OPEN OR CLOSED? THE EVER-CHANGING ANGLESEA RIVER ESTUARY

If you've walked around the Anglesea River estuary, you may have noticed that sometimes it is connected to the ocean and at other times it is closed off by a sand berm. These changes are a natural and important part of how many Victorian estuaries function.

The Anglesea River estuary is known as an intermittent estuary. This means its entrance alternates between sometimes being open to the sea and sometimes being closed by a sand berm. Of the 40 estuaries in the Corangamite Catchment Management Authority (CMA) region, all are intermittently open/closed, except for the Barwon River and Hovells Creek. In fact, around 85% of Victorian estuaries naturally close to the sea from time to time.

Estuary closures occur when ocean waves and currents push sand across the estuary entrance, forming a sand berm. They are most common during drier periods when river flows are low. The timing and duration of openings and closures can vary considerably depending on rainfall, catchment inflows and coastal conditions.

Left: Anglesea River estuary mouth - closed, February 2026. Photo: Anglesea River EstuaryWatch observation #17437. Right: Anglesea River estuary mouth - open, June 2022. Photo: Anglesea River EstuaryWatch observation #14786.

An ever-changing system

Open and closed conditions are both natural and important phases in an estuary's life cycle, supporting different plants, animals and ecosystem processes.

When an estuary is closed, water levels can rise and provide important benefits for wetlands and fringing vegetation. When it is open, fish can move between the estuary and ocean, and mudflats may be exposed, creating feeding opportunities for shorebirds. Different plants and animals thrive under different conditions, making these changes essential for a healthy estuary ecosystem.

The Anglesea River and estuary also experience periods of high acidity caused by naturally occurring acid sulphate soils in the catchment, further highlighting the complex and dynamic nature of this coastal system.

The appearance of an estuary can vary considerably throughout the year. Water colour, clarity and odour may change in response to rainfall, water levels, temperature and other environmental conditions. As highly dynamic systems, estuaries are constantly responding to natural changes in their environment, and these changes do not necessarily indicate poor health.

Human activities can also influence the condition of an estuary. Climate change, changes in land use that alter water flows (including stormwater), and inputs such as litter and pollutants all play a role. Continuous monitoring by citizen science groups such as EstuaryWatch is vital for understanding how the timing and duration of estuary mouth openings and closures are changing over time.  You can view the latest observations made by local Anglesea EstuaryWatch volunteers here.

Why don't we just open it?

When the estuary changes colour, develops odours or looks less healthy than usual, it's understandable that some people might think we need to intervene.

However, these visible changes are often part of the natural processes that occur within an estuary.  Research shows that artificially opening an estuary during periods of low rainfall, low river flows and low water levels can do more harm than good. In these conditions, artificially opening the entrance can worsen water quality, increase the risk of fish deaths and affect vegetation communities. It can also impact the way people use and enjoy the estuary and its surrounds.

When is an artificial opening considered?

There are times when an artificial opening may be necessary, however.

Artificial estuary openings are typically only considered when rising water levels pose a significant risk to public safety and critical assets, such as roads and buildings. Any decision must balance environmental, social and economic considerations, while also assessing risks to public safety and surrounding land.

An artificial opening usually involves digging a channel through the sand berm with an excavator and relying on the natural movement of water to keep the entrance open. If there is not enough inflow from the catchment and ocean swells are large, the opening may be unsuccessful. Careful assessment of water levels and tidal conditions is required before an artificial estuary opening is undertaken.

Corangamite CMA's involvement

Corangamite CMA works closely with Great Ocean Road Coast and Parks Authority (GORCAPA), Surf Coast Shire Council, Traditional Owners, local EstuaryWatch groups, and other partners to support the management of the Anglesea River estuary.

Our role is to provide advice about the estuary’s environmental values and how management decisions may impact water quality, fish, vegetation and habitat. We also help coordinate monitoring to track estuary conditions and ecosystem health.

In most circumstances, our preferred approach is to allow the estuary to function without human intervention, while balancing environmental, social and community considerations.

When rising water levels pose a risk to critical assets or public safety, GORCAPA and Surf Coast Shire consider management options. As the land manager at the river mouth, GORCAPA is responsible for coordinating artificial estuary openings. When an artificial estuary opening is being considered, GORCAPA seeks technical advice from Corangamite CMA, including information on current water quality and conditions, and advice on how to minimise risks to environmental values such as fish, vegetation and habitat.

Our goal is to support healthy, resilient estuaries while helping to protect critical assets and public safety when required. This often means allowing these unique coastal systems to follow their natural cycles of opening and closing, just as they have for thousands of years.

Looking to the future

Artificial estuary openings can help reduce the risk of inundation of assets in some circumstances, but they can’t be relied upon as the only measure for protecting critical assets and public safety. Estuary entrances can close again quickly after opening, particularly when river flows are low and coastal conditions favour the build-up of sand at the entrance. Additionally, conditions are not always safe for excavator contractors to undertake an artificial opening.

As sea levels continue to rise, low-lying land and infrastructure are expected to become inundated more frequently. It is also likely to become increasingly difficult to safely and effectively open estuary entrances, and modelling suggests the effectiveness of artificial openings is likely to decline over time.

These challenges highlight the importance of long-term planning and adaptation. Through the Reimagining the Anglesea River and Estuary (RARE) project, Corangamite CMA and project partners are working with the community to better understand future risks and opportunities, and to explore ways to support a healthy, resilient estuary and a thriving coastal community for generations to come.

Want to learn more?

Visit the Estuaries page of the Corangamite CMA website to learn more, including how you can help monitor your local estuary with other citizen scientists through EstuaryWatch. To read more about the RARE project, visit the project page here.