Geoscience Reference
In-Depth Information
river-bank levees and other structures, nearly all
on tributary rivers. Flows in the 830 km Lower
Murray, below the Darling junction, are controlled
by 10 weirs, levees, wetland regulators and
barrages across the multiple channels connecting
Lake Alexandrina to the Murray Mouth (Walker
et al
., 1992; Walker, 2006). The construction of
dams, weirs and barrages was encouraged by a
dry climate in the first half of the 20th century
and, paradoxically, increased again after flooding
in the 1950s and 1970s. With the return of
drier conditions in the mid-1990s, Basin water
resources in recent years have been well below full
storage capacity. River flows have been confined
to the main channel, with stable water levels in
weir pools, and no effective inundation of the
floodplains since a small flood in 1996.
In 1988, the River Murray Commission,
essentially a flow-regulating authority, was
supplanted by the Murray-Darling Basin
Commission (MDBC), with limited responsibilities
for environmental management. At that time,
resource management was concerned mainly
with storing and distributing water for irrigated
agriculture, but there was growing concern over
changes in riverine environments and declines
of native flora and fauna (Walker, 1992). In
particular, there was a recognition that the
naturally variable regime of low, medium and high
flows is a pre-requisite for most native species,
and that regulated flows favour only some species,
including alien species such as carp. Thus, variable
flows are needed to maintain the spatial and
temporal diversity of the physical environment,
and thereby to maintain biodiversity. In the
1990s, before the drought, the Lower Murray
had experienced a two to three fold reduction
in the frequency of over-bank flows compared
with unregulated conditions (Sharley and Huggan,
1995). Floods were about one-third of natural
volumes; they occurred less than half as often
and lasted for about one-quarter as long. The
effects of reduced flooding are compounded by
salinization of soil and water associated with rising
groundwater, land clearance and irrigation (Jolly
et al.
, 2001).
From 1990, questions over environmental and
management
(Table 29.1). Expert panels were established to
identify 'environmental flow' needs in the Murray
and Lower Darling rivers, including the Lower
Lakes (Jensen
et al
., 2000; Thoms
et al
.
,
2000;
Jones
et al.
, 2002). The panels recommended water
management to restore key elements of the natural
flow regime, including over-bank flows in spring
and summer, and flows to keep the Murray Mouth
open for fish passage and to maintain mudflats for
migratory wading birds.
The last decade
Decline of woodlands
Recruitment to populations of some native plant
and animal species has faltered, leading to declines
in range and abundance and, in some cases,
local extinctions (Walker, 2006). One barometer
of change is the floodplain woodlands, dominated
by the iconic river red gum and black box. The
condition of river red gums especially has declined,
as they occupy habitats that were once flooded
every 2-3 years but now have not been watered
since 1996 (George
et al.
, 2005; Jensen
et al.
,
2008a,b; Williams
et al.
, 2008). The signs of stress
become apparent after 5-8 years without water
(Wen
et al
.
,
2010). As the subsurface freshwater
lenses have been exhausted, saline groundwater
has entered the tree root zones. The stressed trees
release up to 100 times less seed than healthy trees
(Jensen
et al.
, 2008a, b), and there is insufficient
surface soil moisture for seedlings, which take at
least 2 years to develop sinker roots (George
et al
.,
2005).
In parts of the middle and lower Murray Valley,
up to 90% of eucalypt trees have declined or
died since problems were first reported in 1990
(Margules and Partners
et al
.
,
1990; MDBC, 2003,
2005). In these areas the incidence of stressed trees
increased from 52% in 2002 to 76% in 2004. At
Chowilla, South Australia (Figure 29.1), stressed
trees increased from 54% in 2002 to 89% in 2004,
with river red gums most affected (MDBC, 2005)
(Plate 26).
The prospects for over-bank flows in the next
2-3 years depend on rainfall and flow
management. Although intense regional rainfall
events in late 2010 increased soil moisture and
issues
encouraged
new
research
