About the document
This report is the product of five years of monitoring and evaluation in the Gwydir River System under the Commonwealth Environmental Water Office’s Long Term Intervention Monitoring (LTIM) Project. The report evaluates the contribution of Commonwealth environmental water to environmental outcomes in 2018-19 and the previous four years since 2014-15.
The Gwydir River System LTIM Project identified a number of key environmental outcomes achieved through the use of environmental water since 2014-15, including:
- The delivery of environmental water contributed to consistent improvements in water quality.
- A multi-year wetting and drying management strategy is employed to maintain and improve the condition of the wetlands and rivers within the lower Gwydir system. During the LTIM Project, this strategy helped to sustain the ecology of the system, with clear benefits from environmental water. Environmental water was used to provide wetland inundation, stimulate productivity and maintain aquatic refuges in the channels of the lower Gwydir during dry times. Flow events through the system, some containing environmental water, maintained and improved water quality, with thresholds in discharge identified that elicit a different response of pH, conductivity and dissolved oxygen.
- The delivery of environmental water within the Gwydir Selected Area supported a range of habitats, that supported a large variety of waterbirds.
- Waterbirds took advantage of the increased resources following flooding, occupying a range of wetland habitats. High average species richness, density and diversity of waterbirds were observed during periods of higher flow, irrespective of whether it was environmental water (2018-19), or natural flow events in 2016-17.
- Vegetation community condition and plant diversity in the wetlands was driven by patterns of inundation influenced by Commonwealth environmental water.
- Invertebrate communities responded over time due to variations in flow and water quality. Flow events in river channels improved water quality and delivered resources, with flows exceeding 500 ML/assisting in the dispersal of invertebrate species. Following flow events when water levels stabilised, invertebrate densities increased, as they took advantage of the increased resources. In wetlands, invertebrate density increased with time since connection, as water contracted and nutrient concentrations increased, but then decreased as more sensitive taxa were impacted by poor water quality.
- The fish community of the lower Gwydir appeared to fluctuate in composition and abundance, with relatively more native species recorded during and after the largest flow event in 2016-17. At other times, exotic species were dominant.
Implications for the Commonwealth’s Environmental Water Management
The Gwydir LTIM Project has demonstrated that the use of multiple flow types to target a range of wetland and channel outcomes was effective:
- in enhancing wetland inundation,
- increasing river channel productivity by ‘priming’ the system early in the season to promote ecological responses, and
- improving connectivity and maintaining the quality of important low flow refugial habitat during dry times.
The timing of flow events was shown to influence the nature of the response to flow. Flow events delivered in winter/spring appear to improve water quality and stimulate fish movement but induce only minimal primary and secondary production due to lower water temperatures. Flows delivered over the summer/autumn period improved water quality, and promoted primary and secondary production, thus supporting animals higher up the food chain such as fish, frogs and waterbirds. Thus, the seasonal timing of environmental water deliveries needs to be matched with the desired responses to elicit the best outcome of that water.
The fish population in the Gwydir River system appears to be under stress, with many native species in low abundance. This may reflect the carrying capacity of the system in its current state. While some species appear to be breeding and recruiting, others, especially some of the more iconic species such as golden perch, freshwater catfish and Murray cod, are not recruiting sufficiently to improve their populations. Along with providing environmental flows, other options such as habitat rehabilitation, restocking and barrier remediation should be considered to improve the fish communities of the Gwydir Selected Area.
This report is the final one to be produced for Gwydir River System Selected Area under the LTIM Project, with monitoring and evaluation being undertaken from 2014-15 to 2018-19. The data and evaluation report from this project are being combined with those from six other LTIM Project Selected Areas within the Basin, as part of a Basin-scale evaluation of the contribution of Commonwealth environmental water to the environmental objectives of the Basin Plan being led by the Murray-Darling Freshwater Research Centre.
Threatened fish species surveyed in the Gwydir River system during the 2015-16 water year. Olive perchlet (left) and Murray cod (right).