DARLING RIVERINE PLAINS BIOREGION Background Report

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16/08/02 Darling Riverine Plains Bioregion Background Report (Keyte 1994) Figure 2.16 Lower Gwydir Wetlands The construction of Copeton Dam has resulted in a substantial reduction in the number of major floods through this system and stabilisation of the raft. A new raft has formed on the Gingham Channel a few kilometres downstream of the head of the old raft and this may affect the distribution of future floodwaters. The formation of these rafts (accumulations of timber, debris and sediment) is thought to result from the combination of human activities such as the clearing of the upper catchment and natural depositional processes. Currently, with flows of between 2 500 and 40 000 ML/day, 70% of the Gwydir flows into the Gingham and 30% into the Lower Gwydir. Flows of 40 000 ML/day or more produce over-bank flows and the water divides evenly into each watercourse (McCosker & Duggin 1993; Keyte 1994). Significant flooding of the watercourse is irregular and occurs most frequently in January and February. Secondary floods occur in winter. Long term residents recall the Gwydir River entering the wetlands ‘until the River emptied’ but most of the flow is now diverted for irrigation and domestic use before reaching the wetlands. Before regulation the Gwydir wetlands would have received flows sufficient to inundate their core and watercourse wetlands (20 000 ha) in 192 months during a 93 year period or 17% of the time and large flows would have been a maximum of three years apart. With regulation, flows large enough to fully inundate the wetlands occur only 5% of the time, a 70% reduction (McCosker & Duggin 1993; Keyte 1994). The Gwydir wetlands are a flow through system and are likely to be affected by salinity in a similar way to the Macquarie Marshes. The estimated average salinity in the Gwydir as it enters the wetlands is expected to rise to 740 EC in 2100, and the probability of exceeding the threshold of 1 500 EC (this threshold is considered to be where direct biological effects are 48
16/08/02 Darling Riverine Plains Bioregion Background Report likely to occur in river, stream and wetland ecosystems) is likely to change from 3% in 1998 to 12% in 2100 at Pallamallawa. The impact of periods of high salt levels will be similar to that predicted for the Macquarie Marshes, with possible salt scalding where the saline waters become highly concentrated (MDBC 1999). Macintyre, Namoi and Castlereagh River Catchments The Macintyre River (with tributaries known as the border rivers) receives most of its 760 Gl per year of flow from its catchment within New South Wales. Pindari dam in NSW, and Coolmundie and Glenlyon dams in Queensland are major water storages in the Macintyre catchment. About 222 GL per year are drawn from this river, less than for the Gwydir (300 GL/year) and Namoi Rivers (248 GL/year) (DITAR 1996). The Morella Watercourse, Boobera Lagoon and Pungbougal Lagoon wetlands are the remnants of a previous course of the Macintyre River. They are permanent water bodies and provide important habitat for many waterbirds. Boobera Lagoon has great significance to the local Aboriginal people, the Kamilaroi, and is considered to be an exceptionally important Aboriginal site in south-eastern Australia as the local Aboriginal people, the Kamilaroi, believe that Boobera Lagoon is the resting place of the rainbow serpent. Boobera Lagoon plays an important role in the teaching of Aboriginal culture (Environment Australia 2001d). The Namoi River is dammed by Lake Keepit, constructed in 1960. Trials for cotton production began in the Narrabri / Wee Waa area soon after this. Cotton is now extensively grown in the area and 54% of the natural median flow of this river is used for irrigation. This regulation has resulted in the loss of natural variation in flow levels, with periods of low flow very rare, and a reduction in overall flow volume (Inland Rivers Network 1999; EPA 1997). The Castlereagh River rises in the Warrumbungle Ranges in the Brigalow Belt South bioregion, an area of comparatively high rainfall. Most of the upper catchment is sandy and, along some of its length, the river flows through the sand to reappear as pools further downstream. This is one of the least regulated rivers in the bioregion, with no dams and only a few weirs and limited use for irrigation (EPA 2001). 2.4.2 Groundwaters There are two sources of groundwater within the bioregion, the Great Artesian Basin and several alluvial aquifers. The Great Artesian Basin The Great Artesian Basin (GAB) aquifer is a multi-layered, confined system with a large synclinal structure. Much of the artesian water in New South Wales originates in Queensland, but a significant amount is contributed through the Pilliga Sandstones, the primary intake area in New South Wales (Watkins & Meakin 1996, Figure 2.17). The artesian water has total dissolved solids of 500 to 1 500 mg/l, making it suitable for stock and domestic uses but the sodium-bicarbonate-chloride chemistry of this water makes it unsuitable for irrigation (NWCMC 1996). 49
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Appendix A: Mean Monthly Rainfall a
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Major Rangetype Rangetype Physiogra
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Appendix C : Native Flora Other Tha
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APPENDIX E: Fauna Other than Those
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Common 1983 1984 1985 1986 1987 198
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Taxa No Records Dataset Class Arach
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