SWAN RIVER — OXYGENATION
504. Hon LYNN MacLAREN to the minister representing the
Minister for Environment:
(1) On what
basis has the government decided that a third oxygenation plant for the Swan
River is no longer required?
(2) What is the government doing to oxygenate the lower
layers of the Swan River?
(3) What is the current condition of the lower layers of the
river upstream of Dalkeith?
(4) What
measures are being taken to prevent the stratification of the river so that
lower layers no longer struggle to support aquatic life?
Hon HELEN MORTON
replied:
I thank the member for some notice
of the question.
(1) A third
oxygenation plant has not been earmarked for the Swan River. A third plant is
to be built on the Canning River. However, following advice from the Swan River
Trust, a proposed fourth plant on the Canning is no longer considered
necessary. It is expected that the third plant to be built on the Canning River
and the upgrades to the existing plants—scheduled to be completed in
the coming year—will ensure that oxygen relief is provided to all
necessary areas of the Canning River upstream of the Kent Street Weir.
(2) Two
oxygenation plants currently operate on the Swan River at Guildford and
Caversham. These plants are very effective and able to provide relief over a
distance of more than 10 kilometres, though the absolute range is dependent on
tides, water temperature and river flow. These plants have been deliberately
located to address areas most prone to low oxygen events over summer and
autumn.
(3) The Swan
River Trust has advised that deeper waters of the river upstream of Dalkeith
through to East Perth are currently showing localised salinity and oxygen
stratification driven by the upstream movement of saline marine water and the
downstream movement of fresh riverine water.
(4) Salinity
stratification is a common occurrence in estuaries such as the Swan–Canning
system because salt water, entering from the sea via tides, is heavier than
fresh water and sinks to the bottom of the estuary and does not mix with the
fresher surface waters. The major cause of low oxygen levels in the rivers is
the microbial breakdown of organic matter, including algae. The long-term
approach to improving low oxygen levels is to reduce the nutrient and organic
load entering our rivers. Oxygenation plants provide an immediate and localised
improvement in water quality and are one component of an overall catchment-to-coast
approach being delivered via the healthy rivers action plan.