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1. Understanding the Basin and its
Dynamics
John Williams
Introduction
The competing tensions between water extraction for immediate human use and
water essential to the long-term ecological function and sustainability of the
rivers and groundwater systems in the Murray–Darling Basin (MDB) sit at the
centre of public policy debate on water reform. Yet it is much more than this.
The people of the Basin are faced with the enormous challenge of transforming
themselves into more resilient communities. This requires managing and
reconstructing the conict between the climatic and biophysical realities of
the Basin and the earlier private and public policy aspirations of the European
settlers that have dominated for the past 150 years. Water reform is, then, a
social process by which communities work to align land use and economic
industries so that they work more in harmony and within the capacity of the
hydrological and ecological processes operating in the landscape and thereby
are able to harvest a wider range of ecosystem services than they currently
do. For water reform to be embedded in such a process, it is critical, however,
that the dynamics of the biophysical processes operating within the geological
and geomorphic form of the ancient Basin be fully appreciated and understood.
Without an understanding of the Basin’s form and functionality, water-reform
implementation will probably solve one problem while creating several others.
This must be done against a backdrop of climate change which is impacting on
the very high climatic variability that over the past decade has seen a severe,
nine-year drought and a year of large oods. These extreme events — both very
wet and very dry — are what characterise the Basin of the past and they can
be expected to be an increasing part of a climate-change future (Francis and
Hengeveld 1998; Min et al. 2011; Pall et al. 2011).
The biophysical nature of the Basin’s rivers and groundwater systems, coupled
with this climatic variability and change, calls for water reform to be implemented
in innovative ways that will test the fabric of our society and stretch our scientic
knowledge to the limit. For it is management of greater extremes — the oods
and the droughts — in accord with the ecological functional requirement of
these rivers and the increasing demand for water extraction to satisfy human
need that call attention to the need for radical water reform in the light of failure
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of current policy and practice. Present-day problems that confront the Murray–
Darling Basin (MDB) can be related to the way the societal narrative, cultural
values and knowledge have clashed with the climatic and geological history of
the ancient Australian continent.
The MDB heritage we have today is the result of an unfortunate coincidence
between human action and the set of geological and climatic forces that formed
the Basin. Human activity over the past 150 years has exacerbated geological,
hydrological and ecological processes driven by a history of changing and
highly variable climate through time and across the Basin (Williams and Goss
2002). The Basin is ancient. What we see today bears residual features and the
overprinting of a long history of climate changes, involving many sequences
and oscillations between very humid and extremely arid environments.
Clearing of forest and woodland vegetation, in conjunction with the application
of irrigation water, has produced in less than 200 years a change in groundwater
equilibrium and river ow regimes that in many ways mimic the changes that
have resulted in the past from climatic oscillations (Bowler 1990). These changes
brought by Europeans through grazing, clearing of forests and woodlands and
the development of irrigation have resulted in the return of conditions that
existed about 18 000 years ago — once again we have high saline water tables
discharging to rivers. The ow regimes of the rivers have been drastically
changed so that the oodplain ecosystems that drive much of their ecological
function are disconnected, and the ows to ush out salt and refresh the
system are far too infrequent. Innovation, problem solving, and the managerial
capacity of farmers have sustained an impressive productivity growth through
the twentieth century, particularly in cereal production. Great wealth from the
production of food and bre has been fundamental to the wealth and wellbeing
all Australians now share. The Basin has yielded much and has a heritage of
place and natural history that is very important to Australians everywhere.
The Basin is our heartland and holds symbols of our rural heritage, upon which
Australian identity has been built. But now we see much of what has been built
under threat from economic, social and environmental change and decline.
Water reform in the Basin is cast against this background of the ancient bio-
geophysical processes that must be understood and managed while nding new
expressions and narratives within which the Basin’s communities recast and
rebuild more resilient futures. This chapter seeks to examine the nature of the
Basin’s geology, hydrology and ecology, and to weave into this the interaction
of the human aspirations, values and visions that have shaped our communities
and that generate the human and physical heritage within which water reform
must take place.
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Biophysical Background
Geological History and Basin Structure
The Murray–Darling Basin’s streams and rivers sit in a shallow basin, which is
very old, very at, contains large stores of salt, and with respect to groundwater
is very nearly blind in that it has no outlet to the sea.
In geological terms, the Basin has a very ancient foundation. The oldest rocks
(pre-Cambrian), which outcrop in the western margins, date back at least some
600 million years. Most of the Basin has a basement of ancient (Palaeozoic age
of 230–540 million years) rocks that were eroded to a palaeo-plain. Over this
ancient platform, sedimentary rocks formed basins during the Mesozoic age
(some 60–250 million years ago) in the case of the Great Artesian Basin (GAB),
and, later, during the Caenozoic age (less than 60 million years), the Murray
Basin was laid down (Ollier 1995). These two basins are separated by a system of
tectonic warp axes that corresponds to the drainage divides. These are the two
major sedimentary groundwater basins over which the Murray–Darling Basin
catchment is located (Ollier 1995). Within both basins there has been broad
down-warping, subsidence or sinking of these sedimentary rocks. This has
resulted in sedimentary rocks inlling a low-lying, saucer-shaped depression
(Evans et al. 1990), rimed and underlaid by folding and partly metamorphosed
ancient basement rocks. These ancient (Palaeozoic) rocks now form the subdued
mountain ranges around most of the Murray–Darling Basin — apart from the
south-western rim, where concealed basement rocks just beneath the surface
form the Padthaway Ridge. This separates the MDB from the Southern Ocean.
