Georges River
River Flow Objectives explained
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At a Glance |
This section explains each of the River Flow Objectives (RFOs). See the RFOs for each part of the Georges River catchment.
In total, there are twelve coastal River Flow Objectives, each dealing with a critical element of natural flows.
Flow patterns in many rivers have been significantly altered and will not return to natural flow regimes. The NSW Government is not attempting to restore completely natural flow patterns where the community benefits significantly from altered flow patterns. Communities and the Government have identified important areas where we can make adjustments to maintain or improve river health while continuing to benefit from water use.
Water sharing plans (WSPs), where developed, will contain integrated actions and timeframes to achieve objectives and implement identified actions in consultation with the community. Different approaches and outcomes will apply across the different parts of the catchment.
Recommending variations to objectives
The Natural Resources Commission (NRC) will review the role of WSPs in achieving state-wide natural resource management standards and targets after 5 years of the plan being made. As part of this review the NRC will call for public submissions and consider other state-wide policies or agreements that are relevant to the catchment management area. The Minister for Natural Resources will consider the review undertaken by the NRC when deciding whether to extend the duration of the current water sharing plan or to make a replacement plan.
The Minister may also amend plan actions at any time if the plan so allows or if it is deemed to be in the public interest to do so.
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Protect pools in dry times
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Protect natural water levels in pools of creeks and rivers and wetlands during periods of no flow
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During dry times, some streams stop flowing and form pools. Pools and wetlands are refuges for aquatic plants and animals. Pumping water from these areas can make it more difficult for many species to recover after a drought.
Measures to achieve objective
- There should be no water extraction from streams or wetlands in periods of no flow.
Supporting information
- The objective has limited application to this catchment
- Along with social and economic considerations, WSPs should define when and how to protect specific pools:
- in natural wetlands with licensed pumps
- in streams where water flows under the sediment surface
- where these pools have become deeper or shallower.
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Protect natural low flows
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Protect natural low flows
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Water extraction and storage are high in dry times and impose long artificial droughts that increase the stress on aquatic plants and animals.
Measures to achieve objective
- Share low flows between the environment and water users and fully protect all very low natural flows.
- Very low flows: defined as flows below the level naturally exceeded on 95% of all days with flow.
- Low flows: defined as flows below the level naturally exceeded on 80% of all days with flow.
- In streams with little water use, or with important conservation values, minimise risks to ecosystems during low flows.
Supporting information
- The objective has limited application in this catchment.
- Water sharing plans (WSPs) need to identify actions to achieve targets for each stream that will balance local environmental, social and economic considerations.
- In urban streams, the changes made to the natural low-flow regime are generally unknown. Streams may be wetter than natural because of garden watering in the catchment. Alternatively, if inputs to groundwater are reduced, streams could be drier than natural during dry times. Site-specific studies and comparison with undeveloped streams would be needed to determine the amount of change.
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Protect important rises in water levels
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Protect or restore a proportion of moderate flows ('freshes') and high flows
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Rain causes peaks in river flows. This 'pulsing' of flows, and their duration, may trigger migration of animals and reproduction of plants and animals; provide over-bank flows to wetlands and floodplains; shape the river channel; and control water quality and nutrients. Water storage and extraction can alter or remove freshes, inhibiting these vital processes. The height, duration, season and frequency of higher flows are all important.
Measures to achieve objective
- There is little water extraction in this catchment, so a target is not needed.
- In areas of new development, manage stormwater in such a way that there is no increase in the height or flow rate of high flows.
- In areas of existing development, explore opportunities for improving stormwater management to mitigate flooding.
Supporting information
- High flows may cause flooding, which is a significant community concern. Mitigation requires both better stormwater management (increased detention and infiltration) and planning to avoid the problem (relocating houses and infrastructure from flood prone areas and preventing new development in these areas).
- Adequately managing stormwater is more achievable in new developments. It may be costly and more difficult to retrofit in existing developed areas.
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Maintain wetland and floodplain inundation
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Maintain or restore the natural inundation patterns and distribution of floodwaters supporting natural wetland and floodplain ecosystems
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Floodplain and wetland ecosystems develop in response to flow patterns and the nature of the landscape between the river and the wetlands or floodplains. Floodplain works can change the flooding patterns, which will then lead to changes in habitat and vegetation. These changes can be expected to reduce or change the diversity and abundance (or both) of species in the ecosystem. In particular, they can lead to reduced numbers of native fish and to water quality problems.
