Overview

Large flood events are a natural and essential component of riverine ecosystems, but under certain conditions they can also trigger anoxic blackwater events that severely reduce dissolved oxygen levels, resulting in widespread fish deaths and significant ecological impacts.

This project evaluated the extent of the 2022–23 blackwater event, examined the factors contributing to ecological impacts, and assessed the effectiveness of management responses. The findings provide valuable lessons to strengthen preparedness and improve responses to future large-scale blackwater events.

The Challenge

The widespread flooding experienced during 2022–23 delivered substantial ecological benefits across many floodplain systems. However, the inundation of large quantities of accumulated organic matter also created conditions conducive to anoxic blackwater in parts of the Murray–Darling Basin.

The challenge was not simply to document the event, but to understand its spatial extent, severity, ecological consequences, and the effectiveness of the management actions implemented. This required integrating multiple sources of information while recognising the inherent complexity of large-scale natural disturbance events.

Our Approach

The evaluation combined environmental data analysis with expert knowledge and stakeholder engagement to develop a comprehensive understanding of the event and the management response.

The assessment included:

• Collation and synthesis of water quality datasets across affected river systems.
• Analysis of fish mortality records to understand the timing, extent, and distribution of impacts.
• Expert elicitation to identify the primary ecological and environmental factors influencing fish deaths.
• Engagement with environmental water managers and stakeholders to review mitigation measures, operational responses, and opportunities for improvement.

This integrated approach allowed ecological evidence and management experience to be considered together, providing a more complete assessment than either could achieve independently.

Key Findings

The review demonstrated that the impacts of the 2022–23 blackwater event varied considerably across the landscape, reflecting differences in hydrology, water quality, organic matter loads, and local environmental conditions.

Key findings included:

• Water quality and fish mortality data provided a detailed picture of the timing and geographic distribution of blackwater impacts.
• Multiple interacting environmental factors influenced the severity of fish deaths, reinforcing the complexity of predicting ecological responses during large flood events.
• Existing management responses helped reduce impacts in some locations but also highlighted opportunities to improve preparedness, coordination, and mitigation strategies.
• The integration of monitoring data, expert knowledge, and operational experience strengthened understanding of how future blackwater events can be more effectively managed.

Outcome

The project provided an evidence-based evaluation of one of the most significant recent blackwater events, identifying both the strengths of current management approaches and opportunities for improvement.

Its recommendations support more effective planning, earlier intervention where possible, and stronger coordination between monitoring, environmental water management, and emergency response, ultimately improving resilience to future blackwater events.

Impact

This project strengthens future management of blackwater events by:

• Improving understanding of the ecological drivers and impacts of anoxic blackwater.
• Synthesising environmental monitoring data with expert ecological knowledge.
• Evaluating the effectiveness of current mitigation and response strategies.
• Identifying practical improvements for future preparedness and adaptive management.
• Enhancing the capacity of environmental managers to respond to large-scale flood-related ecological events while protecting native fish populations and river ecosystem health.

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