Delivering water

Agreed water shares delivered to states

The following key actions are undertaken to deliver agreed water shares in the River Murray system to the states, including in extreme conditions:

  • regularly assess the water resources of the River Murray System to determine the volume of water available to each Basin state
  • operate structures under the control of the MDBA and determine and review procedures for their efficient and effective operation
  • establish, operate and maintain a system of continuous monitoring of the volumes of stored water, and of flows in the River Murray and from its tributaries
  • liaise with state and federal authorities on matters related to the River Murray System to provide an up-to-date and comprehensive flow of information.

Rainfall and inflows

Rainfall across most of the Basin was above average during 2011–12 (see Figure 9 below), although rainfall and inflow patterns at times differed considerably from the normal seasonal pattern. Most notably, record rainfall and system inflows in the southern Basin occurred during late summer and early autumn in 2012, a time when, in a more typical year, this part of the Basin is drying out. See case study 8 for more information about the big wet of 2010–12.

Figure 10A (below) shows the Basin's rainfall deciles from 1 January to 31 March 2012, compared with the rainfall deciles from 1 April to 30 June 2012 shown in Figure 10B (below).

Basin-wide above-average rainfall generated a total inflow to the River Murray System (including inflows to Menindee Lakes and excluding releases from Snowy Mountains Scheme) of around 16,700 GL in 2011–12, putting the year in the wettest 17% of years (annual exceedance probability, or AEP, of 17%)

Basin-wide above-average rainfall generated a total inflow to the River Murray System (including inflows to Menindee Lakes and excluding releases from Snowy Mountains Scheme) of around 16,700 GL in 2011–12, putting the year in the wettest 17% of years (annual exceedance probability, or AEP, of 17%) (see Figure 11).

Historical River Murray Sytem annual inflows from July 1891 to June 2012

River Murray Sytem daily inflows

In the upper Murray catchment, average rainfall occurred in winter and spring, with the most significant rain event occurring over summer (from late February into early March), when parts of south-east Australia recorded their wettest seven-day period on record. Record seven-day totals included 525 mm at Mount Buffalo, 442 mm at Thredbo Village, 362 mm at Batlow, 354 mm at Falls Creek, 346 mm at Chiltern and 329 mm at Burrinjuck Dam.

This exceptionally heavy rainfall event led to major flooding in parts of southern New South Wales and northern Victoria, including the highest flood levels since 1974 at several locations along the Murrumbidgee River in New South Wales and along Broken Creek on the River Murray in Victoria. A large portion of inflows from this rainfall event was captured in the Hume and Dartmouth reservoirs, resulting in only minor flooding at a few locations along the mid-section of the River Murray. This was the fourth time in 18 months that the Dartmouth and Hume reservoirs prevented significant flooding downstream of Hume Dam.

River Murray System inflows (excluding the inflows to Menindee Lakes and releases from the Snowy Mountains Scheme) totalled 11,700 GL during the year (AEP of 26%), compared with the long-term median of 8,200 GL. However, the inflow pattern this year has differed considerably from the long-term average (Figure 12). Inflows in the spring were relatively low at about 2,270 GL (AEP of 71%), while the inflow over the autumn was extremely high, at about 4,360 GL (AEP of 1%).

The March inflow was the highest on record, double the previous record set in 2010–11, and the inflow in April was the second highest on record. While the period from mid-March to June 2012 was relatively dry, with below-average rainfall, above-average streamflows have persisted because of very wet catchment conditions.

In the upper Darling catchment, significant rainfall events occurred in late November 2011 and in late January – early February 2012. Record peak flows were observed along the Maranoa and Balonne rivers, with very high river levels also recorded along the Paroo, Warrego, Moonie, Weir, McIntyre, Gwydir and Namoi rivers. The large number of rivers flooding simultaneously resulted in major flooding in the Darling River at Bourke, New South Wales, where flow reached 237,000 ML/d in March 2012, the highest flow at Bourke since the 500,000 ML/d recorded in March 1976.

