5.3 Floodplain ecosystems

Floodplains are extensive in the Cooper GBA region and provide habitat for many species, including threatened plant species such as braided sea heath, Schleroleana walkeri and Xerothamnella impervia. Water regimes – particularly the frequency, extent and duration of flooding – control the structure and dynamics of floodplain ecosystems. Flooding provides a significant boost to productivity in the region – in particular, the pastoral industry, which grazes natural pastures on floodplains.

Floodplains are the outcome of a complex set of interactions between flow, sediment regimes and the character of the valley trough (Thoms and Parsons, 2016). Floodplain environments in the Cooper GBA region are conceptualised as mid and lower catchment floodplains. In these environments, energy associated with flows is lower, valleys tend to be wider, and there are high rates of sediment deposition associated with large slow-moving floods that have contributed to the development of very large, wide (greater than 60 km) floodplains. The spatial extent of the floodplain vegetation extent and condition endpoint in the Cooper GBA region is 25,283 km 2 (about 19%), of which almost 30% overlies areas that are prospective for the development of unconventional gas resources ( Figure 12 ). Existing disturbance, including existing infrastructure for agriculture, tourism and oil and gas development (393 km 2) and seismic surveys (145 km2), affects about 2% of floodplain areas in the Cooper GBA region (Geological and Bioregional Assessment Program, 2021m).

The frequency and duration of flooding is an important characteristic of floodplain environments as it controls vegetation growth and the potential growing period including flowering and seed set. Floodplain ecosystems are less diverse than riparian, wetland or dryland ecosystems, containing just 6 regional ecosystems in Queensland (Queensland Herbarium, 2018a) and 5 mapped ecotypes in South Australia (Hobbs et al., 2017). Vegetation in these ecosystems is characterised by low, often sparse or open, chenopod and other shrublands, and low woodlands. Floodplain vegetation provides habitat for protected fauna and flora prioritised for the assessment, such as the grey grasswren, and plants such as braided sea heath, Scleroleana walkeri and Xerothamnella parvifolia.

Cooper Creek has an extensive floodplain containing braided channels that lead to long travel times and large transmission losses, with over 75% of the water flowing into the system from the Barcoo and Thompson rivers lost by the time it reaches the Queensland – South Australian border (Jarihani et al., 2015). Transmission losses support terrestrial vegetation and pasture used for grazing, as well as filling lakes and recharging shallow groundwater. Cooper Creek floodplain supports an agricultural grazing industry worth $65 million per year with single large floods increasing the value up to $150 million (Phelps et al., 2007). Satellite monitoring of water use was used to determine to where Cooper Creek floodwaters go ( Box 9 ).

Box 9 When Cooper Creek floods, where does all the water go?

To trace where over 75% of Cooper Creek floodwaters go before they reach the Queensland – South Australian border, remotely sensed measurements of actual evapotranspiration were separated into open water, riparian, floodplain and rain-fed areas ( Figure 14 ). Water use by open-water areas represent direct evaporation losses from river channels and permanent waterholes. Riparian vegetation uses a combination of water sources derived from rainfall and infiltration from bank recharge from river channels and permanent waterholes, as well as overbank flooding. Floodplain vegetation relies on soil moisture from rainfall and overbank flooding. Outside of the floodplain, dryland vegetation relies solely on rainfall.

For the period 2000 to 2018, 77% of actual evapotranspiration losses were from floodplain areas, associated with overbank flooding where it supports pasture growth. The remaining 23% of actual evapotranspiration losses were from fringing riparian vegetation or as evaporation from open water in waterholes. In other words, high ecological value wetlands, waterholes and fringing riparian vegetation account for less than a quarter (23%) of total water use on Cooper Creek floodplain.

FIGURE 14 Classification of open water, riparian, floodplain and rain-fed areas based on water use

Based on water use, there is one obvious dominating floodplain in the Cooper GBA region, stretching south-west of Windorah in Queensland almost to the South Australia border near Innamincka. This floodplain also stretches north-east of Windorah but not as far as its south-west extension. There are secondary floodplains, one in the far east of the Cooper GBA region, near Quilpie in Queensland, and the other north of Moomba in South Australia.

Data: Geological and Bioregional Assessment Program (2020d)

Element: GBA-COO-3-685


Overland flow obstruction can change flow paths on the floodplain and in small flood runners. This can cause reduced flooding, which is of ‘potential concern’ in up to 6% of the floodplain vegetation areas. Reduced flooding in these areas could also affect less than 4% of the agricultural productivity , Channel Country SEA condition and Coongie Lakes Ramsar wetland endpoint areas, as well as habitat for the grey grasswren and the Australian painted snipe, and areas where braided sea heath and Sclerolaena walker are mapped.

Direct disturbance at the surface is of ‘potential concern’ in up to 29% of floodplain areas (refer to the soil compaction , vegetation removal and vehicle movement node descriptions). Indirect impacts that could spread beyond the resource development area, disrupting natural processes, are of ‘potential concern’ in up to 30% of floodplain areas (refer to the competition and predation , ecosystem burning , and habitat degradation, fragmentation and loss node descriptions).

Localised stressors, such as dust generation , soil compaction , overland flow obstruction and vegetation removal , are managed by site-based protocols and controls. Regional-scale stressors include those that may lead to increased ecosystem burning, such as vehicle movements that can promote the spread of fire in the landscape, or competition and predation, such as the proliferation of the artificial water sources that support and can facilitate the spread of invasive herbivores and predators; in the region.

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