Full lake for winter
The Murray Darling Basin Authority has confirmed Lake Mulwala will not be drawn down this winter with the latest weed survey report showing there is no need to do so.
Lake Mulwala is sometimes drawn down during the winter when regular surveys of the aquatic weed Egeria densa point to a need to expose the lake bed to air and frost to control the weed growth.
In early April, MDBA General Manager Andrew Reynolds told the Yarrawonga Chronicle, “when last surveyed in 2016 we did not expect to need to lower the water level this year, but weed growth appears to have been greater in the past year than we have seen previously”.
Mr Reynolds said community advice corroborated the suggestion of increased weed growth so a final decision on whether the lake would need to be lowered this year would hinge on a weed survey to be undertaken later in April.
Late last week an MDBA spokesperson provided detail to the Yarrawonga Chronicle of the most recent weed survey.
“This year’s survey of Lake Mulwala again sampled 165 sites distributed across the lake. An underwater video camera was used to detect Egeria as well as native aquatic vegetation in the lake,” the spokesperson said.
“The survey has shown that while Egeria has increased over the last year, there is no need to lower Lake Mulwala at this time.
“The results are consistent with our aim of keeping the lower basin around Mulwala and Yarrawonga relatively weed free for recreation. “
The spokesperson pointed out that aquatic vegetation was important to the lake’s
ecosystem, including to provide habitat for fish, so allowing some regrowth was beneficial.
“While the survey showed an increase in Egeria in the mid and upper reaches of the Lake, it also revealed an increase in native species.
“With the overall abundance at low to medium levels, a lowering of the lake again is not yet warranted.
“Given the increase in Egeria, a lowering of Lake Mulwala in 2018 may be beneficial. MDBA will consider the Egeria situation along with other local and system-wide factors before advising of a final decision in the lead up to winter 2018.”
Lake Mulwala was last lowered during the winter of 2015. That year the decision was made early in February because the draw- down was required to facilitate work on the Yarrawonga Weir wall as well as assist with weed control.
In an economic boon for Yarrawonga Mulwala the advanced notice of the drawdown allowed time to plan more than an estimated $1 million in public and private works in and around the lake.
Works included the construction and repair of major retaining walls, building of around 30 jetties, erosion control works and the maintenance of boat ramps.
The works were co-ordinated in partnership with the MDBA by Goulburn Murray Water and many residents took the opportunity to obtain permits for jetties while a moratorium was temporarily lifted.
Ordinarily a decision on whether or not to lower Lake Mulwala isn’t made until April, once the MDBA has the latest weed survey results.
Lake Mulwala was lowered in 2008, 2009, 2011 and in 2015.
Egeria densa, commonly known as dense waterweed, is a non-native plant that forms dense clumps in Lake Mulwala.
Victorian Department of Primary Industries expert Dr Tony Dugdale told the Chronicle in 2011 it was “unlikely Lake Mulwala will ever be rid of Egeria densa”.
“Not unless something completely out of the ordinary or unexpected happened. Drawing down the water in the lake to expose the Egeria to air is the most effective and readily available method of control,” Dr Dugdaale said.
Once an area of Egeria is exposed to air and frost, it then takes an undefined period of time for the weed to recover which in turn dictates when the next drawdown is needed.
This winter Lake Mulwala will remain full after the MDBA determined there was no need to lower the water as a weed control measure.
These are immature strands of Egeria densa, a water weed native to South America but now at home in Lake Mulwala. The strands would be double this size by maturity.