration curves for Lachlan River at (a) Cowra
and (b) Booligal for regulated and unregulated conditions.
reduced by around an order of magnitude (at Q95
from 0.1 mm day)1 to 0.007 mm day)1; Fig. 9). While
the low flows are lower, they are nevertheless
persistent and this is a feature of this type of
regulation. Where flows are diverted out of the
system, releases are made to provide for riparian
users and these tend to be more reliable than the
natural low flows they replace. The ratio Q90/Q50 is an
Fig. 9 Daily flow duration curves for the Snowy River at Dalgety before and after the construction of the Jindabyne Dam.
2003 Blackwell Publishing Ltd, Freshwater Biology, 48, 1147–1160
In this paper we have described the characteristics of
low flows in Australian streams. There is reason to
believe that this pattern of flow variability, or something like it, has persisted for a very long time on the
Australian continent. In discussing the strategies used
by Australian freshwater biota in coping with low
flows, Lake (1995) points out that the ‘richness of
refugial strategies is in contrast to the paucity
elsewhere… Such adaptive strategies have had a long
evolutionary development in Australia.’ One of the
major drivers of climatic variability in Australia is
the ENSO effect and Kuhnel et al. (1990) have shown
the extent and magnitude of its impact on streamflow
variability. Nicholls (1989) points out that climate
variability in Australia is amplified by ENSO such
that variability is higher than in other areas of the
world with equivalent climates not influenced by
ENSO. He uses a range of animal adaptations to
climate variability to argue that much of the Australian biota is adapted to variability and that this
adaptation is more complete than in other areas of the
world. The conclusion can therefore be drawn that the
influence of ENSO in the Australian climate is of
sufficiently long standing to have influenced the
course of species evolution.
The influence of flow variability and low-flow
behaviour on fish distribution can also be seen at the
level of individual river systems. As pointed out
above, flow tends to become more reliable with
distance downstream (and increasing catchment area)
in a river system. A number of studies of fish
distribution in Australian river systems have pointed
to a downstream increase in fish species richness
(Lake, 1982; Hortle & Pearson, 1990; Gehrke & Harris,
2000).
Our discussion of the low-flow behaviour of
Australian rivers has not been exhaustive; there
are many techniques and procedures for analysing
low flow behaviour which we have not discussed,
particularly spell analysis (Donald, Nathan & Reed,
1999), which may be of particular interest to river
1158
T.A. McMahon and B.L. Finlayson
ecologists. Typical flow sequences consist of many
small low-flow events and few major ones (Fig. 5).
The probability of these events can be determined
from the flow record and for this reason we
encourage aquatic ecologists interested in studying
the impacts of low flows on in-stream biota to locate
their study sites near gauging stations with a long
flow record so that the periods they have studied
can be located in the context of the long-term
hydrological record of the stream. Where ecological
studies are located in ungauged reaches or catchments, it is possible to estimate characteristics of the
flow behaviour (R.J. Nathan, unpublished data;
Nathan & Weinmann, 1993; McMahon et al., 2002)
although this lacks the precision of real flow
records.
The difficulty in determining whether a significant low-flow period has started, and of predicting
how long and how severe it will be, has meant that
these periods are not well described ecologically
(Lake, 2000). Lake (2000) has argued that there is a
need to improve understanding of disturbance in
aquatic ecosystems, especially faced with global
climate change, which is predicted to have a
significant impact on the frequency of extreme
events (Whetton et al., 1993). While this may be
true, the identification of a climate change signal in
the Australian streamflow records has so far proved
elusive (Gan, McMahon & Finlayson, 1990; Chiew &
McMahon, 1993). Flow forecasting is becoming
available which will allow some level of prediction of impending low-flow periods (Chiew et al.,
2000).
Drought is a poorly defined word and many
discussions of drought get bogged down in unproductive attempts to define it. The point we have tried
to make here is that the flows of natural river
systems encompass a range of flow conditions from
extremes of high flows to extremes of low flows.
These are not disasters when viewed from the
perspective of the natural ecosystem. They may
cause major disturbances in those systems but any
communities or species unable to cope with that
magnitude and frequency distribution of flow events
would seem to have no place in the system in the
long term. Ecological processes in rivers are controlled by flow variability (Puckridge et al., 1998) and
low flows are an inherent part of the pattern of
variability.
Of much more significance in this context is the
impact of river regulation on low-flow behaviour.
Unlike changes driven by climate change, the impacts
of river regulation on flows are dramatic, usually
quite sudden, and clearly discernible in the records.
Even if the most extreme predicted climate change
scenarios were to eventuate, they would be small in
the scale of their impact and rate of onset when
compared with the changes that have already
occurred as a result of regulation. It is for this reason
that a better understanding of the impact of low flows
on river ecosystems is needed. There is little that river
managers can do to deal with climatically driven
changes to flows in unregulated systems but on
regulated rivers, where there are major changes
already in place, the capacity exists to manipulate
flow patterns so as to provide ecological benefits.
In our regulated rivers, we have replaced a natural
distribution of low flows with an artificial distribution
that lacks many natural low-flow events. In the
terminology of this special issue, some of the
‘droughts’ have been removed from the regulated
rivers and replaced with what we term ‘antidroughts’ – periods that have higher flows than
would be expected naturally. Furthermore, in some
periods, high natural flows have been replaced by
periods of considerably lower flows as a result of
water extraction or stream regulation. We contend
that ‘drought’, however defined, is not a problem for
the management of Australian freshwater ecosystems,
though there is clearly scientific interest in understanding the way these systems cope with such events
and recover from them. The issues for management
are the ‘anti-droughts’ and induced droughts that
now characterise most, if not all, of our regulated
rivers.
Acknowledgments
We are grateful to our colleagues Drs Senlin Zhou and
Murray Peel who processed data for us and to the
Department of Natural Resources (Queensland), Ecowise Environmental Ltd, Melbourne Water Corporation, Theiss Environmental Services and the Water
and Rivers Commission (WA) for providing streamflow data. The final form of this paper owes much to
the efforts of Drs Paul Humphries and Darren
Baldwin of the Murray-Darling Freshwater Research
Centre, Albury.
2003 Blackwell Publishing Ltd, Freshwater Biology, 48, 1147–1160
Low flows in Australian rivers
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