NSW has around 130 estuaries (coastal rivers, lakes and lagoons), with 80 of them prone to closing off from the sea.

Many coastal lakes and lagoons are naturally closed for long periods, until either a flood breaks them open or they are manually opened by machinery.

This helps to explain some of the community’s concerns about closed coastal lakes and the management of their entrances.

What an entrance…

Coastal lakes and lagoons close over when sand from the sea blocks the entrance channel where it crosses the beach. This can often cause community concern, particularly over issues such as:

Heavy equipment such as excavators are sometimes used to open coastal lakes. The decision to open a coastal lake depends on many factors, but is mostly done to relieve foreshore flooding.

Water quality

Just because a lake is closed to the sea does not necessarily mean that it has poor water quality. Many closed lakes have good water quality often suitable for swimming and water sports. However, after heavy rainfall the water quality can worsen as a result of ‘dirty’ runoff entering the lake from creeks and drains.

Some people believe that digging out the entrance of a coastal lake will flush out all the ‘dirty’ water and replenish the lake with ‘clean’ seawater. However, opening the lake will not always improve the water quality due to the limited tidal movement or ‘flushing’ in coastal lakes. Unpleasant smells (like rotten egg gas) can also arise from the exposed mud and rotten vegetation when lakes are opened.

Fish and prawns

Coastal lakes that close support a unique range of plants and animals that have adapted to those conditions. For example, fish and prawns are adapted to survive for years in lakes that have closed to the sea. Some fish do need to migrate to the sea to lay their eggs, but others actually reproduce in the estuary. Fish can also migrate from other estuaries to restore stocks, even if the lake is only open for a short period of time.

Some of the most productive estuaries for commercial fishing are those that are closed for long periods.

These are often more productive than the permanently open estuaries. In some cases, manually opening lakes has caused fish kills. The rapid lowering of water levels can stir up mud and plant matter, causing oxygen levels to drop and fish to suffocate.

Drowned plants

Plants growing around the edge of lakes act as important buffers, filtering out pollution before it enters the lake. Certain plants growing around coastal lakes may die from waterlogging if they are close to the water’s edge. This loss of vegetation can be managed by:

Ensuring there are sufficient replacement plants on the landward side, and Planting more suitable species that can tolerate wet, salty soils, such as salt marshes, which are already endangered.

Rock walls

Some people believe that the only way to ‘fix’ their local lake is to permanently open it to the sea with the aid of rock training walls. However, lake entrance works are costly (millions of dollars) and can have a number of undesirable effects, including:

• Changing the ecology of the lake

• Creating tidal currents that can cause floating fish larvae to settle in inhospitable areas of the estuary, and

• Altering tidal flushing that may lead to poorer water quality in backwater areas.

When should a lake be opened?

Opening coastal lakes before the water level is high enough results in an ineffective opening, which can close rapidly. This will have limited benefit to water quality and fish migration, and doesn’t allow the surrounding wetlands to absorb sufficient water.

It also allows more marine sand to enter the lake entrance, creating larger sand bars and future blockages. There is a clear trend that opening lakes at the highest possible water level will produce a longer lasting and well scoured entrance opening, which will lead to improved lake flushing.

The time an entrance remains open will depend on a number of factors including rainfall, size of the catchment, size of the lake, location of the entrance and sand movement along the coast.

Managing lake entrances

Scientists conclude that maintaining, as much as is possible, natural openings of coastal lakes is the best way to manage them. Although some lakes need opening more often to prevent serious foreshore flooding, others should be left to open naturally.

With the assistance of the State Government an Entrance Management Policy can be developed for estuaries that need regular opening. These policies set conditions under which the lake entrance can be manually opened.