Geoscience Reference
In-Depth Information
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floods. Floodplain wetlands vary from lakes
and billabongs to incised creeks, flood-runners,
anabranches and claypans, supporting allied species
of crayfish ('yabbies',
Cherax destructor
), freshwater
mussels (
Velesunio ambiguus
)andturtles(
Chelodina
longicollis
). The separation of channel and wetland
species is blurred somewhat by the presence of
impoundments, particularly weir pools, where
river and floodplain species may cohabit (Walker
et al
., 1992). These species are accompanied by
diverse waterbirds, fish, frogs, reptiles, mammals
and other fauna and flora (Jennings, 2009). All are
reliant on small to medium floods which inundate
all or most of the floodplain. A key mechanism
is the replenishment of soil moisture and shallow
water tables by these flood waters, to provide
water reserves during dry periods of low flow. The
natural floods and droughts are reflected in 'boom
and bust' cycles of recruitment.
The ecological consequences of the changed
regime have become increasingly apparent in the
last 30 years, especially in the Lower Murray,
South Australia. In 1990 there was clear evidence
of declines in the range and abundance of
some native flora and fauna, increases of alien
species (particularly common carp,
Cyprinus carpio
),
salinization of soil and water, and degradation
of wetlands (including Ramsar-listed wetlands at
Chowilla and the Coorong, Lower Lakes and
Murray Mouth). Some of these effects were
described 20 years ago at the conference on
River
Conservation and Management
in York, 1990 (Walker
et al.
, 1992).
Since 1990, degradation has increased, despite
major investments by governments in management
by local communities and state and federal agencies
(Walker, 2006), and new threats are emerging,
including exposure of acid sulphate soils, river-
bank slumping and the need for dredging at the
river mouth (Kingsford
et al.
, 2011). The decline
has been intensified by severe, widespread drought
in 2002-2010. In 2009, the
Sustainable Rivers
Audit,
a Basin-wide assessment of river ecosystem
'health', showed that 20 of 23 valleys in the Basin
were in 'poor' or 'very poor' health (Davies
et al
.
,
2010). The condition of the Lower Murray, in
particular, was rated as 'poor'.
Under the Australian Constitution, sovereignty
over Murray-Darling Basin water resources is
vested in four states (Queensland, New South
Wales, Victoria and South Australia). There have
been attempts to transfer control to a central
authority, facilitated by a new Water Act (2007).
This requires consumptive use to be within
prescribed, long-term sustainable limits, intended
to reserve sufficient water to maintain key
ecological functions. The Act created the office of
the Commonwealth Environmental Water Holder
to manage environmental flows, and provided
for the establishment in 2008 of a new Murray-
Darling Basin Authority (MDBA) with greater
responsibility for environmental management, but
still without complete control over flows in
the system. In October 2010, the MDBA began
community consultations on a 'Guide' to a draft
Basin Plan that proposed 'Sustainable Diversion
Limits', intended to define a new balance between
agricultural and environmental needs (MDBA,
2010) (http://www.mdba.gov.au). The initial Basin
Plan proposals were met by strong public criticism,
particularly in regard to the likely social and
economic impacts of reduced water allocations for
irrigation, and at present these are under review.
The coincidence of over-allocation of resources
and persistent drought in 2002-2010 contributed
to a long-term crisis of water supply in the Murray,
particularly the Lower Murray. Although there has
been significant respite since September-October
2010, with high seasonal rainfall and flooding river
flows, the underlying problem of over-extraction is
not resolved. Concerns are deepened by projected
declines
in
rainfall
and
runoff
associated
with
global climate change (CSIRO, 2008).
A developing crisis
Retrospective
The Murray has been regulated since the late 19th
century, with escalations after 1920 and again
after 1950 (Maheshwari
et al
.
,
1995), and is now
controlledbymorethan100damsandmanyweirs,
