The Darling Anabranch Adaptive Management Monitoring Plan - M/BUS/239
MDFRC Technical Report 11
The following information is extracted from the tender documentation provided for the project (Contract No: MD 163.07, Prepared by: NSW Dept of Commerce). The Great Anabranch of the Darling River (henceforth the Darling Anabranch) is an ancestral path of the Darling River, which extends 480km from its off take on the Darling River (50 km south of Menindee, NSW) to its downstream junction with the Murray River (15 km west of Wentworth, NSW). The Darling Anabranch is a naturally ephemeral stream which under natural conditions would only receive inflows when flows in the Darling River reach 10,000ML/day. Following construction of the Menindee Lakes system in the 1960's, an annual replenishment flow of 50,000ML/yr was provided to the Anabranch properties. A series of 17 block banks and weirs were constructed across the Darling Anabranch and were used to pond the replenishment flow. Landholders use about 3,000 ML/annum from these weir pools, with the balance accounted for by transmission losses, evaporation, riparian vegetation, wildlife and seepage. Little of the replenishment flow reaches the Murray River. The current water management regime has led to an increase in sedimentation, salinity, contributions to groundwater and has provided conditions that favour some non-native fish species. Darling Anabranch water quality is poor compared to both the Darling and Murray Rivers. This is due to both the poor water quality of the replenishment source water (Lake Cawndilla) and the subsequent ponding of this water in the weir pools. Salinities are high due to the Lake Cawndilla source water being more saline than the other Menindee Lakes, evaporation during ponding and highly saline groundwater intrusions. Blue-green algal blooms are common during summer in many of the Anabranch weir pools. The Darling Anabranch Management Plan (Nias 2002) proposed to supply the stock and domestic requirements of the landholders via a pipeline with an adaptively managed environmental flow. The Environmental flow is to be provided from Lake Cawndilla into the Darling Anabranch channel resulting in an 'end of stream' flow. The existing block banks (in stream structures) are to be modified or removed to reduce the ponding of the environmental flow water. For the past 40 plus years an annual replenishment flow of 50,000 ML at a maximum of 900 ML/d has been released from Lake Cawndilla (Menindee Lakes) when available. This water was maintained behind the various block banks / regulators and achieved little to no end of system flow into the Murray River. The Darling Anabranch Project aims to create a more natural wetting-drying cycle through the release of an environmental flow of approximately 60,000 ML at a maximum rate of 2,000 ML/day every 2 to 3 years on average (when water is available in Lake Cawndilla). The release will take place over about 6 weeks depending on the lake outlet capacity, stipulated discharge rate, and desirable ramping up and ramping down periods. With the various block bank / regulators removed an end of system flow is anticipated to be achieved. Operation of the environmental flow is via releases from Lake Cawndilla through the Packers Crossing Regulator (approx 30 km south of Lake Cawndilla). Investigations are currently being undertaken to determine the most appropriate downstream structure (Oakbank Regulator or Warra's Dam) to control end of system flows into the River Murray. There is no control structure (except the natural channel) for flows from the Darling River. It is preferable that the release time is varied over the years, rather than occurring in the same months every time. There is some potential for environmental flows in the Darling Anabranch to be generated through the Darling River during periods of moderate to high flows (> 10,000 MLday-1 ), if environmental flows are unable to be to be delivered from Lake Cawndilla.