Lachlan River System
Image: River Red Gum Woodland, Juanbung in Spring 2021. Photo credit: Fiona Dyer
About the Lachlan
The Lachlan River flows through the traditional lands of the Wiradjuri, Nari Nari, Mutthi Mutthi, Ngiyampaa, and Yita Yita Nations forming songlines and dreaming tracks. The Lachlan rises near Gunning north-east of Canberra and travels 1400 km to the Great Cumbung Swamp near Oxley in NSW. As the fourth longest river in Australia, it is largely disconnected from the rest of the Murray-Darling Basin, only joining the Murrumbidgee River during large flood events.
The river system sustains diverse landscapes and species. During floods, large areas of water cover floodplains, wetlands, and ancient river pathways, bringing the landscape to life. Birds, other animals, and dormant plants emerge, while small wetlands are replenished, helping trees like black box and river red gums survive the dry periods.
The construction of dams and the use of water for irrigation have altered the river's natural flow and impacted wetland health, especially those at the river's end. These changes have transformed the character of the river and its floodplains.
Environmental water is used in the Lachlan River System to support the significant ecological values across the catchment, particularly those most affected by changes to flow regimes.
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Drone image of the Great Cumbung Swamp in November 2023. Photo credit: Will Higgisson
Larval fish monitoring at Wallanthery. Photo credit: Rhian Clear
Rod Ubrihien collecting water quality data in Lake Brewster. Photo credit: Rod Ubrihien
Our Work
Learn about the key activities taking place in this Area.
Native Vegetation
Native Vegetation monitoring and evaluation will focus on groundcover vegetation and lignum shrubland on the lower Lachlan floodplain, and tall emergent marshlands within the Great Cumbung Swamp. It will also assess the extent and duration of inundation caused by environmental water.
Native vegetation will be assessed using a combination of water observations from space, data collected using drones (lignum shrublands and tall emergent marshlands) and field-based methods (groundcover vegetation) to evaluate the role of environmental water in maintaining and improving native vegetation.
Native Fish
Native Fish monitoring will focus on how environmental water contributes to maintaining or improving native fish populations in the Lachlan Area. Field sampling will involve electrofishing at 20 fixed main-channel sites in early autumn (March/April) each year. Sites will be established across a stream length of 600 km, spanning two reaches (10 sites per reach): the mid-Lachlan River from Forbes until upstream of Brewster Weir, and the lower-Lachlan River around Hilston.
Native Waterbirds
Waterbird monitoring will be conducted in the event of bird breeding and/or in cases where environmental water is used to support waterbirds. Event-based ground-surveys will occur at a minimum of 10 survey sites over four trips, timed around the watering event. This theme will also draw on data collected through complementary monitoring programs such as annual spring aerial and ground waterbird surveys conducted by NSW DCCEEW and UNSW.
Cultural Outcomes
The Lachlan team supports a culturally safe and staged approach in Flow-MER. As such, the program's expected outcomes will mature and evolve with the building of new relationships and trust as it is built over the life of the program. We are working in collaboration with the Aboriginal Water Committee to the Lachlan in the planning of the Cultural Outcomes Theme, and have engaged members of the committee as our Cultural Advisors. We are also continuing to build our relationship with the Down the Track program, and continue to support environmental and cultural education camp-out weekends.
River Flows and Connectivity
The River Flows and Connectivity Theme in the Lachlan will evaluate what environmental water contributed to the flow regime:
- In the riverine system by evaluating the timing, duration and magnitude of flow events attributed to Commonwealth environmental water (CEW) using gauged flow data and daily water accounting, and;
- In the creeks and floodplain system by estimating the timing, duration and extent of inundation attributed to CEW using satellite imagery.
Knowledge Exchange and Community Engagement
The Lachlan Team lead a range of activities designed to share knowledge and information to the Flow-MER Program team, operational stakeholders, key landholders and stakeholder groups outside of the delivery of the Program. We write a quarterly newsletter which is shared digitally and printed to provide updates to landholders and the local community. We also provide training and education days providing an opportunity to talk to the local community and youth about the importance of flow variability and the role of environmental water.
