The River Murray System – a constantly challenging and changing environment
By Jarrod Eaton
Posted: 22 November 2016
River Murray System inflows have significantly improved compared to what was being observed towards the end of the 2015-16 water year when inflows were trending towards historically low levels. Since June 2016, there has been a remarkable turnaround in the inflow conditions due to a very wet winter bringing widespread rainfall across the southern Murray-Darling Basin and also importantly across the northern basin.
The Barwon-Darling River systems in the northern basin have experienced a significant drought over the past 3 years with the Menindee Lakes storage reducing to less than 5% of its capacity. The other River Murray storages had also declined to low levels and without the rainfall and inflows experienced between June to October 2016, the outlook for the River Murray was very poor. A complete reversal of this outlook happened with substantial rainfall and inflows filling storages and creating an unregulated flow event for the first time since 2012.
Unregulated flows occur when the volume of water exceeds the capacity to regulate flows. These unregulated flow events provide substantial benefit to floodplain landscapes.
The floods upstream of South Australia will result in a peak flow of around 95 gigalitres per day at the South Australian border in late November to early December. Estimating peak flows is very difficult as no two floods are the same. High flows of this nature do not occur very frequently. The last time a similar high flow occurred at the South Australian border was in 2011 and prior to this in 1993 when the flow peaked at 110 gigalitres per day. These high flow to flood events provide substantial benefits to the floodplain, river channel, Lower Lakes, Coorong and Murray Mouth.
The Department for Environment, Water and Natural Resources is undertaking a comprehensive survey of selected shack areas between Cadell to Mannum to investigate the impacts of high flows in these areas. This information will be important for inundation mapping projects in these areas and DEWNR staff have been out measuring water level heights using differential global positioning systems and collecting detailed information on infrastructure within these selected shack areas (Figure 1). Some shack areas are already experiencing the impacts of inundation (Figure 2).
Figure 1 – DEWNR staff surveying shack areas
Figure 2 – Impacts of flooding in a shack area below Lock 1
These floodplain flows have been significantly altered by river regulation and occur much less frequently. It is important to maintain the integrity of such flow events to provide benefits for water dependent ecosystems, bird and fish breeding opportunities. As the high flows spread across floodplains they also bring important nutrients and carbon back into the main river channel which is important. One impact of the large and irregular floodplain flows is blackwater, this occurs when organic material breaks down in the water and removes oxygen. There is a substantial natural blackwater event along the mid to lower River Murray and lower Murrumbidgee River at the moment producing extremely low levels of dissolved oxygen. Blackwater has a strong unpleasant smell and can cause distress and death to fish and crustaceans. Several small fish kills have been reported in the Murrumbidgee River and Lower River Murray. There is little that can be done to manage blackwater on this scale, however small operational changes to provide refugia for native fish are being implemented where practicable to provide some relief from the blackwater.
Figure 3 – Blackwater entering Lake Victoria (source SA Water)
Communications on this high flow event has been extremely important to ensure that river users can make timely decisions, but also to minimise the impacts on tourism. Now is great time to visit the River Murray and see it in high flows. But, all water users should take necessary safety precautions and be aware that the River Murray under high flows can be a dangerous place with many hazards such as submerged jetties being a prominent feature around shack areas.