As a result, the Menindee Lakes began prerelease/flood operation on 14 December 2011, under the day-to-day management of the New South Wales Government, and continued for the remainder of the year. Total inflows to Menindee Lakes were 4,870 GL (AEP of 13%), more than five times the long-term median of 945 GL. At the end of June 2012, the lakes remained surcharged. The releases have provided a second consecutive year of much-needed water to floodplains along the Lower Darling and Great Darling Anabranch as well as parts of the floodplains along the River Murray downstream of Wentworth.

Active storage

A second consecutive year of higher-than-average rainfall resulted in inflow conditions that pushed water storages in the River Murray System to very high levels at the end of June 2012. Total MDBA active storage has been well above the long-term average since December 2010 (see Figure 13).

MDBA active storage, June 2000 to June 2012

State diversions, River Murray system: 1991-92 to 2011-12

Total MDBA active storage on 30 June 2012 was 7,945 GL, including about 1,438 GL in Menindee Lakes. This is the highest recorded end-of-June active storage since the construction of Dartmouth Reservoir in 1979.

At 30 June 2012, the following volumes are available for use in the Murray in 2012–13:

  • about 180 GL of water in inter-valley trade accounts in the Murrumbidgee and Goulburn valleys
  • 230 GL of River Murray Increased Flow (RMIF) environmental water (stored in the Snowy Mountains Scheme)
  • 59 GL of water in the Upper States Drought Account.

Water shares for New South Wales and Victoria in MDBA storages at the beginning and end of 2011–12 are shown in Table 4.

Table 4. Water shares for New South Wales and Victoria — end June 2011 and June 2012

Storage at end June 2011 (GL) [a]

Storage at end June 2012 (GL) [a]

Storage

NSW

Vic

Total

NSW

Vic

Total

Dartmouth Reservoir

1,186

1,304

2,490

1,664

1,692

3,355

Hume Reservoir

1,404

1,404

2,808

1,434

1,434

2,869

Lake Victoria

241

241

482

241

241

481

Menindee Lakes [b]

978

978

1,956

959

959

1,918

Total [c]

3,809

3,927

7,736

4,296

4,323

8,619

a. Data relates to total storage.

b. Menindee Lakes releases at 30 June 2012 were being managed by NSW as part of flood operations. MDBA may later call on water from Menindee Lakes when flood releases cease and downstream demands increase. MDBA will cease to be able to call on water from Menindee Lakes when the storage volume next reduces to less than 480 GL.

c. Accounts are based on the best available data, which may contain some unverified operational data that could change in the future. Figures are rounded to the nearest GL.

State water allocations, diversions and carryover

Murray Valley water allocations started higher in 2011–12 than in recent years.

South Australia started the year with a 100% allocation, the first time since 2002–03. The water sharing plan (WSP) for the NSW Murray and Lower Darling Regulated Rivers water sources recommenced in July 2011, having been suspended since 2006 because of the severe drought. The New South Wales high security allocation started at 97% and New South Wales general security access licence holders, while not receiving an initial allocation, had access to a carryover volume equivalent to 77% of entitlement.

In Victoria, high reliability water shares started with an allocation of 21% compared with starting allocations for the previous four years of zero. By mid-November 2011, allocations had increased to 100% for NSW high security and general security access licence holders and 100% for high reliability water shares in Victoria.

On the Lower Darling River, both general and high security water holders had a 100% allocation for the entirety of 2011–12.

Despite allocations reaching 100% by mid-November, the total amount of water diverted by Basin states was relatively low, at about 3,300 GL, compared with other high allocation years over the past 20 years when diversions ranged from between 4,000 and 5,000 GL (see Figure 14 above). The unseasonably high rainfall in many irrigation areas meant that less irrigation was required to meet crop and pasture water requirements.

