The Impacts of Water Regulation and Storage on the Basin's Rivers

Introduction

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The massive construction program of river regulatory and water storage structures of the past one hundred years (Figure 1) has made possible the Basin's large irrigation industry and much of its other economic activity.

Figure 1 Rivers, regulatory structures and major storages in the MDB

It has also resulted in major changes to the Basin's aquatic ecosystems and, in particular, its rivers (Maheshwari et al. 1995; Finlayson et al. 1994). These were made very clear in the study of water use within the Basin, An Audit of Water Use in the Murray-Darling Basin (see The Cap). In brief, the study demonstrated that

• there have been substantial changes to the flow regime of the rivers of the Basin;
• these changes are continuing; and
• unless there are major changes to current management policies, these changes will continue into the future (MDBMC 1995, 26).

This page outlines the main features of the changes that have taken and continue to take place.

Overall reduction of river flows

Of the water that would have originally reached the sea from the Murray-Darling Basin, over two-thirds is now diverted from its rivers each year. Mean outflow from the Murray to the sea has been reduced from some 12,300 GL per year under natural conditions to 4,900 GL per year, or as low as 40 per cent of natural flows according to some sources. The mean figures, however, are influenced too much by the large floods. The median annual flow to the sea (i.e. the flow that is exceeded in 50 per cent of years), is now only 27 per cent of the natural median flow. From a figure of around 3,000 GL in 1930, diversions now total over 11,000 GL (Thomson 1994, 8). On some occasions, as in 1981 and for a number of months in 1995, water has ceased to flow to the sea, though in this case it was partly due to drought conditions. Further, as Figure 2 indicates, the increase in diversions has been particularly marked since the late 1950s, with over 90 per cent of the diverted water used for irrigation.

Figure 2 Total Murray-Darling Basin annual diversions (excluding Queensland), 1930-31 to 1990-91 (source: Thomson 1994)

The mean annual flow of the Basin's river system is less than half of the long-term mean under natural conditions. A comparison of natural and the current regulated conditions shows that reduced overall flows are now experienced in virtually all of the Basin's major rivers (Table 1). In dry years, the differences between the figures are even higher. Also, diversions have increased since these figures were compiled, especially along the Rivers in the northern parts of the Basin. The only significant exceptions to the reduced flows are in the upper reaches of the Murray and Murrumbidgee rivers where flows have been increased as a result of diversions through the Snowy Mountains scheme (MDBMC 1995, 14).

One very significant result of the reduced flows though out the Basin is that the rivers are now in a state of drought (as defined by river levels) for more than 61 years in every 100 compared with 5 years per hundred under natural conditions (MDBMC 1995, 19). This is a particular issue on the lower reaches of the river system, especially for the Coorong and the mouth of the River Murray. On the other hand, regulation has eliminated most of the extreme low flows, though they are still a feature of the Darling system. Without regulation, the Murray may well have ceased to flow during the drought of 1982-83.

From a Basin-wide perspective, there has been a shift from a steady variation in mid-range flows under natural conditions to a dominance of very low flows and occasional high flow events (Thomson 1994, 10).

This is the flow regime pattern of an arid region river system (Walker 1992).

An illustration of the impact of diversions on an individual river is provided by the Murrumbidgee since the 1930s (Figure 3).

Figure 3 Annual diversions from the Murrumbidgee River, 1930-31 to 1990-91 (source Thomson 1994)

Median annual outflows from the Murrumbidgee are currently only 25 per cent of the natural outflow despite the extra supplies from the Snowy Mountains scheme. Flows that were only experienced in the worst droughts under natural conditions are now expected in 57 per cent of years (MDBMC 1995, 21-23).

Reduction of flows in the Darling is a particularly controversial issue and one that has been complicated by the droughts of recent years. It is a complex situation that has often been over-simplified and one that, at times, has become very emotional (Marshall 1993). However, the increased diversions are a fact. In 1960, diversions from the Barwon-Darling and the New South Wales and Queensland tributaries were 50,000 ml; in 1990-91, they were 1.4 million ml. The increase has been particularly marked over the last 20 years (Figure 4).

Figure 4 Annual diversions from the Darling River, 1930-31 to 1990-91 (source Thomson 1994)

As a consequence,

Median annual outflows from the Darling are currently 60 per cent of the natural outflow. Flows that were only naturally experienced in the driest 10 per cent of years are now expected in 27 per cent of years (MDBMC 1995, 25).

The increase in diversions has been primarily due to the expansion of the cotton industry and the use by growers of large on-farm water storage. As a result, there has been much conflict along the Darling and especially along some of its tributaries, between graziers, conservationists and irrigators (Figure 5 illustrates the situation for the Gwydir).

Figure 5 Annual diversions from the Gwydir River, 1930-31 to 1990-91 (source Thomson 1994)

The non-irrigators hold the irrigators responsible for the reduced flows; the irrigators say that seasonal conditions over recent years are to blame (Osborne 1995). The reduced flows, especially from small to medium sized floods, have severely impacted on graziers in southern Queensland and northern New South Wales who have lost the beneficial effects of the small to medium sized floods. Many of the areas wetlands, such as along the Gwydir and Gingham, have also been affected (see Wetlands).

Changes to flood flows

Increased storage and regulatory structures have resulted in a significant reduction of the numbers of flood flows. Whilst the very big floods, such as that of April-May 1990, are affected to only a limited extent (see Surface Water Resources), the small to medium sized floods have been virtually eliminated on most rivers.

Changes to seasonal flows

Regulatory and storage structures have been constructed and are managed essentially in the interests of irrigation. As a result, quite apart from the overall reductions in flow, the seasonal flow regimes are different to those under natural conditions. However, they have changed in different ways in different parts of the Basin (MDBMC 1995, 14-25).

