Intermittently closed and open lakes and lagoons

Intermittently closed and open lakes and lagoons open when water levels rise. This usually happens after rain when more water flows into the lake or lagoon from the catchment.

Once it reopens, the force of the water can rapidly scour a channel through the beach to the ocean.

It closes when ocean waves and tides push sand from offshore into the entrance. This gradually closes the entrance channel.

Flooding and drying are natural are part of the natural process of Intermittently closed and open lakes and lagoons.

Estuaries form the transition zone between river and ocean environments. Intermittently closed and open lakes and lagoons are one example of an estuary system.

Intermittently closed and open lakes and lagoons are important ecosystems for sustaining fish, birds, marine life, vegetation and wetlands.

They are also important cultural use places for Traditional Owners of this land.

Council opens the lake’s entrance to reduce the impacts of flooding. After it rains, water levels can rise in a closed lake. This can lead to an increased risk of flooding of footpaths and parks upstream.

The entrance barrier is manually opened to ‘drain’ it to the ocean and lower its water levels.

We must carefully plan the opening around tides, swell and rainfall conditions. We don't want it to have a negative impact of the environment.

Heavy equipment such as excavators are used to open it by digging a channel through the entrance barrier to the ocean. On occasion an Intermittently closed and open lake or lagoon may be required to be opened in response to an environmental pollutant. 

Intermittently closed and open lakes and lagoons should be left to function as naturally as possible. We must abide by our Queensland Government permit conditions. 

We are only able to open the entrance if there is:

• going to be a risk to public safety through flooding upstream
• a significant decrease in water quality.

When our permit conditions allow us to open the lake entrance, we do this when the tides are right. This gives us the time needed to dig a suitable channel with the best chance of it staying open.

Sometimes sand closes off the entrance for weeks, months or even years at a time. It's hard to predict how long the entrance will stay open for because there needs to be a lot of water flowing out of the lake to keep it open.

A closed lake can have many benefits – for Currimundi Lake a spring/summer closure means less midge larvae and a lower lakeside biting midge population. This is because the midge breeding habitat, at the high tide mark, is flooded and it disrupts the breeding cycle.  

There is no need to be concerned when an Intermittently closed and open lakes and lagoons is closed. We keep a close eye on the water quality.

There are often concerns over unpleasant smells and odour, flooding of low-lying areas and the health of fish stocks.

Just because it is closed, it doesn’t mean it has poor water quality or should be opened.

Even when the entrance is closed water still circulates around the lake. A tidal exchange also happens through the sand.

After heavy rainfall, the water quality in a closed lake can temporarily decline. This is because of runoff containing sediment, nutrients and pollutants entering from upstream waterways and stormwater drains.

When Intermittently closed and open lakes and lagoons are closed for long periods some plants around the lake edges may die from prolonged waterlogging.

This is a natural process. Foreshore plants do change over time as water levels change.

We regularly monitor water quality to check it comply with the Queensland Water Quality Guidelines. Values are compared with the South East Queensland regional guideline values for physico/chemical indicators (slightly to moderately disturbed waters) – Enclosed coastal water type.

Intermittently closed and open lakes and lagoons are very complex environments. There is still more to learn about how opening their entrances affects fish and their habitats.

Many types of estuarine fish and prawns breed in ocean or coastal waters. They then enter estuaries and Intermittently closed and open lakes and lagoons as larvae and juveniles.

Some fish need to migrate to the sea to complete their breeding cycle. Others can reproduce in the estuary without needing an open entrance. Long periods of closed entrances may limit the number of certain fish species. Some fish and larvae can still enter  from the ocean:

• during short openings
• when seawater flows over the entrance during high tides and storms.

Estuarine fish have adapted to the changing conditions in Intermittently closed and open lakes and lagoons, like varying water levels and lower oxygen and salinity. They can survive for many years in a closed lake without needing the entrance to be open.

Ongoing fish surveys show that Sunshine Coast Intermittently closed and open lakes and lagoons are in good health.

• Prevent fertiliser, green waste, animal waste or litter from entering stormwater drains in your catchment which flows into the lake affecting water quality.
• Make sure you only use designated paths when accessing the lake edge.
• Use sediment barriers when carrying out earthworks on your property to prevent sediments entering the water.
• Limit boatwash to prevent streambank erosion.

Unauthorised openings of Intermittently closed and open lakes and lagoons are illegal and may result in prosecution.