RIVER MURRAY BARRAGES- ENVIRONMETNAL FLOWS
FULL REPORT
REPORT SUMMARY
Overview
This report, commissioned by the Murray-Darling Basin Commission (MDBC) and edited by Anne Jensen, Michael Good, Prudence Tucker, and Martine Long, evaluates the environmental flow requirements for the Lower Lakes (Lake Alexandrina and Lake Albert) and the Coorong in South Australia. Published in June 2000, it addresses the ecological impacts of the River Murray barrages, constructed to manage water flow and salinity, and proposes strategies to mitigate environmental degradation. The study was conducted by a scientific panel with input from experts in hydrology, ecology, and water management.
Background
The MDBC initiated this study as part of a broader environmental flow management program for the River Murray and Lower Darling River. The barrages, built between the 1930s and 1940s near the Murray Mouth, regulate water levels and prevent saltwater intrusion into the freshwater lakes. However, they have significantly altered natural flow regimes, affecting the ecology of the Lower Lakes, Coorong, and Murray Mouth.
Key Issues
The scientific panel identified four primary issues driving environmental degradation:
1. Reduced Estuarine Area: The barrages have diminished the estuarine zone, disrupting the transition between freshwater and saltwater habitats.
2. Changed Water Regimes: Reduced and altered flows have led to static water levels, increased turbidity, and sedimentation.
3. Freshening of Brackish and Saline Habitats: The shift to freshwater conditions has reduced biodiversity in estuarine ecosystems.
4. Reduced Habitat for Aquatic Plants: Low water throughput and high turbidity have limited aquatic vegetation.
These issues have broader ecological consequences including:
- Increased frequency of Murray Mouth closures (from 1 in 20 years to 1 in 2 years).
- Decline in estuarine fish populations and commercial fisheries.
- Loss of habitat for migratory birds protected under international agreements (CAMBA and JAMBA).
- Increased algal blooms, including toxic cyanobacteria, due to nutrient accumulation.
Current Status
- Flow Reduction: Total flow through the Murray system is now one-third of the median unregulated flow, with significant implications for the Coorong and Murray Mouth.
- Biodiversity Loss: Aquatic plant communities, fish, and bird populations have declined due to altered hydrology and habitat loss.
- Sedimentation: Reduced flows have increased siltation, particularly in the Murray Mouth and Mundoo Channel.
- Economic Impact: The commercial fishery in the remnant estuary, though smaller in area, matches the value of the larger freshwater lakes ($1.1 million annually).
Ecological Needs and Opportunities
The report assesses ecological needs across various domains:
- Bird Ecology: Improved water level management to support migratory bird habitats.
- Fish and Invertebrates: Enhanced fish passage and restoration of estuarine conditions to boost recruitment.
- Phytoplankton: Reduced nutrient loads and better flow management to control algal blooms.
- Hydrological Management: Strategies to mimic natural flow regimes, including flushing of lakes and Coorong water level adjustments.
Opportunities for improvement are categorized by timeframe:
- Short-term: Release water from Lake Alexandrina in summer to reduce sediment and nutrient loads; improve barrage operations for fish passage.
- Medium-term: Develop predictive models for sediment transport and adjust barrage operations accordingly.
- Long-term: Implement catchment management to reduce external sediment and nutrient inputs; adopt an ecosystem-based management approach.
Recommendations
The scientific panel proposed actionable recommendations:
- Barrage Operation: Automate gates to allow flexible flow management and fish passage.
- Coorong Water Levels: Maintain seasonal variability to support estuarine ecosystems.
- Flushing of Lakes: Increase through-flow to reduce turbidity and sedimentation.
- Enlarge Estuary: Restore a larger estuarine area to enhance biodiversity.
- Erosion and Sedimentation Control: Use riparian buffers and manage sediment inputs.
- Further Investigations: Research sedimentation, bird ecology, fish dynamics, and phytoplankton to inform management.
Economic Valuation
A contingent valuation by Baker and Pierce (1997) estimated the annual value of the lower River Murray region at $85.9 million, with a present value of net benefits at $186.1 million and a benefit-cost ratio of 74.38, highlighting the economic justification for improved management.
Implications
- Management Agencies: Must integrate ecological flow needs into operational strategies.
- Upstream Water Management: Upstream flow allocations should consider downstream ecological requirements.
- Community Consultation: Engage stakeholders to ensure sustainable outcomes.
Conclusion
The report underscores the urgent need to address the ecological decline of the Lower Lakes and Coorong caused by the River Murray barrages. By restoring natural flow regimes and estuarine conditions, the proposed strategies aim to enhance biodiversity, support fisheries, and maintain the region’s ecological and economic value. Implementation requires coordinated efforts across short, medium, and long-term horizons, supported by further research and adaptive management.