Readers of the Colac Herald will have seen a recent front-page article with calls from a local farmer to start the release of water from Lake Colac into the Lough Calvert Drainage Channel at the northern end of the lake, near Irrewarra. His concerns related to flooding of his and others property on the northern shores of the lake, given the highwater levels currently in the lake as a result of three years of above average rainfall. Given the impact of previous releases on salinity levels in the Barwon River, the Committee asked for a briefing from the CCMA.
The drainage scheme was initiated after the floods in the early 1950’s to minimise flooding around the lake and to divert water through the Lower Lough and via the Birregurra Creek to the Barwon River. Other goals were to:
- maintain a level of water in Lake Colac to allow various recreational activities
- to limit impact on downstream users and biological systems
- as well as ensuring there was no residual surface water in the Lower Lough by the end of September.
There are written operational rules that mean water cannot be released from the lake unless the water elevation exceeds 116.6 m and salinity measured in the Barwon at Winchelsea, between July and September, is less than 2500 ECU. There is an overflow that operates if the water elevation exceeds 117.4 m.
In 1995, 10 GL of water was released from the lake into the drain and again in 1996, 9 GL of water was released. Given the salinity in the lake then was recorded at 2000 ECU and is now closer to 4000 ECU, this resulted in about 20,000 tonnes of ‘salt’ being delivered into the Barwon with impacts on the biodiversity in the river being noted as far downstream as Winchelsea. Currently, depending on the amount of water to be released, similar impacts could be expected in the Barwon. When full, to lower the lake level by 0.5 m, 15 GL would need to diverted into the channel. Given this could occur over a period of 180 days, it would mean the equivalent of 83 ML/day being released whilst the channel has a capacity of up to 150 ML/day. Such a diversion would send 37,500 tonnes of ‘salt’ into the Barwon. If a draw-down of 0.25 m was required these figures can be halved but still have the potential for significant impacts downstream. One also has to consider that there are very high levels of nitrogen and phosphorous in the lake water and these would impact on the development of blue-green algal outbreaks as far downstream as Geelong, where such events are already causing major disruption to recreational events like rowing.
Other impacts would include erosion in the Birregurra Creek, which has occurred with previous releases and the impact on riparian revegetation. It is all very well for landholders upstream of Birregurra Creek and the Barwon to raise concerns about the impact of local flooding but they cannot overlook the impact on the Barwon of large releases of water from Lake Colac. We need to be mindful of all the consequences of any release and just not what is impacting our own backyard.
Despite recent winter rains, the forecast of a drier and warmer three months ahead hopefully will mean the trigger points are not reached and a release will not be required. We can only wait and see what the weather brings.
In a recent follow up from the CCMA, they will only release about 40 ML/day which is the same amount entering Lake Colac from the two creeks and the treatment plant. They have telemetry to measure these flows in real time.
By Trevor Hodson
Photo: Fishing on Lake Colac – courtesy of Otway.biz