The New South Wales volume of carryover water this year was reduced markedly, to around 700 GL, down from 1,600 GL last year, caused mainly by the reintroduction of the WSP for the NSW Murray and Lower Darling Regulated Rivers water sources, which reduced the maximum allowable carryover from 100% (while the WSP was suspended) to 50 % of entitlement. In Victoria, the carryover into 2012–13 is about 1,400 GL, higher than last year's figure of 1,200 GL.

Flow to South Australia

This year was the first since 2002–03 that South Australia began the year with its full entitlement of 1,850 GL assured. Additional dilution flow has been delivered to South Australia since 1 August 2010, because the total volume of Hume and Dartmouth reservoirs exceeded 2,000 GL and the volume of Menindee Lakes exceeded the required monthly trigger volumes. Additional dilution flow is likely to continue well into 2012–13.

Unregulated flow to South Australia began in September 2010 and continued until the start of November 2011. It recommenced briefly in late December continuing into early January 2012, before resuming in February and continuing for the remainder of the water year. Between October 2011 and March 2012, the flow to South Australia included more than 350 GL of environmental water.

The total annual flow across the South Australian border, including additional dilution flow, unregulated flow, environmental water and traded water, was about 10,200 GL (AEP probability of 20%) compared with 15,100 GL last year. The long-term median annual flow to South Australia is 5,200 GL.

The Murray component of the Snowy Mountains Scheme

The Snowy Scheme terms of operation are defined in the Snowy Water Licence (SWL), which sets a minimum release that must be achieved by the licensee (Snowy Hydro Limited, or SHL) over the course of the Snowy water year, which runs from 1 May to 30 April.

The required annual release (RAR) volume may change throughout the course of the Snowy water year and Snowy Hydro Limited is free to release volumes in excess of the required release.

The Snowy Water Licence was amended in October 2011, to give MDBA partner governments the right to build a callable drought reserve in the Snowy Scheme and to remove SHL's obligation to make good previous reductions in the minimum release volume allowed as a result of low inflows to the Snowy Scheme.

Partner governments were also given the right to call River Murray Increased Flows (RMIF) from the Snowy Scheme under certain conditions, and SHL given the flexibility to make prereleases on the required release of subsequent years. Before October 2011, this flexibility was only available in drought sequences.

In 2011–12 the required annual release volume was 176 GL on 1 May 2011. This volume increased during the year, with a final obligation of 477 GL. The SHL released a total of 770 GL in 2011–12; the 293 GL released in excess of the RAR in 2011–12 reduces the RAR for 2012–13.

Operating the River Murray System

System operations during 2011–12 were similar to those in 2010–11 and were mainly determined by high inflows rather than high demands for water. While operations still aimed to maximise water availability, the frequent high inflows meant that operations were concentrated on delivering environmental outcomes and safely passing high flows through storages without increasing peak flows.

Upper Murray system

At the start of 2011–12, storage was 2,444 GL in Dartmouth Reservoir (63% of capacity) and 2,812 GL at Hume Reservoir (94% of capacity). Storage in Dartmouth Reservoir increased to 87% capacity by June 2012.

Moderately wet conditions and the high initial storage in Hume Reservoir meant that water did not have to be transferred from Dartmouth to Hume Reservoir. Releases from Dartmouth Reservoir were held near to minimum (200 ML/d) for most of 2011–12, with some exceptions:

  • four entitlement releases for power generation by AGL Hydro during June and August
  • three short pulses of flow in December and January for water quality management in the Mitta Mitta River
  • two periods during January and February when release was varied from between 200 ML/d and 300 ML/d to maintain the flow at Tallandoon at about 600 ML/d to assist access for local diversions.

At Hume Reservoir, the release to meet downstream demands over the irrigation season was relatively low because of high inflows from downstream tributaries and reduced demand across the irrigation areas following good rainfall. The release, for demand purposes, only exceeded 20,000 ML/d on one day in November and for one week in early January — at both times, this release included water for environmental purposes. The highest release was 25,000 ML/d, which occurred in August for four days as part of flood management when the reservoir was spilling as it approached full supply level.