Measures to achieve objective
- In management plans and actions for waterways, include strategies to:
- maintain, restore or mimic natural inundation and drying patterns in natural and semi-natural wetlands and remaining native floodplain ecosystems
- ensure adequate access for native fish to and from floodplain wetlands.
- Flooding patterns should not be altered without proper environmental assessment.
Supporting information
- Property protection issues need to be taken into account.
- Maintaining wetlands and floodplains will improve water quality and reduce downstream flooding.
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Mimic natural drying in temporary waterways
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Mimic the natural frequency, duration and seasonal nature of drying periods in naturally temporary waterways
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In urban areas, the preponderance of hard surfacing (e.g. paved or concreted areas) and garden watering can cause streams and wetlands to be 'wetter' than natural. In streams and wetlands that would dry out naturally if these impacts were absent, this can create problems in maintaining habitat, vegetation, nutrient cycling and signals for breeding. It can also lead to a high water table and associated salinity problems. Natural wetting and drying cycles produce diversity of habitat and, therefore, high species diversity.
Measures to achieve objective
- Identify any streams where unnatural flows have greatly reduced drying-out periods. Assess the potential short- and long-term environmental, economic and social effects of this change, and of possible management alternatives.
- Decide what (if any) action is appropriate to implement this objective in streams and wetlands on a case-by-case basis, after giving due consideration to local views.
Supporting information
- This objective has limited application in this catchment.
- There are few streams where drying periods differ significantly from natural conditions. In urban streams, the change to the natural low-flow regime is generally unknown. Streams may be wetter than natural because of watering in the catchment; alternatively, if inputs to groundwater are reduced, streams could be drier than natural during drying times. Site-specific studies and comparison with undeveloped streams would be needed to determine the change.
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Maintain natural flow variability
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Maintain or mimic natural flow variability in all streams
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Australia's rainfall and river flows are naturally variable. The way we currently store and divert river water can reduce natural pulsing of water down rivers and maintain artificially high or stable river heights. In urban areas and other places where the ability of the land to absorb or detain rainfall is reduced, more water runs off rapidly, so water levels will rise higher. These changes often create problems with streambank stability, biodiversity and signals for breeding and migration.
Measures to achieve objective
- Identify streams with unnatural flow variability and develop actions to mimic natural variability.
- Identify streams with potential to have or cause flow variability problems and take early action.
Supporting information
- Changes in variability occur mainly in controlled rivers, but may occur in certain locations or situations in uncontrolled rivers and streams, such as urban streams. In these areas, the objective should be addressed through stormwater management plans.
- This objective may also affect the achievement of Water Quality Objectives.
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Maintain natural rates of change in water levels
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Maintain rates of rise and fall of river heights within natural bounds
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Unnaturally fast changes in water level can occur in creeks with hard-surfaced catchments (such as in urban areas) or in streams that receive large releases from dams or that have large pumping capacity relative to normal stream flow. If water levels fall too fast, water does not drain properly from riverbanks and they may collapse. Migration of aquatic animals may also be restricted by such sudden falls in river height. Very fast increases in flow and stream height can endanger people or aquatic organisms and contribute to bank erosion.
Measures to achieve objective
- Identify locations where water levels often rise or fall faster than they would naturally. Identify the reasons and impacts. Remedial action requires case-by-case assessment.
- Identify potential problems and take early action.
Supporting information
- Local observations, such as bank erosion, may indicate a need for action.
- In urban streams, solutions may require increasing on-site detention or infiltration.
- Of all uncontrolled stream categories, this objective applies most to urban streams. Consideration should be given to prioritising the rehabilitation of channelised streams.
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Manage groundwater for ecosystems
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Maintain groundwater within natural levels and variability, critical to surface flows and ecosystems
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Some shallow groundwaters are directly linked to flows in streams and wetlands. They may provide base flows in rivers during dry periods and may be primary sources of water for wetland, floodplain and riparian vegetation. Also, serious depletion of groundwater in dry times may lead to unnatural recharge of groundwater from surface waters the next time the river is flowing. Lowering groundwater in many coastal areas may expose acid sulfate soils.
Measures to achieve objective
- Implement the State Groundwater Policy (DLWC 1997a, 1998b).
- Identify any streams or ecosystems that may depend on high groundwater levels, and assess whether impacts on these may be caused by changed recharge rates or excessive pumping.
- Identify long-term trends or changes in groundwater levels that are likely to threaten ecosystems or the quality of ground- or surface-water.
- Determine appropriate action to keep groundwater level changes within acceptable bounds.
