All river flows were well below the long-term average in 2017 and 2018. A continuation of these conditions will have consequences for the ecosystem health and amenity of rivers in the ACT. All discharges downstream of storage reservoirs met the environmental flow requirement; this took place despite the significantly reduced rainfall and river flows in 2017 and 2018.
Indicator assessment legend
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.
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
The ecological condition and functioning of rivers are strongly linked to natural flow regimes. A flow regime is the timing, size and duration of river flow events. It is a key driver of river and floodplain wetland ecosystems, influencing river morphology, biodiversity, and the processes that sustain aquatic ecosystems. Modification of natural flow regimes may affect biodiversity, alter riverine habitat, and facilitate the invasion of exotic species. Aquatic plant and animal species have evolved life histories directly in response to the natural flow regimes. Healthy river flows are also required to support human activities and needs such as domestic water supply, irrigation and recreation opportunities.
The natural flows in ACT rivers 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.
In the ACT, natural flows have been altered by water resource development such as the presence of dams and other barriers, regulation of flow, diversion or extraction of in-stream flows, and channel modification. Rainfall interception for farm dams can also impact on natural flows but this is minimal in the ACT with farm dams estimated to reduce run-off to surface water by approximately 1%.Kalisch, D. and R. Argent, 2019, Integrated Water Accounts for the Canberra Region 2013–14 to 2016–17, Australian Bureau of Statistics and the Bureau of Meteorology, Canberra.
Land use also influences river flows; for example, highly urbanised catchments and cleared agricultural lands can quickly increase river flows due greater rainfall run-off. Other impacts on river flow include fire and plantation forestry. These activities can both increase and decrease flows through the removal of vegetation which increases rainfall run-off, and regeneration which increases the uptake of water.
Condition and trends
River discharge is measured at gauging stations throughout the ACT. Annual data is presented for the 2015 to 2018 period only. For the Murrumbidgee River, annual discharges were well below the longterm average in 2017 and 2018 for gauging stations located upstream and downstream of the ACT (Figures W14 and W15). This followed two consecutive years of annual discharges higher than longterm average flows from 2015 to 2016. This trend in annual discharge relative to long-term flows was also evident for the Molonglo River (Figure W16).
Figure W14: Annual discharge for the Murrumbidgee River at Lobbs Hole, 2015 to 2018.
Figure W15: Annual discharge for the Murrumbidgee River at Halls Crossing, 2015 to 2018.
Figure W16: Annual discharge for the Molonglo River at Oaks Estate, 2015 to 2018.
Discharges for the Cotter River and Paddys River also had annual discharges that were well below the longterm average in 2017 and 2018 (Figures W17 and W18). But unlike the Murrumbidgee and Molonglo rivers, only annual discharges in 2016 were above the long-term average.
Figure W17: Annual discharge for the Cotter River at Gingera, 2015 to 2018.
Figure W18: Annual discharge for Paddys River at Riverlea, 2015 to 2018.
Annual discharges were lowest in 2018 due to the lack of rainfall (see Indicator CC1: Climate trends). This resulted in annual discharges significantly lower than longterm averages. For example, the annual discharge in Paddys River was just 7% of the longterm average; Molonglo River 15%; Murrumbidgee at Lobbs Hole 17%; Murrumbidgee at Halls Crossing 19%; and Cotter River 24%.
These reduced discharges have consequences for ecosystem health as well as the amenity of the ACT’s waterways. The low rainfall and river flows in the ACT’s drinking-water catchments has also impacted on the ACT’s water resources (see Indicator HS5: Water resources).
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 2018 period, annual discharges for the Murrumbidgee River leaving the ACT were much higher than those upstream of the region (Figure W19). This occurred despite the significantly reduced river flows in 2017 and 2018. Reasons for increased discharges include contributions from the Cotter River and Molonglo River, as well as discharges of treated effluent from the Lower Molonglo Water Quality Control Centre.
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.
Figure W19: Comparison of annual discharge for the Murrumbidgee River flowing into the ACT (Lobbs Hole) and leaving the ACT (Halls Crossing), 2015 to 2018.
Environmental flows describe the quantity and timing of water required to maintain the health of aquatic ecosystems affected by water resource development. In the ACT, environmental flow requirements are specified in the Environmental Flow Guidelines, an instrument under the Water Resources Act 2007.Water Resources Act 2007 (ACT), found at https://www.legislation.act.gov.au/a/2007-19/, accessed 20 October 2019.
In heavily used river systems, such as water supply catchments, environmental flows can be delivered in ways that protect specific components of the flow regime, to help keep stream ecosystems healthy, or to provide conditions required for aquatic fauna life histories (such as fish breeding). Environmental flows can be provided through releases and spills from reservoirs (for example, the Cotter, Murrumbidgee and Queanbeyan rivers) and through restrictions on the amount of water that can be withdrawn.
Over the 2015 to 2018 period, all discharges downstream of storage reservoirs met the environmental flow requirement, including discharges below the Cotter Dam (Figure W20), Bendora Dam (Figure W21), Corin Dam (Figure W22) and Googong Dam (Figure W23). Environmental flow requirements were met despite the significantly reduced rainfall and river flows in 2017 and 2018. For the Cotter, Corin and Googong dams, downstream flows greatly exceeded environmental requirements.