Environmental and irrigation releases from October to February resulted in Hume Reservoir being drawn-down to 63% by late February 2012. However, the large rain event in February–March resulted in the storage regaining about 820 GL (27% capacity). During this event the peak daily inflow to Hume Reservoir was more than 120,000 ML/d, one of the highest daily inflows on record. The full capture of this event by storing it in Hume Reservoir prevented major flooding at Albury–Wodonga, in contrast with major flooding in neighbouring rivers and streams.

At 30 June 2012, the volume in storage at Hume Reservoir was 2,869 GL (95.5 % capacity) — effectively the reservoir was full — and spill release had begun in early June to provide limited airspace to reduce the risk of possible flooding.

Mid-Murray

Flood operations were undertaken at Yarrawonga Weir in August 2011, with high flows from Hume Reservoir and from the Kiewa and Ovens river catchments. The peak release was 52,500 ML/d on 22 August. In late February to early March, 2012, 264 mm of rain fell at Yarrawonga, causing very high local inflows as well as inflow from the Kiewa and Ovens river catchments. Releases were greater than 25,000 ML/d from 1 to 11 March, including a peak release of 62,100 ML/d (below minor flood level) on 7 March.

The Barmah Choke was not a major constraint on the delivery of water to downstream users during 2011–12. The rule preventing trade of allocations from above to below the choke has been relaxed since September 2007. We review the relaxation each fortnight, but it has remained uninterrupted.

Inflow from the Goulburn River totalled 1,875 GL for the year (AEP of 27%), measured at McCoys Bridge, with a peak inflow briefly reaching the minor flood level on 9 March 2012.

At Torrumbarry, the flow exceeded 25,000 ML/d for two periods between late July and early September, and for most of March, resulting in overbank flooding into the Gunbower and Koondrook–Perricoota forests. A peak flow of 33,300 ML/d was recorded on 28 August, with another peak of 30,800 ML/d on 23 and 24 March (minor flood level occurs at about 39,000 ML/d).

The Murrumbidgee catchment experienced major flooding during March and April 2012. The peak flow on the Murrumbidgee River at Wagga Wagga was about 440,000 ML/d on 6 March, the highest flow since the record flood in 1974 when flow at Wagga peaked above 490,000 ML/d. The March 2012 flood was greatly attenuated as it travelled downstream — at Balranald, the Murrumbidgee River only reached minor flood level for around 10 days, with a peak of 29,500 ML/d on 25 April. Total inflow from the Murrumbidgee River during the water year was in excess of 2,400 GL (AEP of 10%).

At Euston, the flow was greater than 29,000 ML/d from late July to late September 2011 and again from mid-March to mid-May 2012. The peak flow during the year was 40,200 ML/d on 18 September (minor flood level occurs at about 88,000 ML/d).

Downstream of the confluence of the Murray and Darling rivers, the flow at Wentworth averaged 29,100 ML/d during the water year. The flow was above 10,000 ML/d throughout the year, except for a short period during November 2011, with a peak flow of 58,400 ML/d from 4 to 7 May 2012 (minor flood level occurs at about 87,000 ML/d).

Lake Victoria

Operations at Lake Victoria throughout 2011–12 were consistent with the Lake Victoria Operating Strategy. In mid-July 2011, the lake was filled to 25.78 m Australian height datum (AHD); at this time, sufficient water was in transit upstream to allow operations to draw the lake down to minimise time spent at high lake levels and to provide a small boost to the South Australia flow, taking it to about 34,000 ML/d.

In August 2011, the lake level was raised and lowered, and in the second half of September it was raised to 26.1 m AHD (84% capacity) as conditions became drier. During the second half of October and early in November, the lake increased to 100% capacity (27 m AHD) and remained close to full until early December. The lake was then drawn-down to around 26 m AHD (83% capacity) by early January 2012, where it remained until early February.

