Water bodies say the likelihood of a blackwater event is highly likely, after severe flooding hit parts of the catchment at the weekend.
More than 200mm of rain was recorded to have fallen across the region since storms rolled in on Friday and operators at Goulburn Valley Water have already taken steps to treat affected river water for consumption.
Water management authorities are monitoring water quality in the Goulburn River.
Blackwater could form if leaf litter and other debris is washed into tributaries such as Castle, Pranjip, Hughes and Seven creeks before flowing to the Goulburn River.
As well as discolouration within the water, if temperatures are warm, oxygen levels in the water could drop, possibly affecting native fish, water bugs and other aquatic life.
Goulburn Broken Catchment Management Authority environmental water manager Simon Casanelia said the impact of this blackwater event would not be as severe as that earlier in the year.
Flooding in January this year led to an event that caused a large number of native fish to die in the Goulburn River around Shepparton and Mooroopna.
Ms Casanelia said cooler forecast temperatures would help reduce the likelihood of a low dissolved oxygen event.
‘‘Since the rainfall, dissolved oxygen in the Goulburn River around Shepparton dropped from 7mg/litre to around 4mg/litre, and has increased to 5mg/litre (yesterday) morning,’’ Mr Casanelia said.
‘‘When dissolved oxygen gets below 3mg/litre, it starts to impact on aquatic life. We do, however, still have significant volumes of water moving into and down the system carrying significant amounts of organic matter.’’
GV Water’s quality manager Mark Putman said the organisation had already taken steps to treat the water, which has increased dirt and other organic material that had to be removed.
Mr Putman said blackwater posed challenges in removing dirt and colour from drinking water and more chemicals were needed to treat the water.
Some 100mg of aluminium sulphate is being added to the water, with the normal amount being about the 25mg to 30mg mark.
He said the additional chemicals used could impact the water’s taste and odour.
‘‘The positive is that the dilution effect of last year’s event was a lot less than what it is this time, because the Goulburn has already been flooded and there is a constant movement,’’ Mr Putman said.
‘‘The taste and odour of the water is probably being impacted now, but we are starting to see a slight improvement in the water quality, so it’ll be an up-and-down scenario for the next few days.
‘‘We’re continuing to treat the water and are providing increased surveillance when necessary to ensure it remains safe to drink.’’
In the event of fish deaths, the community has been urged to phone the EPA on 1300372842.