Whilst the Murray Basin is very large, the sedimentary rocks are relatively thin.
The maximum thickness is 600 m, found over the region of most subsidence,
while at the margins of the Basin the thickness of the sediments is less than
200 m. They form a pancake-like veneer over the older basement rocks (Evans
et al. 1990). Because the sedimentary rocks are quite thin, the Basin has a
relatively small capacity for groundwater storage. This saucer-shaped structural
conguration with subsidence just south of the centre, covered by a thin layer
of sediments, has important implications for the nature of the Basin and the
way surface and groundwater must be managed. The MDB is essentially a closed
groundwater basin within which groundwater drainage is directed internally
towards the central subsidence and thicker sediments, rather than towards the
side where the Murray connects to the sea.
Because the Basin is blind and because the sediments in which groundwater can
be stored are relatively thin and thus oer a relatively small storage capacity as
the sedimentary rocks are largely water saturated. Thus, there is little capacity
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in the groundwater system for the storage of additional recharge. Thus, if the
groundwater systems receive increased recharge as they have since European
settlement, the additional recharge cannot escape. The water tables must rise.
These features all point to a most important conclusion. Minimal groundwater
recharge will drive a rapid water table rise, and because the Basin is essentially
blind and therefore has a small discharge capacity, the response of groundwater
levels to reductions in recharge rate will be very slow. When the additional
recharge is reduced, the water table fall will be very slow largely determined
by the small discharge capacity via the Murray or by evaporation from land-
surface discharge regions in the depressions and lakes of the landscape. Thus,
groundwater systems can be lled easily, but must empty and discharge very
slowly. This is a most unfortunate fundamental fact about the MDB and it is
essential to understand that the Murray River needs to have large ushing ows
to carry salt to the oceans where it came from (Evans et al. 1990).
About 40–60 million years ago, the central area of the MDB subsided as the
eastern highlands were uplifted. This subsidence formed two distinct regions of
sedimentation and later groundwater accumulation. The southern area is known
as the Murray Groundwater Basin, which is not fully synonymous with the
catchment but it does underlie a great deal of it. The northern area, over which
the Darling River and its tributaries now ow, is the southern part of the Great
Artesian Basin. The climate of the early Tertiary (40–60 million years ago) was
very much wetter than at present and the Murray Basin then contained large
swamps and bogs, and thick sediments that were laid down in broad valleys.
With increasing subsidence and eastern highlands uplift, stream dissection and
incision in the highlands resulted in sand and gravel deposition in fans as the
rivers entered the plains.
During the Miocene (26–7 million years ago) the sea level rose relative to the
land, and the inland sea covered the south-western corner of the Murray
Groundwater Basin. Marine materials were deposited in sand sheets. In the
past two million years, the sea retreated, leaving a succession of stranded beach
ridges and relic coastlines. Following the sea’s retreat, a huge freshwater lake
developed, as there was a blockage across the Murray. During the Quaternary
glacial period about two million years ago, the climate became very arid, with
dry and windy conditions prevailing. Another set of dunes — this time Aeolian
— was built by the wind action. In the past 30 000 years, a thick blanket of ne
alluvium has been laid down over coarse sediments in the old bedrock of the
central area of the Murray Basin. A similar process took place in the Darling.
The sea once occupied the Mallee and most of the Murray Basin, extending to
Balranald in New South Wales, with thin reaches stretching to Kerang, Victoria,
at its peak, before retreating from about three to four million years ago. Whilst
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the salt associated with this intrusion has long since left the Basin through
leaching, the retreat of the sea established the ultimate gradient and outlet for
the Basin and the modern (past 500 000 years) landscape development of the
Basin. This retreat of the sea had a number of other important consequences.
Not only did the climate dry from the extensive wet rainforest period (12–30
million years ago), but earth movements dammed the Murray outlet to result
in the huge Lake Bungunnia. The lake formed about 2.5 million years ago and
continued to exist for about two million years, until about 500 000–700 000
years ago when the outow point was deepened suciently to drain the lake
and permit the Murray River to cut a deep gorge through earlier sediments to
provide an outlet to the sea.
Modern Features of the Landscape, Waterways and
Vegetation
Within the time frame of the Murray–Darling’s origins, there are four factors
(Evans et al. 1990) that control the modern landscape features
1. the low level of tectonic activity over long periods
2. a strong east–west gradient of increasing aridity
3. the marine inuence on the south-western corner of the Basin
4. the prevailing south-westerly winds.
Compared with other continents, Australia has been remarkably free of volcanic
or mountain-building activity in recent time. While the Australian continent
has drifted north from Antarctica over the past 60–80 million years, very minor
changes in topography have occurred (Bowler 1990; Ollier 1995). The Great
Dividing and Flinders ranges and the extensive plains between were already
present from at least 20 million years ago. These ranges are very subdued
features compared with the mountains of other continents. The late-Quaternary
(past million years) history of the Murray–Darling Basin has been of minor
tectonic movement and the evolution of landforms under increasingly arid
conditions (Wasson 1987). The major subdivisions such as the Eastern Upland,
Cobar Plains, Murray and Upper Darling basins have largely remained as they
are, but within these, landform changes have occurred to produce the rivers,
dunes, alluvial plains and slope colluvium as they are today (Wasson 1987).
The closed nature of the Murray Basin results in a strong interaction between
groundwater and surface water. In the west, the River Murray is an ecient drain
providing the natural pathway for removing groundwater and its dissolved salts.
In fact, the lower sections of the river have always been a salt drain. The changes
