Indicator assessment
CONDITION
TREND
DATA QUALITY
The ACT’s rivers experienced flows well below the long-term average during the drought conditions from 2017 to 2019. Annual river flows were lowest in 2019 with cease-to-flow periods occurring for the Murrumbidgee and Molonglo rivers. These extremely low flows impacted on aquatic ecosystem health as well as the amenity of the ACT’s waterways. The high rainfall years following the drought period resulted in very high flows that were significantly greater than the long-term average from 2021 to 2022. Despite these wetter conditions, the ACT’s annual river flows remain at risk from severe drought periods.
Over the 2019 to 2022 period, annual flows downstream of all ACT water supply reservoirs were greater than the environmental flow requirement despite the significantly reduced rainfall and river flows in 2019.
CONDITION
Environmental condition is healthy across the ACT, OR pressure likely to have negligible impact on environmental condition/human health.
Environmental condition is neither positive or negative and may be variable across the ACT, OR pressure likely to have limited impact on environmental condition/human health.
Environmental condition is under significant stress, OR pressure likely to have significant impact on environmental condition/ human health.
Data is insufficient to make an assessment of status and trends.
TREND
DATA QUALITY
Adequate high-quality evidence and high level of consensus.
Limited evidence or limited consensus.
Evidence and consensus too low to make an assessment.
Assessments of status, trends and data quality are not appropriate for the indicator.
Introduction
For information on this indicator see Background: Water.
River flows are monitored at gauging stations throughout the ACT. With the exception of the Molonglo River, the data presented here are for the ACT’s main natural river systems and does not include flows for Canberra’s heavily modified urban waterways. Information on urban waterway flows can be found in the 2022 Investigation into the State of the Lakes and Waterways in the ACT, this includes information on the of the Molonglo River flows.
The period from 2015 and 2022 saw extremes in weather conditions across the ACT, from the hot and dry conditions between 2017 to 2019, to the very wet conditions from mid-2020 to 2022 (see Climate Change). These extremes had a significant impact on river flows in the ACT and also impacted on the ACT’s water resources (see Human settlement — HS5: Water resources).
River flows
Monitoring results show that the ACT’s annual river flows are highly variable, characterised by generally dry conditions punctuated by wet years which replenish water storages and river systems. Flows also vary seasonally with higher flows usually occurring in the winter and spring months. Prolonged periods of drought conditions such as from 2017 to early 2020 are the most concerning for river flows in the ACT. Long periods of low flows significantly impact on the ecological condition and the functioning of aquatic ecosystems, biodiversity, and human needs such as domestic water supply, irrigation and recreation opportunities.
For the Murrumbidgee River, annual flows upstream (Lobbs Hole) and downstream (Halls Crossing) of the ACT were well below the long-term average from 2017 to 2019 (Figure 14). This followed two consecutive years of annual flows that were higher than long-term average flows from 2015 to 2016. From 2020 to 2022, the Murrumbidgee River experienced flows significantly above the long-term average with 2021 and 2022 having very high flows that were between three times to nearly five times higher than the long long-term term average. Annual flows in 2022 were the highest recorded for the Murrumbidgee River since 1980, both upstream and downstream of the ACT.
Figure 14: Annual flows as a percentage of the long-term average for the Murrumbidgee River at Lobbs Hole, the Murrumbidgee River at Halls Crossing, the Molonglo River at Oaks Estate, the Cotter River at Gingera, and the Paddys River at Riverlea, 2015 to 2022.
Data sourced from: Environment, Planning and Sustainable Development Directorate.
The Molonglo River at Oaks Estate showed similar patterns to the Murrumbidgee River with annual flows that were well below the long-term average from 2017 to 2019, followed by high flows that where around five times higher than the long-term average in 2021 and 2022. The 2022 flows were also the highest since 1980.
The Cotter River at Gingera and the Paddys River at Riverlea both had annual flows that were well below the long–term average from 2017 and 2019. But unlike the Murrumbidgee River, only annual flows in 2016 were above the long-term average. The 2021 and 2022 flows in the Cotter and Paddys rivers were also very high, from two to over three times above the long-term average. The 2022 flows for the Cotter River were the highest recorded since 1980.
The drought conditions from 2017 to 2019 had significant impacts on the ACT’s river flows, aquatic health and river amenity. Annual river flows were lowest in 2019 with the Paddys River flows just 5% of the long-term average, the Molonglo River 8%, the Murrumbidgee River at Lobbs Hole 13%, the Murrumbidgee River at Halls Crossing 16%, and the Cotter River 17% of the long-term average. The impact of drought on the 2019 and early 2020 river flows also included:
- Cease-to-flow to flow periods occurred for the Murrumbidgee River at Lobbs Hole, the Cotter River, the Paddys River and the Molonglo River
- Halls Crossing flows reduced to as low as 71 megalitres a day in late 2019 and early 2020. Much of this flow was from treated effluent discharges from the Lower Molonglo Water Quality Control Centre
- For the Murrumbidgee River, the 2019 annual flows were the third lowest recorded since 1980 at Lobbs Hole and the fourth lowest for Halls Crossing
- For the Cotter River at Gingera, annual flows in 2019 were the second lowest recorded since 1980, and
- For the Paddys River at Riverlea, the 2019 flows were the lowest recorded since 1980.
