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Managing Lake Cathie

Lake Cathie waterways
Perch Hole Photo: Teena Northey

Lake Cathie is classified as an ICOLL - an Intermittently Closed and Open Lake or Lagoon, and is one of about 70 such coastal lakes and lagoons located along the coast of NSW. ICOLLs are complex systems that have adapted to varying water levels and varying salinity levels. They are usually highly biodiverse and support a large range of fauna and flora. They are also very sensitive to human disturbance. Lake Cathie and Lake Innes are well studied lakes with much in depth scientific research and modelling completed over the years.

The management of the Lake Cathie system is the responsibility of a number of stakeholders including:

  • Port Macquarie Hastings Council;
  • National Parks and Wildlife Services (NPWS);
  • Office of Environment and Heritage (OEH); and
  • NSW Fisheries and Department of Industry (DPI - Lands).

Under the NSW Coastal Management Act 2016, the preference for management of this type of lake is to maintain natural processes as much as possible.

The opening of ICOLL entrances is often the subject of much debate and enquiry. While the Coastal Management Act prefers no modification of ICOLL entrance occur, sometimes ICOLLs are opened manually to the sea to reduce the impacts of localised flooding. When water levels rise in a closed ICOLL following rainfall, flooding of urban and rural development adjacent to the lake or lagoon foreshore, can occur. Port Macquarie Hastings Council are quick to act when these flooding issues occur and last opened the entrance to Lake Cathie in July 2018 when high water levels were reached.

Many NSW ICOLLS are currently experiencing drought conditions, and as a result many communities, experiencing the dry-out ICOLL environment, are asking for these lakes to be opened. Usually, as there are so many stakeholders involved in the management of these systems, there is an Opening or Entrance Management Policy. For Lake Cathie any manual or ‘artificial’ opening of the lake is undertaken according to the Lake Cathie Opening Strategy. The Strategy has been in place for several years but it has been thoroughly tested by scientific data and a recent extensive study and hydrological model developed in 2014. This model demonstrated that the Opening Strategy reflected the best management for the lake. (The model and research also examined widening the bridge and separating Lake Innes to allow it to return to a freshwater lake. None of these conditions were recommended as they will have a negligible impact on the health of the lakes).

Opening Strategy

The Lake Cathie Opening Strategy outlines that the lake is only to be opened when the lake water level is at, or exceeds, 1.6 metre AHD or falls below 0.2metre AHD with high salinity if optimal conditions allow. Currently the water level of the lake has fallen below this level, however there are concerning circumstances that have meant the managing stakeholders have decided not to open the lake. 

One of the concerning circumstances is the large amount of red weed (rhodophyta) that is currently present in the local coastal waters. If the entrance to Lake Cathie was opened at the moment, it would close in a very short period of time (a matter of weeks) due to the lack of sufficient volumes of water within the lake and surrounding catchment to keep the entrance open. Onshore sand would be transported into the lake entrance and would quickly close the lake entrance. Coastal waters entering the lake could contain large amounts of red weed, which, when the lake closed, would be trapped inside the lake. The red weed would then decay and would make water quality conditions within the lake much worse than they are at present. This would endanger aquatic fish and other lake fauna and flora. Optimal conditions are not prevailing.

Port Macquarie Hastings Council are keeping a close watch on the lake and the local flora and fauna that are supported by the lake. There was a small fish kill in January which was investigated by DPI and was found to be largely the result of high water temperatures and resulting low dissolved oxygen. There were similar fish kills in NSW around this time also. Since this time no further fish kills have been reported. The local bird life have however, been enjoying the exposed foreshores and extensive mud flats which are abundant with food.

Port Macquarie Hastings Council are seeking permission from all the managing stakeholders to do a study of the Acid Sulphate Soils around Lake Cathie and Lake Innes while the lake sediments are exposed due to the current drought conditions. We will have additional information to understand what is happening in the immediate catchment and within the lake systems themselves, once this work is completed. Council are also in regular contact with the key managing stakeholders and other Councils with similar ICOLLs in their areas of governance. Representatives from the Office of Environment and Heritage recently visited and confirmed nature should be let to run its course and not interfering with the lake is the best management option at this time. 

Water and algal level testing

The water quality of Lake Cathie is being regularly monitored in three locations and to date, the test results have consistently shown that the water quality is suitable for recreational use, such as swimming. In addition, further in-depth water quality testing is taking place to better understand conditions within the remaining waters of the Lake. If test results should ever fall to a ‘Poor’ recreational level, the community will be notified immediately that it is unsuitable for swimming. Algal levels within the lake water show that it is still environmentally healthy.

A closed ICOLL, low water levels and brown water do not necessarily mean that Lake Cathie is dying, needs revival or is unhealthy. There is a drought at the moment with the Port Macquarie Hastings area under level one water restrictions. Many water systems up and down the coast are experiencing similar drought conditions with high evaporation and low water levels. Many ICOLLS are looking similar to Lake Cathie and are in the same condition with extensive evaporation and low water levels. The good news is that the lake and the fauna and flora it supports, will get ‘back to normal’ with some quality rainfall. This means that we can all enjoy the many recreational activities that this water system presents having seen how it copes in drought conditions and surviving well!

This page was last updated on: 30 May 2019