DARLING RIVERINE PLAINS BIOREGION Background Report

16/08/02 Darling Riverine Plains Bioregion Background Report
the wetland habitat. The allocation is generally insufficient to stimulate breeding so is timed
to occur with natural triggers such as rainfall events (Kingsford & Thomas 1995).
The Macquarie Marshes are one of the largest inland semi-permanent wetlands in southeastern
Australia (Map 20). They cover approximately 200 000 hectares when flooded and
are about 100 km long and 30 km wide. They have a flow through system rather than being a
terminal basin. Water quality is increased by the anabranch flow, which acts as a filtration
system and promotes a diverse and productive biological system (Wolfgang 1998). In 1818
when Oxley was exploring the area, the Macquarie Marshes were probably at least twice the
size they are today. In 1863 a newspaper reported that ‘The whole of the country between the
Merri Merri [Creek, 24 to 41 km east of the Macquarie River] and the Macquarie [River] was
one sea of water with scarcely a dry foot of ground’ (quoted in Kingsford & Thomas 1995).
The 1874 flood was reportedly even larger, but the largest on record was in 1955. These
marshes contain extensive reed beds, Eucalyptus camaldulensis (red gum) swamps, open
lagoons and Muehlenbeckia florulenta (lignum) covered gilgaied floodplains.
A range of significant Aboriginal sites have been recorded within the Macquarie Marshes
including oven mounds. The Macquarie Marshes is the only area where oven mounds have
been recorded outside the Murray Valley. Aboriginal culture in the area was influenced by
the changing water regimes of the Macquarie Marshes (NPWS 1993).
The impact of rising salinity levels will be localised in areas where evaporation concentrates
the salts into pools or depressions. At the point where the Macquarie River enters the
Marshes, the expected average salinity for the year 2100, is 2 110 EC, well above the
acceptable ANZECC (1999) trigger levels (DLWC 2000). Monthly salinity data from the
Macquarie River at Narromine indicate that in 1998 the river had a 4% chance of exceeding
the 1 500 EC threshold but extrapolation of this data indicates that this threshold will have a
23% chance of being exceeded in 2100, mostly during low flow periods. This could have a
serious effect on in-stream biota and recruitment of biota into and between wetlands. This
would result in a loss of species diversity and declines in abundance of species (MDBC
1999).
Gwydir River Catchment and Gwydir Wetlands
The Gwydir River is regulated by Copeton Dam which supplies irrigation water for the cotton
industry. The Gwydir Valley is the largest cotton producing area in Australia and water
supplies from Copeton Dam are over allocated. The dam rarely has sufficient supplies to meet
demand resulting in significantly reduced water flows in the river and a subsequent increase
in the usage of off-river storage to capture water during periods of high flow (EPA 1997).
Downstream of Moree the Gwydir divides into two major arms, the Gingham in the north and
the Lower Gwydir in the south (Figure 2.16). Towards the end of these watercourses the
water forms a delta with intermittent and semi-permanent wetlands. The distribution of flows
into these wetlands is naturally dynamic. Prior to 1936 most small floods flowed into the
Lower Gwydir and only the large flows reached the Gingham. This has changed with the
development of the Gwydir Raft near the Tyreel Regulator. The raft is an accumulation of
deposited timber debris and sediment which began to form in the early 1900s and effectively
blocked the original river channel. The raft has continued to steadily accumulate and now
extends for about 30 km. By the mid-1940s water flowed more readily to the north, increasing
flows into the Gingham system (McCosker & Duggin 1993).
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