In February, projected high inflows permitted the lake level to be lowered for the remainder of the season. However, during March the lake level was temporarily increased when higher river flows were mitigated to assist downstream construction works. The flow to South Australia peaked at around 60,000 ML/d during April, including some water released from Lake Victoria to draw the lake down to 24.1 m AHD (52% capacity) which provided an opportunity to conduct a lakebed survey for Aboriginal cultural heritage material. Refilling of the lake began in early June 2012; at the end of the month, Lake Victoria was at 71% capacity and sufficient flows were in transit to enable its rate of filling to be slowed.

The operation of Lake Victoria during 2011–12 provided favourable hydrological conditions for the continued growth of many juvenile spiny sedge (Cyperus gymnocaulos) plants that had propagated in 2010–11. As well, we observed modest development of new spiny sedge bulbils following drawdown over recent months. However, many river red gum (Eucalyptus camaldulensis) seedlings present on the foreshore last year did not survive the summer inundation. Red gums fared better at higher elevations where inundation did not last as long.

Menindee Lakes, Lower Darling River and the Great Darling Anabranch

Total inflows to Menindee Lakes between July 2011 and June 2012 were about 5,000 GL (AEP of 13%) compared with the long-term median annual inflow of about 945 GL. The high inflows caused by two major rainfall events in northern New South Wales and southern Queensland.

The first event occurred in late November 2011, when heavy rainfall triggered large flows in the Gwydir and Border rivers and the Namoi catchment. These large flows caused a peak flow in the Darling River at Bourke of 73,000 ML/d and a peak inflow to Menindee Lakes of around 36,000 ML/d.

The second event, in late January and early February 2012, generated high flows in the Namoi, Gwydir, Moonie and the Balonne–Culgoa–Bokhara river systems. The Darling River experienced major flooding from this event, with a peak flow at Bourke of around 240,000 ML/d (major flooding) and a peak inflow to the Menindee Lakes of near 60,000 ML/d. The NSW Office of Water has overseen the daily flood operations at the lakes since 14 December 2011. At the end of June 2012, Menindee Lakes were still surcharged, storing 1,918 GL (111% capacity).

Releases from the lakes, measured at Weir 32, were pulsed by the NSW Office of Water and varied between 200 ML/d and 1,000 ML/d from July to December 2011. The pulses were aimed at encouraging water oxygenation in the Lower Darling to reduce the likelihood of fish kills. From December, the release was increased to pass floodwater; by mid-April 2012, the release reached close to 36,000 ML/d. Releases along the Lower Darling River were gradually reduced to 500 ML/d throughout April, May and June to minimise potential riverbank damage.

The past year was the second consecutive year of substantial flows to the Great Darling Anabranch. Flow passed into the anabranch from late December 2011 until the end of May 2012, rejuvenating riparian and aquatic ecosystems and connectivity for fish passage through to the River Murray. Return flows to the River Murray began around the end of April and are likely to continue for some months.

Lower Lakes and barrage operation in South Australia

Releases through the barrages continued throughout the year. However, these releases were restricted during short periods when downstream water levels in the Coorong exceeded upstream water levels in Lake Alexandrina because of high tides and/or storms. It is estimated that in excess of 7,000 GL of water flowed to the Southern Ocean during 2011–12, compared with the estimated long-term average of 4,900 GL. The peak release from the barrages was about 76 GL/d in late May 2012.

In winter and spring 2011, water releases through the barrages were managed to 'actively' vary the level of the Lake Alexandrina from between 0.55 m and 0.85 m AHD, to export salt from Lake Albert. Improving water quality in Lake Albert continues to be problematic because of its single narrow connection to Lake Alexandrina. Passive water exchange between the lakes is primarily driven by wind seiche that causes changes in the lakes' water levels which forces water through the narrows. By actively varying the water level in Lake Alexandrina, we aimed to replace higher salinity water from Lake Albert with fresher water, reducing Lake Albert's salinity level.

