OPINION: What is the cause for Swan Bay mangrove dieback?

DEAR News Of The Area,

AS a twenty three year resident of Swan Bay, a regular river kayak paddler and a daily walker to the river mouth, I was interested to read Gordon Grainger’s thoughts in last week’s edition about salinity, sand movement and dredging of the eastern (short cut) river entrance.

I suspect the dieback of grey mangroves in Swan Bay is unrelated to either water salinity or sedimentation.

The dieback is restricted to a two to three kilometre section of the river upstream from Tea Gardens.

Although the dieback is quite extensive there are large sections within the broader affected area which are showing no signs at all of dieback.

If salinity was a cause I would expect the dieback to be uniform and be evident along the whole length of the river.

Similarly, sedimentation seems to be an unlikely cause.

The dieback commenced suddenly and simultaneously across the affected area, suggesting a single issue or event may have been the cause.

There is certainly no sign of any sudden build up of sediment which might have triggered it.

Symptoms of dieback first appeared in April/May of 2021.

It is possible that flood rains in the catchment – Bulahdelah received 614 mm rain in March 2021, including 281 mm on 22nd March alone – may have triggered a localised blackwater event.

It’s unfortunate that possible causes were not better investigated when the dieback first became evident.

Regarding the eastern (short cut) entrance, it will be interesting to see if any different pattern of sand deposition develops over time or whether the entrance continues to silt up as it has done after each of the recent dredging programs.

While designating the eastern entrance as a navigational channel sounds appealing it is unlikely that the NSW government would accept the need for two navigational channels into the Myall River and the one requiring less dredging and maintenance, the northern channel, will continue to be favoured.

Hawks Nest.

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