The impacts of dams and sewage treatment plant discharges on the ACT’s river flows
In addition to rainfall and temperature trends, the natural flows of the ACT’s rivers have been altered by water resource development such as the presence of dams (both within the ACT and NSW) and other barriers that regulate flows, diversion or extraction of in-stream flows, and channel modification.
The flows of the Murrumbidgee, Molonglo and Cotter rivers are all impacted by large dams. For the Cotter River, impacts on flows from the Corin, Bendora and Cotter dams are most likely greatest during average rainfall periods when much of the dam inflows are retained for the ACT’s drinking water supplies. However, the environmental flows released do ensure that the Cotter River has some flow to maintain ecosystems and biodiversity and is especially important during severe drought periods.
The Murrumbidgee River receives an average of 22 gigalitres of water annually from Tantangra Dam. This is less then 10% of the predicted inflows into Tantangara dam. Much of this flow is delivered in spring and for considerable periods of time there are no water releases from the dam. Modelling of potential river flows for the Murrumbidgee River upstream of the ACT estimates that Tantangara Dam significantly reduces river flows. The difference is particularly apparent for the winter and early spring months, with river flows up to five times lower due to the lack of water releases from the dam. Modelling also shows that the impacts of Tantangra Dam is especially severe during drought. For example, the minimal water releases from the dam during the drought period of 2018 to 2019 meant that actual water flows in the river were vastly lower than what would be expected in the absence of water regulation. This resulted in the Murrumbidgee River cease-to-flow event that impacted areas of the ACT and upstream NSW in 2019. For more information see case study on the impact of Tantangara Dam on the Murrumbidgee River flows.
Molonglo River flows are reduced by Googong Dam which retains water for community supply. The biggest impacts occur in wet years following drought periods when water is retained to increase storage levels. This can reduce Molonglo River flows by up to 60%. However, during the severe drought years from 2017 to 2019, environmental flow releases from the dam did ensure that there was some water available for the Molonglo River, although volumes were low.
The Murrumbidgee and Molonglo Rivers also received water from sewage treatment plants. For the Murrumbidgee River, discharges from the Lower Molonglo Water Quality Control Centre can contribute a significant proportion of river flows in severe drought periods. For example, in late 2019 and early 2020, much of the flow in the Murrumbidgee River downstream of the ACT was from treated effluent discharges. This is also the case for the Molonglo River which receives an annual average daily flow of between seven and ten megalitres of water a day from the Queanbeyan Sewage Treatment Plant upstream of the ACT. During the severe drought in 2019, nearly 40% of river flows came from the Queanbeyan Sewage Treatment Plant.
More information on the impacts of Googong Dam and the Queanbeyan Sewage Treatment Plant on the Molonglo River can be found in the 2022 Investigation into the State of the Lakes and Waterways in the ACT.
Water flows entering and leaving the ACT
To ensure that the ACT is not having a detrimental effect on ecosystem health and water supply downstream of the region, it is important that the volume of water leaving the ACT via the Murrumbidgee River should be comparable to that entering the region. Over the 2015 to 2022 period, annual discharges for the Murrumbidgee River leaving the ACT were much higher than those upstream of the region (Figure 15). This occurred despite the significantly reduced river flows from 2017 to 2019. Reasons for increased discharges include contributions from the Cotter River and the Molonglo River, as well as discharges of treated effluent from the Lower Molonglo Water Quality Control Centre.
Figure 15: Comparison of annual discharge for the Murrumbidgee River flowing into the ACT (Lobbs Hole) and leaving the ACT (Halls Crossing), 2015 to 2022.
Data sourced from: Environment, Planning and Sustainable Development Directorate.
These results show that the ACT’s additions to Murrumbidgee River flows are vital for downstream ecosystem health and water supply, particularly during low flow periods.
Environmental flows
Over the 2019 to 2022 period, annual flows downstream of storage reservoirs were greater than the environmental flow requirement. This includes the environmental flow releases below the Cotter, Bendora, Corin and Googong dams. Environmental flow requirements were met despite the significantly reduced rainfall and river flows from in 2019 and early 2020. In 2021 and 2022, there were very high volumes of water releases from all dams with ACT storages at 100% capacity for large periods of time.
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