At Albury, downstream of the Hume Reservoir, not only is the average annual flow in the Murray 10 per cent higher than under natural conditions (as a result of diversions through the Snowy Mountains scheme), the seasonal flow patterns are virtually the reverse of those under natural conditions (Figure 6).

Figure 6 Median monthly flow at Albury, in GL per month (source: MDBMC 1995, 14)

At Yarrawonga, downstream of two major irrigation offtakes, Mulwala Canal and Yarrawonga Channel, the regulated regime is again very different to the natural one, though not the complete reversal experienced at Albury (Figure 7).

Figure 7 Median monthly flow downstream of Yarrawonga Weir, in GL per month (source: MDBMC 1995, 15)

"At Euston, which is well downstream of the major diversions and of all the tributaries other than the Darling, flows from rivers without storage largely maintain the seasonal pattern of flow but at a much reduced volume compared with natural conditions" (MDBMC 1995, 16) (Figure 8).

Figure 8 Median monthly flow at Euston Weir, in GL per month (source: MDBMC 1995, 16)

By the time the Murray reaches the barrages at the mouth of the River, there is little difference in the patterns of flow regimes under regulated and natural conditions. However, as indicated earlier in this chapter, the critical difference is the reduction of flows, the effect of which is particularly significant in dry years (Figure 9).

Figure 9 River Murray flow over the barrages during dry and wet sequences of years (source: MDBMC 1995, 17)

The same situation applies to the Goulburn, Broken, Campaspe and Murrumbidgee Rivers (Figures 10 and 11).

Figure 10 Flow in the Murrumbidgee River at Balranald during dry and wet sequences of years (source: MDBMC 1995, 21)

Figure 11 Murrumbidgee River median monthly natural and current flows, based on the data for the period 1982 to 1991 (source: MDBMC 1995, 23)

Natural flows in the Darling River system are much more variable than those in the Murray-Murrumbidgee system. However, here too there have been no significant impacts on the seasonal flow regimes as a result of the regulation that has taken place. The main impact has been in terms of the reduced overall flows (Figure 12).

Figure 12 Darling River flow into the Menindee Lakes during dry and wet sequences of years (source: MDBMC 1995, 23)

There are other features associated with the changes to seasonal flow patterns as a result of regulation. Firstly, the naturally low summer flows are replaced by the rivers running at bankfull state for the supply of water for irrigation. Secondly, as such water is frequently released from the lower levels of storage's, it is as much as 10°C cooler in summer than under natural conditions. Such lower water temperatures have significant effects on aquatic fauna (see Water Quality). Thirdly, as a result of variations in flows with releases from storages, river levels can change very rapidly, both up and down. Such changes in flows and levels are very much greater than occur under natural conditions. They have impacts on fauna and river bank stability.

Some of the consequences

River flows and flow regimes are critical to river ecosystems. The changes outlined above have had dramatic impacts on the environmental health of the Basin's aquatic ecosystems and the rivers in particular. It has been suggested that the Basin has been "drained to death" (The Australian, October 20, 1995). Some scientists have stated that rather than simply placing a cap on water use, demand must be reduced by 20 per cent.

Other pages consider in more detail some of the consequences of river regulation and water storage's (see also The Living Murray Program).

Conclusion

As one of the Murray-Darling Basin's former Commissioners has stated, the water audit's message is plain: the amount of water presently taken from the rivers is not ecologically sustainable and a new balance between the environmental requirements and the consumptive use will have to be struck (Toyne 1995).

This is essential for the long term viability of not only the aquatic ecosystems and rivers, but also virtually all economic activity within the Basin.

References

DBBRC (1995): Impact of Water Use on Flows in the Condamine/Balonne/Culgoa River System. Dumaresq-Barwon Border Rivers Commission, Brisbane.

Finlayson, B.L. et al. (1994): "Effects of reservoirs on downstream aquatic habitat". Water, 21(4), 15-20.

Maheshwari, B.L. et al. (1995): "Effects of regulation on the flow regime of the River Murray, Australia". Regulated Rivers: research and management, 10, 15-38.

Marshall, A. (1993): "The disappearing Darling". The Land, May 27, 12-13.

MDBC (1990): The River Murray System: the regulation and distribution of River Murray waters. Murray-Darling Basin Commission, Canberra.

MDBMC (1995): An Audit of Water Use in the Murray-Darling Basin. Murray-Darling Basin Ministerial Council, Canberra.

Osborne, A. (1995): "Irrigators not to blame for no flow". The Land, May 18.

Thomson, C. (1994): The Impact of River Regulation on the Natural Flows of the Murray-Darling Basin. Technical Report 92/5.3. Murray-Darling Basin Commission, Canberra.

Toyne, P. (1995): "Water use and environmental flows in the Murray-Darling Basin". In Proceedings of the Water Use and Environmental Flows Workshop, 22-23 August 1995. Murray-Darling Basin Commission, Canberra.

Walker, K.F. (1992): "A semi-arid lowland river: the River Murray, Australia". pp. 472-492 in The River Handbook, Volume 1, edited by Calow, P.A. & Petts, G.E. Blackwell Scientific, Oxford.

Table 1: Mean and median annual flows during natural and current conditions since 1892 (source: Water Audit Study, Murray-Darling Basin Commission, Canberra)

* The Gwydir River end of system flows are greater under current conditions than under natural conditions because of the large amount of re-regulation channelisation that has occurred. Under natural conditions, the majority of the flow went down the Gwydir River and Gingham watercourse to the wetlands; water only went down the Mehi, Moomin and Carole Channels under high flows. With development, flows are being regulated down the Mehi, Moomim and Carole Channels which join up with the Barwon River