Supporting information
- WSPs should identify areas and actions needed, particularly where there are threatened ecosystems.
- This objective also covers areas of acid sulfate soils, where draining can mobilise acid and toxic metals, affecting surface ecosystems.
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Minimise effects of weirs and other structures
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Minimise the impact of instream structures
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Most instream structures (e.g. weirs) convert flowing water to still water, thus altering habitat and increasing the risk of algal blooms or other water quality problems. Barriers restrict the passage of plant propagules (e.g. seeds) and animals. This can be a major problem for species that depend on upstream and downstream migration at particular times in their lifecycle.
Measures to achieve objective
- Implement the NSW Weirs Policy (DLWC 1997b).
- Identify (and act to reduce) the impacts on native fauna of other in-stream structures that impede the movement of water (e.g. floodgates, tidal barriers) .
Supporting information
- The NSW Weirs Policy (DLWC 1997b) and weir review process helps set priorities and assess management options.
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Minimise effects of dams on water quality
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Minimise downstream water quality impacts of storage releases
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Many dams release water from the bottom of reservoirs, where temperatures and dissolved oxygen are low and nutrient concentrations are high. These differences in water quality can affect the river down-stream for hundreds of kilometres. For instance, many native fish will not breed in colder water.
Measures to achieve objective
- Determine whether water quality from dam releases limits achieving WQOs in unregulated streams and, if so, develop appropriate actions.
Supporting information
- This objective may apply to releases from urban stormwater detention basins or floodgates, particularly with respect to dissolved oxygen, acidity and algae.
- This objective is not generally a priority in coastal catchments, but information available for particular catchments may indicate a specific problem.
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Make water available for unforeseen events
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Ensure river flow management provides for contingencies
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River systems can sometimes be affected by unforeseen or irregular events-such as algal blooms or the start of bird-breeding seasons. As river flows are a major determinant of many of these processes, a water-quality or environmental problem can sometimes be alleviated by better managing river flows.
Measures to achieve objective
- There is no specific target.
Supporting information
- This objective does not apply in this catchment.
- Protecting or mimicking elements of a natural flow regime is preferable to making water available for unforeseen events.
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Maintain or rehabilitate estuarine processes and habitats
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Maintain or rehabilitate estuarine processes and habitats
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Coastal lagoons, estuaries and river mouths often change naturally in response to storms or tides. Flood-mitigation structures, weirs and other works also affect estuaries by limiting tidal flow, changing salinity conditions or altering water levels. Development of estuarine areas can also disturb acid sulfate soil deposits, which may release large amounts of sulfuric acid and toxic metals into the estuarine environment.
Upstream management of rivers also affects estuaries and lagoons. Stormwater carries nutrients, organic matter and sediments. Scouring as a result of flooding can affect the opening and closing of coastal lakes and lagoons. Reduced freshes and flooding in estuaries severely depletes food sources for estuarine species. These effects can contribute to a decline in the number and abundance of species that use estuaries as habitat, nursery grounds or both.
Measures to achieve objective
- Dredging beyond the minimum needed to maintain navigation channels should be subject to environmental assessment before being allowed to proceed.
- Minimise draining or disturbance of areas of potential acid sulfate soils.
- Ensure that water-based activities have minimal impact on fish habitat.
- Ensure that tidal wetlands continue to receive tidal flushing; and minimise the impact of flood levels and gates, roads and other barriers (see also RFO Maintain wetland and floodplain inundation).
- Deal with other processes affecting (or potentially affecting) estuary health-e.g. the impact of increased urbanisation.
- Protect fish habitats-such as protected areas, seagrass beds and mangroves-from pollution and increased sedimentation or erosion. Polluted sites should be rehabilitated.
Supporting information
- Dredging, digging or drainage could expose acid sulfate soils. As treatment of acid sulfate soils is expensive and relatively untested, the priority is to minimise exposure of these soils.
- Sedimentation is a problem in most enclosed bays that receive stormwater, and the community frequently calls for dredging. The objectives for dredging should be clearly established, adequate environmental assessment made, and a management plan for the proposal written (and implemented) covering aspects such as minimising the impact on water quality and disposal of sediments. This is particularly important, as many of the sediments-especially those adjacent to industrial (or former industrial) sites-are contaminated by heavy metals, persistent chemicals and hydrocarbons.
- Marinas and extensive moorings may affect water circulation patterns or aquatic habitats. This may expose acid sulfate soils and contaminated sediments, and affect the water quality and tidal flows generally.
This page was published 1 May 2006