The open pit development at MRM required 5.5 kilometres of the McArthur River and 3 kilometres of Barney Creek to be re-channelled as the zinc, lead and silver ore body to be mined lay directly under the river. The new channels were completed in 2008 and open to waterflow prior to the 2009 wet season.

The McArthur River is over 300 kilometres long. At the reach nearest to the mine, the river is at best a 15 metre wide, slow flowing creek for 10 months of the year. At an average of one-in-five years, it overflows the channel which can be ua to 15 metres deep, and opens to a flood plain. These floods are generally shallow, slow moving bodies of water that do not last long.

Barney Creek is a small left bank tributary of the McArthur River which flows for about 5.5 kilometres around the northern side of the mine site. There are no permanent pools of water in Barney Creek and it only flows for relatively short periods following rain events. It can, however, break its banks and open to the floodplain in a one-in-two year rainfall event.

Rivers can often change their course naturally but have also been rechannelled for centuries as part of development projects. In the resources industry, the environmental science associated with rechannelling is well advanced. There are now principles and recommendations set by the Australian Coal Association Research Program (2002) paper “Monitoring and Evaluation Program for Bowen Basin Diversions” and “A Rehabilitation Manual for Australian Streams”. We have met these recognised standards in the design for the McArthur River and Barney Creek.

The design of the river rechannelling also included the recommendations of an independent adviser to the Northern Territory Government to ensure the rechannelling will be secure and prevent erosion and contamination in all weather conditions.

The new channels replicated the form and function of the existing river and creek and were planned to a great level of detail to ensure they:

• remain stable for all flood events and throughout the mine life and beyond
• will not be subject to higher than natural levels of erosion and sedimentation
• enable the river to, in parts, continue to flood naturally into the plain without affecting the mine facilities
• re-establish the river vegetation so there is no fragmentation of fauna habitats
• allow fish to continue to naturally pass through the channels.

As part of our development planning, we studied the McArthur River and Barney Creek in great detail. A helicopter survey of 40 kilometres of the McArthur River channel was supported by on the ground fieldwork. During the fieldwork, specialists walked along a 10 kilometre stretch of the river and creek to map vegetation and record the formation of the channel and river flows.

The new channel design for both the McArthur River and Barney Creek copied the natural environmental conditions wherever possible. This includes the type of soil and rock used on the base and slopes, the depth and width of the watercourses, the speed of water flow and flood characteristics.

The main differences are in the use of bedrock and artificial rock riffles in some locations on the river bed to help prevent eroded sediment flowing downstream.

Riffles are ‘U’ shaped rocky structures which mimic the existing bedrock outcrops, reducing the speed of water flow, and in doing so, help revegetation along the channel banks. These rock riffles are similar to already naturally-occurring rock bars and will not disrupt fish passage.

Studies of the habitats both in-stream, bank-side and in the vicinity of the river were also carried out. Results of these studies were considered in the design. The studies looked at 10,100-metre-long sites along the mine reach of the McArthur River and two sites in Barney Creek.

Other areas of interest

Factsheet 15: 2007 Mine Management Plan – River Rechannelling (0.2Mb)

Xstrata Zinc Australia Sustainability Report 2010 (2.2Mb)