Tweed Shire Council
TweedRiver showing signs of good health
Oxygen and acidity levels normal despite recent flood
Thursday, 24 January 2008
Tests have revealed that dissolved oxygen and pH levels in the TweedRiver are normal following the recent floods.
This is in contrast to the nearby RichmondRiver, where fishing has been banned after tens of thousands of fish and prawns have died as a result of severely depleted oxygen levels.
Tests were conducted from the BarneysPointBridge at Banora Point to upstream of Murwillumbah by Council’s Waterways and Coast Coordinator Tom Alletson and Floodplain Officer Marty Hancock.
Mr Alletson said dissolved oxygen and acidity levels were found to be normal in all samples.
Salinity tests revealed that the river was completely dominated by fresh water as far downstream as Chinderah. This will have a significant impact on the behaviour of aquatic life, but will not result in massive fish kills.
“We also collected water samples to test for the metals aluminium and iron. This will give us more information about acid sulphate soil runoff,” Mr Alletson said.
While the results were reassuring, Mr Alletson said Council would not claim the results showed the Tweed was better managed than the Richmond.
“The RichmondRiver fish kill is a symptom of the geography of the river as it’s a much larger catchment which extends further inland than the Tweed,” he said.
“The Richmond has vast areas under shallow inundation during floods. The water gets very warm, depleting oxygen levels, in addition to the effect of rotting vegetation, which also consumes oxygen.
“It is believed that the fish kills occur when vast quantities of deoxygenated water from the floodplain displaces the healthy water in the river,” Mr Alletson said.
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“We have a significant floodplain as well but the extent and duration of the flood, combined with differences in land use mean we have on this occasion scaped the devastating deoxygenation experienced in the Richmond.” Mr Alletson said.
In 1987, fish kills of a similar scale were experienced in the TweedRiver, and these were linked to runoff from acid sulphate soils.
Mr Hancock said that since this time Council has worked with the sugar industry and researchers from a number of universities to implement world’s best practice in the management of acid sulphate soils.
“There is still a risk that we may see acid runoff from some drains within the Tweed floodplain. This issue has not been solved but we have advanced knowledge and strategies to deal with acid sulphate soils,” Mr Hancock said.
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