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Myall Lakes System Experiencing Natural Cycle


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A scientist who has been watching and studying the Myall Lakes system for a number of years said today the current conditions of low salinity, low temperature and ‘dirty’ water being experienced in the Myall Lakes and Myall River is an entirely normal, though not very frequent, occurrence.

Acting Manager of Department of Environment and Climate Change’s (DECC) Waters and Coastal Science Section, Dr Peter Scanes said, the channel conditions at the mouth of the Myall River are not contributing in any way to the recent fish kill caused by winter disease in the Bombah Broadwater or the prolonged 'fresh' being experienced in the Myall Lakes and Myall River.

"The data is very clear that there is no reduction in the total volume of water leaving the Myall Lakes or the Myall River, just a temporary increase in the amount of fresh water flowing into Port Stephens."

"The current situation is not a result of the system being blocked or choked at its mouth. If this were the case, then the lake should be getting deeper as time goes on, which is not happening."

"Low salinity conditions at Tea Gardens are infrequent but quite natural events. In 1997 the salinity in the Myall River at Tea Gardens was very low for an extended period, similar to the conditions that are present now."

Dr Scanes said, unlike many other NSW estuarine systems, the Myall Lakes system is a naturally slow flushing system and this combined with higher than average rainfall over the past two years, is essentially the cause of the current situation.

"The Myall Lakes system can be described as a large “retention basin” capturing flows from the Myall River and then slowly releasing these flows into the Port Stephens estuary. This is a naturally slow process due to the constriction of the lower Myall River which is long (22km) and narrow."

"It is estimated that it could take 750-800 days to flush the entire volume of the Myall Lakes. That is, the natural hydrology of the Myall Lakes is one of water retention and slow release to the ocean."

The other significant factor has been rainfall over the past couple of years, not just the past few months, which has saturated the catchment around the lakes.

"Many people may remember the minor floods in the Myall Lakes this year following storms in autumn and winter. There was up to twelve times the normal volumes of freshwater delivered to the lake system and this has forced saltwater down the lower Myall River."

"Even after the rain and flooding has stopped, large volumes of freshwater continue to drain out of the saturated soils of the catchment and this is keeping saltwater a long way downstream. River flows have been higher than normal for four months and this is why 'fresh' conditions have persisted."

"Simply, the high volume of fresh water still going downstream is much greater than the normal volume of saltwater pushing its way upstream with the tides."

"When river flows drop to normal or base flow conditions we should see a slow increase in salinity in the Myall River and the Broadwater as the tidal salt water works its way back upstream. If rain continues, freshwater conditions will persist. Most recently the rains in early September have again raised the level of the river and the lakes."

Dr Scanes encourages anyone who would like to find out more about the processes that are causing the current conditions in the Myall Lakes and Myall River, the long term outlook for the health of the Myall Lakes and the cause of the recent fish kill to look at the following information and links:

Information regarding the fishkill and current conditions in Myall Lakes (myalllakesfishkill.pdf, 49KB)

Water Quality Improvement Plan - Draft (Great Lakes Council)

Dr Scanes said, "We should be concentrating on improving catchment management using processes such as the current Water Quality Improvement Plan which will improve the health of the Broadwater and not get diverted by the current obvious, but natural, water conditions at Tea Gardens."

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