Overall, the operation of the lakes in 2011-12 to improve salinity levels in Lake Albert was largely successful. While salinity levels fluctuated over the year, the overall trend was downwards, with Lake Albert's salinity levels being reduced from an average of 5,700 EC in July 2011 to less than 3,900 EC in June 2012.

Commonwealth environmental water was used during late spring and early summer 2011 to permit higher releases through the barrages. These higher releases provided fish passage and delivered nutrients to the Coorong's mudflats; they also improved the Coorong's salinity level and helped its environment recover (particularly the important native submerged plant Ruppia tuberosa). Lake levels were lowered in December to maximise flow to the Coorong to maintain water levels during this period of high irrigation demand.

From late summer, flows from the barrages increased because of flood flows from the Darling River and later from the upper Murray and Murrumbidgee catchments. In autumn and winter 2012, management of the lakes focused again on improving the salinity levels in Lake Albert by actively cycling water levels.

Delivering environmental water

During 2011–12, MDBA River Operations assisted with the delivery of environmental water held by the Basin states, The Living Murray and the Commonwealth Environmental Water Holder, to target a range of environmental outcomes. Our assistance included coordinating the entry of environmental water from tributaries into the River Murray System.

About 424 GL of environmental water was released from Hume Reservoir between mid-October 2011 and early February 2012. These releases were timed to maintain water levels in key colonial waterbird breeding areas and other wetlands in the Barmah–Millewa Forest during periods of lower flows. The releases also provided small flow pulses in the river to assist fish migration and spawning along the length of the River Murray. A large portion of this flow also contributed to higher flows into South Australia, including the Coorong, over spring.

Over 350 GL of environmental water was provided across the South Australian border over summer to stimulate and maintain fish breeding and recruitment, particularly of large-bodied native species, throughout the lower River Murray channel. The higher flow also provided higher releases through the barrages to improve salinity levels in the Coorong and to support keeping the Murray Mouth open.

Extensive flooding along Broken Creek, Murrumbidgee River and Billabong Creek and within the Koondrook–Perricoota and Gunbower forests this year caused parts of the river system to be affected by blackwater. However, these events were much less severe than the blackwater events that occurred last year when the drought first broke.

Blackwater events occur naturally when organic matter is washed from floodplains into rivers by floods. This matter breaks down and the river water becomes discoloured and, at times, deoxygenated, resulting in fish kills. The 2011–12 River Murray blackwater event caused a small number of observed fish deaths but also provided nutrients for the river system, which will promote the growth of many aquatic organisms.

Australian government and New South Wales and Victorian government agencies collaborated with the MDBA in implementing blackwater dilution actions along the River Murray and its associated streams. In some cases using Murray Irrigation Limited infrastructure and the provision of specific flow rates at key locations enabled the dilution blackwater as it returned to the river from the floodplain.

River Operations helped construct environmental works such as fishways and regulators by adjusting flows and weir-pool levels where feasible. Our forecasts of the timing and magnitude of flood peaks and recessions enabled state constructing authorities and their construction contractors to make more informed river management decisions.

At Yarrawonga Weir (Lake Mulwala), the lake level during winter 2011 was held to around 2.5 m to 3 m below full supply level, to help control the non-native aquatic weed 'dense waterweed' (Egeria densa) that had recolonised some shallower parts of the lake. We have successfully undertaken this operation several times in recent years. The lake was refilled by early August 2011, and monitoring in March 2012 found that the abundance and distribution of dense waterweed was the lowest since monitoring began in 2008. The weed now occupies ~1% of the volume of the lake, compared to ~60% in June 2008.

Please see case study 9 for more information about the dense waterweed control program and its outcome.

Back to top