Research
Our research over 2024 to 2026 focuses on blue-green algae outbreaks in Lake Brewster and the lower Lachlan. This project will identify and quantify the key drivers of blue-green algae outbreaks such as temperature and flow velocity and develop management protocols aimed at improving environmental water use to reduce the occurrence, severity, and impact of blue green algae outbreaks.
Key Insights
Environmental water planning in a complex system
The delivery of environmental water is set within the context of a river system that is being managed for multiple outcomes. River operators must balance the need to provide water for irrigation and other consumptive demands, manage dam levels to limit flood risks, conserve the water resource for future needs, deliver precise water levels for environmental outcomes, and account for water travel times (e.g. the travel time between Lake Brewster and Booligal amounts to 16 days) and evaporation/ riverbank seeping losses in the system.
Managing flow for Murray cod recruitment
Over the past 10 years, the greatest number of Murray cod larvae have been caught in very low flow years (2014, 2015, 2018 and 2019) with very few larvae caught in flood and high flow years. Numbers of juvenile Murray cod are generally high in the years following good larval catches (in 2015 and 2020) but were notably low in 2016 and high in 2021. During high flow years, hypoxic conditions have led to significant losses of Murray cod through fish kills in 2016–17 and again in 2022–23. In combination these conditions have resulted in a steady decline in the biomass of Murray cod over the past 10 years. The rate of fall of Spring pulses should be managed if the falling limb occurs during the peak Murray cod spawning window (mid-September to mid-October). Data collected through this program suggests that a minimum flow threshold during nesting supports Murray cod spawning and recruitment to juveniles.
The importance of environmental water during dry periods to maintain groundcover vegetation
High rainfall events during dry periods result in a flush of terrestrial-dry native and exotic species (as was observed in Autumn 2020). These are often the conditions with the greatest abundance of exotic species. During these dry periods the floodplain is driven by climate – mainly rainfall and temperature. Floodplain inundation results in a shift from a terrestrial driven system to an aquatic driven system – dominated by floodwater (depth, duration and extent among other factors) with a shift in the groundcover from a terrestrial-dry to aquatic-respondent species dominated system.
Sites that receive environmental water between the larger flood events contain increased diversity, abundance and percent cover of native amphibious and submerged species during large scale (natural) flooding. This is not observed at sites that do not contain environmental water which record no or very low percent groundcover– highlighting a lack of viable seedbank or asexual plant species. This result highlights the important role that CEW has and continues to play in reinstating the small and medium sized flow events during drier periods, which have been removed from the hydrograph to maintain viable and productive floodplains and wetlands.
Environmental water can be used effectively to underwrite risk of potential losses of water resources
Environmental water has been used in the Lachlan in underwriting the potential loss of future stored volumes caused by management actions. A watering action in 2022 maintained water levels in Lake Brewster to manage for pelican breeding requirements rather than water resource storage. Here, by underwriting the risk of loss of water resources in the potential occurrence of spills from Wyangala Dam higher up in the catchment, critical nesting habitat was maintained throughout the breeding event. This action involved early, and regular communication with river operators, informed by science and on-ground monitoring, which was at the centre of the success of this action. A total of 30,000 pelicans were counted, making it the largest breeding event in Lake Brewster in over 30 years.
Environmental water can provide large within-channel pulses for small-bodied fish without risk of significant carp recruitment
Over the past ten years, environmental water has been used effectively to deliver large fresh flow conditions in the Lachlan, engaging billabongs and flood runners, while not resulting in widescale inundation of the floodplain.
Our data has shown that these large within-channel flow pulses increase productivity and benefit small-bodied fish, with some of the greatest numbers of native fish such as Australian smelt and Carp gudgeon recorded following these events with minimal risk of triggering a significant carp spawning event.
Our Team
The Centre for Applied Water Science, University of Canberra has been engaged by the CEWH to conduct Flow-MER activities in the Lachlan River System from July 2024 to June 2029.
University of Canberra
Partners
Adam Kerezsy
NSW Government