Is it worth keeping the Murray mouth open 90% of the time, asks John Cox

10 December 2011

Some serious discussion needs to take place on whether the costs of keeping the Murray Mouth open for 90% of the time are worth the benefits, particularly when Charles Sturt with his keen eye said that most Australian rivers "naturally fail before they reach the coast, and exhaust themselves in marshes or lakes; or reach it so weakened as to be unable to preserve clear or navigable mouths, or to remove the sand banks that the tides throw up before them".

This environmental outcome is one of Minister Burke's most publicised reasons for reducing irrigation allowances.  What he does not mention is that this outcome requires about 7,000 GL of water flowing out to sea in an average rainfall year.

Modelling carried out for the previous Guide indicates that 5,000 GL will flow out to sea through the Murray mouth in a normal rainfall year and by reducing irrigation allocations by 3,000 GL per year then this would increase flows to 7,000 GL per year.  That is, a 3,000 GL cut in irrigation allocations, which is similar to the 2,750 GL per year in the proposed basin plan, would increase flows out of the Murray mouth by about 2,000 GL and leave just 1,000 GL for environmental watering plans within the basin itself.

I doubt very much whether the environmentalists who lobby  for more water to prevent environmental degradation of the Murray Darling basin realise that two thirds of these water savings flow out to sea and have little effect upon the environment within the basin.

Despite the prominence of this environmental outcome for the Murray mouth there is very little explanation or technical details in the report to justify the benefits and costs of this increase in flows through the mouth. 

If the proposed plan was really serious about environmental outcomes in this area then it should have incorporated the recommendations of a major study by environmentalists for the Murray Darling Basin Commission in the year 2000 called "River Murray Barrages Environmental Flows - an Evaluation of Environmental Flow Needs in the Lower Lakes and Coorong".

This report does not recommend more freshwater flows but states that the main factor driving environmental degradation in the Lower Lakes and the Coorong has been the construction of the artificial barrages which have reduced the size of the River Murray estuary to 11% of its natural size.  They state:

"The primary change for the Lower River Murray since the construction of the barrages and the dramatic reduction in flows has been the loss of the vast estuarine system and the consequent loss of biodiversity and ecological resistance.  The ecosystems are at risk of degrading into a uniform condition, having low diversity and interfering with the natural productive capacity of the region, particularly from a fishery perspective".

They state that the value of commercial fish in the remnant estuary is equal to the value from the whole of the freshwater lakes, which is nine times as large.

They recommend moving the barrages up to the real mouth of the Murray near Wellington so as to instigate a tidal prism which would be nine times larger than the present prism and would help to keep the mouth to the sea open.  As an interim measure they recommended increasing the size of the estuary by cutting a channel from Lake Albert to the Coorong and subjecting this lake to tidal changes.

All of these actions would lead to a much greater improvement in the environment of the Lower Lakes and the Coorong than keeping the sea mouth of the Murray open for 90% of the time.  It would also save most of the senseless waste of 7,000 GL of fresh water into the sea, an amount which is of the same order of magnitude as the 10,873 GL of sustainable diversions recommended in the proposed plan.

One of the spurious reasons given for these planned flows to the sea is that it will flush salt from the system.  Low river flows between 2007 and 2010 show up the lack of logic in this argument, as salt levels at the Mannum takeoff for Adelaide water did not rise above tolerable levels when no water went over the barrages in 4 years.  The reason for this is that there have been 18 salt interception schemes constructed along the Murray in the last 40 years with a capacity for removing 1,500 tonnes of salt per day and this has led to a continual improvement in salt levels in the lower reaches of the Murray in SA.

 The proposed plan says that flows are needed to flush 2,000 tonnes from the system but it is probably no coincidence that this is the amount of salt in the Lower Lakes in their present freshwater state.  There would be no need for any flushing of salt under an estuarine scenario.

The lack of integrity of environmentalists in lobbying for cuts in irrigation water is most evident in their treatment of the Lower Lakes of the River Murray.  Despite pushing for a return to natural flows in the rivers and basins of the Murray Darling basin, there have been no suggestions of returning the artificial fresh water lakes formed by the barrages back to their natural estuarine conditions, even though this action will have a remarkable improvement in environmental outcomes for these Lakes. 

 Even worse, in times of drought and low river flows, when these lakes would have naturally reverted back to sea water, they use the low freshwater levels to complain of environmental damage to the ecology of this lakes area and then lobby to obtain the 800 GL needed for the 1 metre of evaporation from these 2 to 3 m deep lakes.  This fresh  water for evaporation would not be required under natural estuarine conditions.

Modelling shows that most of the water that is cut from irrigation allocations in the proposed plan ends up in the Lower Lakes to keep them in an unsustainable freshwater state and to keep the Murray Mouth open.  The environmental benefits of this water are quite minimal and other measures, such as converting these lakes to an estuarine environment, would produce much better environmental outcomes.  It would, moreover,  give the River Murray an estuary like other major rivers around the world as well as saving precious fresh water that would otherwise flow out to sea.

John Cox is a citrus grower in the Waikerie area of SA.