Commonwealth Environmental Water Office Long Term Intervention Monitoring Project Gwydir River system Selected Area – Five Year Evaluation Report

Abstract

The Gwydir catchment, located in the northern Murray-Darling Basin, extends from the Great Dividing Range west to the Barwon River. Downstream of Moree, the system fans out into a broad alluvial nearterminal floodplain. Numerous anabranches and distributary channels characterise the lower half of the Gwydir catchment, with the Mehi River and Moomin Creek to the south, and the Lower Gwydir River, Gingham Watercourse and Carole Creek to the north. Commonwealth environmental watering targets channel, wetland and floodplain assets including the Lower Gwydir, Gingham and Mallowa wetlands with expected environmental outcomes downstream (west) of Tareelaroi Weir on the Gwydir River. Climatic conditions were dry for most of the Long Term Intervention Monitoring Project (LTIM Project), especially since 2017 when rainfall has been well below the long-term average. Above average rainfall during August - October 2016 stimulated the largest natural flow event through the system, but since then, flows have been dominated by regulated releases from Copeton Dam. Commonwealth environmental water was delivered to the channels of the lower Gwydir system and wetlands via a number of watering actions, cooperatively, as a combination of both Commonwealth and State managed water. This report considers the combined influence of both Commonwealth and State managed environmental water sources on hydrologic and environmental responses. Environmental watering in the Gwydir catchment is delivered under a multi-year wetting and drying strategy to protect and maintain the condition of the wetlands and rivers within the system. During the planned ‘wet’ years, such as 2014-15 and 2018-19, watering aims to inundate core wetland areas and improve their condition and resilience to dry phases. During the intervening ‘dry’ years, environmental watering is more targeted at smaller scale wetland watering and to provide in-stream flows to support native fish populations and protect critical refuge habitat. The largest inundation event during the project was achieved in the Lower Gwydir and Gingham wetlands in 2014-15 as a result of targeted environmental water deliveries. During this time 6,342 ha of wetland was inundated including key vegetation communities, which remained inundated for four to six months. Maximum measured inundation was observed in the Mallowa system in April 2016 (901 ha) as a result of inflows containing environmental water. Key Responses to Flow Water Quality • Water quality throughout the Gwydir Selected Area was generally outside the ANZECC guideline trigger values but this did not manifest in any detrimental ecological consequences for river and wetland biota. • Delivery of environmental water consistently improved water quality in rivers and wetlands, reducing mean pH and conductivity (salinity). • Warmer temperatures generally increased primary production and dissolved oxygen levels in rivers and wetlands. In rivers, larger flows tended to have increased turbidity that led to reduced dissolved oxygen levels. Ecology • Hydrology and its influence on water quality is the primary driver of invertebrate communities in the Gwydir wetlands. High nutrient levels and primary production following inundation correlate with the highest invertebrate diversity and densities, demonstrating the role of inundation with environmental water in stimulating aquatic food webs. Water quality deteriorated after >50 days inundation and invertebrate taxa with a higher tolerance to poor water quality dominated. • Waterbirds took advantage of the increased habitat and food resources following flooding. High average species richness, density and diversity of waterbirds were observed during periods of higher flow/inundation, irrespective of whether it was environmental water (2018-19) or natural flow events in 2016-17. • The fish community of the lower Gwydir system is under stress. Variations in the native and exotic species composition followed annual variations in flow, with native species increasing in number during the wetter 2016-17 year. While the prolonged dry conditions were not always positive for native fish, the Northern Connectivity Event and Northern Fish Flow were essential actions that reconnected channel habitats, improved water quality and promoted fish movement among the channels of the lower Gwydir system. • The delivery of environmental water to the Gwydir wetlands during the 2014-15 water year increased the richness and cover of vegetation communities. For example, the delivery of environmental water reduced the cover of the exotic lippia species from 60% to 3% and increased native water couch cover from 10% to 81% in the lower Gwydir wetlands. Water couch dominant communities are fundamental to maintaining the character of the systems Ramsar sites. Water couch dominant communities persisted for the next 4 years following inundation highlighting the long-term benefits of environmental water despite the prevailing dry conditions seen throughout the northern Murray-Darling Basin in recent times. Implications for Commonwealth Environmental Water Management • The environmental watering strategy employed in the Gwydir Selected Area uses multiple flow types to target a range of wetland and channel outcomes. These include flows targeted at wetland inundation, flows to increase river channel productivity, stimulate food webs and ‘prime’ the system early in the season to promote ecological responses, and flows to improve connectivity and maintain the quality of important low flow refugial habitat during dry times. The LTIM Project has demonstrated that this approach to environmental watering is producing positive environmental outcomes for the lower Gwydir system. • Through the LTIM Project monitoring, a number of potential thresholds were identified that influence different aspects of the Gwydir Selected Area’s water quality and ecology. These included discharge thresholds that influence water quality variables such as pH, conductivity and dissolved oxygen, discharge thresholds that negatively impact flow sensitive invertebrate taxa, and inundation frequencies that promote native vegetation recruitment and growth. • The fish population in the Gwydir River system remains 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 for sustainable 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.