COLLIE Shire Council was facing some issues of control over the Collie River, Professor Peter Davies noted at the last Weeds and Waterways Advisory Committee meeting.
“You are asking the river to do a lot of work with all the different values hanging on it”, he observed.
Professor Davies, who is director of the University of WA’s Centre of Excellence in Natural Resource Management, was enthusiastic about Collie Shire Council’s attempts to control the aquatic weeds Typha orientalis (bulrush)and Marsilea mutica (nardoo).
While ion Collie he inspected the river with advisory committee chairman Glyn Yates and former chairman Peter Piavanini.
He praised the council’s scientific weed control trials, testing excavation, grappling hooks, solarisation and herbicide as weed control methods.
The river was no longer a natural water course, he told the meeting. As a result of the dredging, which followed the 1964 floods, “you would need a flow the size of Sydney Harbour to have a flood”, he said.
Professor Davies said the weeds had taken off because a hydroponic situation resulted from the boards along the river which created a series of ponds, hence still water.
There was no shade because trees had been removed or could not reach over the widened waterway, warming the water.
There was no annual scouring and there was a constant flow of nutrient inputs from farms and town.
“A female plant can produce quarter of a million seeds a year,” Professor Davies said.
To stop those plants reproducing, river managers needed to reduce the nutrients, have more natural flows (not dam the water behind river stops) and more shade. These weeds did not grow under the bridges crossing the river, he pointed out.
Flows could be reinstated by removing the stop boards and installing riffles to maintain water velocity.
“Nutrient is a catchment issue,” he said.
Light on the water was another local issue.
Tackling all of these issue required different timeframes.
Re-establishing shade trees along the river was a long-term project.
Herbicide use or grappling hooks to tear out the weed were short-term.
“You can remove the nardoo when flows are high enough,” he said.
“We have targets we should aim for so that nardoo won’t grow.”
He recommended pulling out the river stop boards and replacing them with riffles. This would create much-needed turbulence and aeration of the water and also would not halt fish migration as the stop boards did.
River campaigner Ed Riley warned that if the Roberts Rocks boards are removed, Minninup Pool will recede “by a third at least.”
He agreed with the riffle concept but said they did not allow river managers to “play god” as river stops did.
Mr Riley warned there was a lot of concern about using herbicides on the river and said council staff had been made aware of earlier concerns about Reglone, the herbicide trialed this year near the Collie golf course.
Professor Davies responded that rifles made sense ecologically but admitted “you have community expectations” about water levels.
The meeting was told the most successful weed control trials were solarisation and herbicide.
Where excavation and grappling hooks had been tried, there was no longer any evidence that action had been taken.
Professor Davies said a 2007 report recommended being careful with herbicides but chemicals were certainly the cheapest direct cost metyhod of controlling aquatic weeds.
The river had already been exposed to a lot of herbicides and insecticides with no loss of species, “though there has probably been a change in where they are distributed”.
If the council scaled up the use of herbicide, decisions would have to be made on using it carefully.