DARLING RIVERINE PLAINS BIOREGION Background Report

16/08/02 Darling Riverine Plains Bioregion Background Report
from the South Eastern Highlands, Brigalow Belt South and South Western Slopes
Bioregions. These rivers are predominantly fed by winter rainfall and flooding is more likely
to occur in winter-spring.
Under a natural flow regime, the Barwon-Darling system receives an average of 35% of its
water from the border rivers (MacIntyre, Dumaresq, and Boomi), 25% from the Namoi River,
20% from the Condamine-Culgoa, 5% from the Macquarie-Bogan and 10% from the Gwydir.
Modelling by DLWC has estimated that flows from the Namoi have been reduced by 70%,
from the Condamine-Culgoa by 45% and from the Border Rivers by 45%. Total irrigation
diversions from the Barwon-Darling system and tributaries exceed the natural median flow at
Menindee (Thoms et al. 1995).
The highly variable rainfall within the bioregion is reflected by flow variability, a feature of
the Darling-Barwon system. Annual variations in flow range from 0.04% to 911% of the
long-term mean, and discharges from the major rivers in the basin are highly skewed with a
large proportion of average flows being recorded in very wet years and during major floods.
This river system mostly floods in summer but flow is intermittent downstream of Louth
(Lloyd et al. 1994). The Macquarie River has the most variable flow of 18 selected rivers in
the Murray-Darling Basin. Annual flows vary from 2% to 940% of the mean, and only about
one third of the flow entering the Macquarie Marshes flows through to the end of the system,
except during floods (Kingsford & Thomas 1995; DWR 1991).
Flooding is an important natural process that replenishes the floodplain with water and allows
the large scale cycling of nutrients and biota. Inundation of the floodplain boosts invertebrate
populations, breeding activity in waterbirds and fish, initiates growth and regeneration of
riverine vegetation and creates extensive areas for colonisation by aquatic plants. Floodplains
generally contain a mosaic of habitats ranging from those that are permanently wet to those
that are rarely inundated. These all have different defining attributes and support distinctive
communities. They are critical habitats for the biota in the floodplain and rivers (Lloyd et al.
1994).
Flood behaviour in the Barwon-Darling is complex with no two floods being the same and
with a variety of localised forms of flooding. The relative contribution of the tributaries to
this river system varies between floods. Mostly, the largest inflow is from the Namoi River
but in the 1990 floods, the greatest inflow was from the Bogan River (most of this water was
carried overland to the Bogan from the Macquarie River in the flood period). The Castlereagh
River carries the Namoi and Barwon overflows as well as runoff from its own catchment to
the Macquarie that then flows into the Barwon River. In 1998 extensive floodplain inundation
occurred along the Darling between the Bogan and Warrego River junctions (DLWC 1999).
Wetlands
There are 4 810 defined wetlands covering 533 439 hectares in the DRP (Table 2.2).
Wetlands are highly significant for the maintenance of biodiversity, regularly supporting
significant breeding populations of waterbirds. They are also important refuges when drought
occurs in other parts of arid and semi-arid Australia (Pressey, 1988). Parts of three wetlands
within the DRP have been recognised by the Convention of Wetlands of International
Importance (Ramsar Convention): the Gwydir wetlands west of Moree (Gingham
watercourse and Lower Gwydir wetland), the Macquarie Marshes north of Warren and the
Narran Lake system north-west of Walgett. In addition to these wetlands, the Menindee
Lakes, Talyawalka Anabranch and Teryawynia Creek, Morella watercourse-Boobera lagoon-
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