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Office of the Commissioner for Sustainability and the Environment



Issue: Catchment Quality

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How have the ACT's catchments changed?

ACT catchments were subjected to two extreme events during this reporting period – drought and bushfire. Urban development and management have ongoing effects.

This drought, at its most severe in 2002–03, brought a decline in ground cover in most areas and it also reduced flows in most ACT rivers. Impacts include a general decline in water quality in unforested catchments, and a reduction in the amount of water released for environmental flows in the Cotter River.

In many of the ACT’s catchments the effects of the drought were compounded by bushfires at Christmas 2001 and, more significantly, January 2003.

The consequences of changes to soils, vegetation cover and catchment hydrology will be evident for many years. This is particularly the case for the forested catchments of Cotter River and Gudgenby–Naas Rivers. Until January 2003, these forested catchments had the best water quality in the ACT. Other areas such as pine plantations in Stromlo Forest (Molonglo catchment) were devastated. Many of the benefits of long-term management were obscured or destroyed by the scale of damage.

The drought and 2003 bushfire caused a decline in river health (as measured by aquatic macroinvertebrate populations). Overall, water quality was affected by increased sediment and nutrient loads, decreased oxygen concentrations, higher water temperatures, reduced flow, and increased algal populations.

Of ongoing concern is that runoff from suburban development in Gungahlin, Dunlop and south Tuggeranong continues to degrade water quality in Ginninderra Creek and Tuggeranong Creek. A previous recommendation about sediment and erosion control (from the 2000 State of the Environment report) needs to be implemented effectively.

While Canberra’s urban lakes are designed to trap sediment and other pollutants, other functions such as recreational activities and visual amenity suffered as a result. Lake Burley Griffin was closed to recreation more frequently than normal, mostly due to high levels of bacteria. On one occasion this resulted from an accidental spill of about 120,000 litres of sewage upstream of the lake in Jerrabomberra Creek catchment.

There are some positive trends. By the end of the reporting period, riparian areas unaffected by the 2003 fires were in good condition after revegetation or erosion abatement. This suggests that riparian rehabilitation activities can be associated with improvements in local conditions.

Also, monitoring of the Murrumbidgee River showed that the ACT had minimal adverse impacts on the quality of water leaving the Territory.

After the fire

Some 165,000 hectares were burned by the January 2003 bushfires, affecting five of the nine ACT river catchments – all or almost all of the Cotter, Gudgenby–Naas and Paddys River catchments, and parts of the Molonglo and Murrumbidgee River catchments.

Although the full impact of the bushfires and subsequent rains has not been assessed in detail, it is already obvious that impacts included massive loss of vegetation, followed by soil degradation, erosion, sedimentation and poor water quality downstream.

For example, it is estimated that the post-fire flow of inorganic sediment (soil) into Corin Dam in 2003 was equal to 17 years’ mean annual input, and the amount of organic matter (ash and charcoal) was equivalent to 27 years’ input. In some small subcatchments, the amount of sediment eroded from hillslopes exceeded the estimated loss of 1000 tonnes per square kilometre in the peak erosion phase of the nineteenth century.

In the Cotter River catchment alone, sheet erosion has increased from contributing 7% of the sediment to contributing more than half. The scale of erosion was partly because of the loss of vegetation in bogs, fens, and floodplains, which usually act as sediment ‘traps’. In addition, when soil organic matter burns and is lost, soils become 'hydrophobic' or water-repellent so rainfall does not infiltrate the soil and runoff increases in yield and velocity.

For all affected catchments, water quality is likely to suffer for years from sediment and debris transported from actively eroding hillslopes that have been burnt. In the Cotter River catchment, much of this will be caught up in the water supply dams. In catchments without dams, the extra sediment and debris may degrade water quality for some time until vegetation re-establishes – this will be seen as increased turbidity, bedloads and channel snags.

Of areas assessed, some 840 kilometres of streambanks were changed because of fire damage to native and introduced riparian vegetation.

Some water quality monitoring stations were burned, and the ACT Government Analytical Laboratories were damaged, with the resulting loss of some important long-term water quality records for Murrumbidgee recreational sites and urban lakes.

Catchment snapshot

  • Forested catchments – Cotter and Gudgenby–Naas River catchments – in good condition, despite reduced flow due to drought, until the January 2003 bushfires. Native vegetation and pine forests completely burnt, with rain eroding soils and washing iron- and manganese-laden sediment into streams and dams.
  • Urban catchments – Ginninderra and Sullivans Creeks, and Tuggeranong Creek (in the Murrumbidgee River catchment) – poor condition because of construction-induced erosion in new areas, worsened by the effects of drought and heavy rains. Lakes closed because of bacteria.
  • Rural catchments – Paddys River, and Jerrabomberra and Woolshed Creeks – increased grazing pressure from native animals and livestock (all catchments) reduced ground cover during the drought, with complete loss of vegetation cover in the burnt catchment (Paddys River). Declining catchment quality due to reduced flows and later heavy rains washing soil and fire-debris off hillslopes into streams.
  • Mixed – Molonglo and Murrumbidgee River catchments – poor condition due to effects of drought and fire in parts of the catchments, although values of some water quality indicators were good in the Murrumbidgee until then. Decline in water quality as post-fire rain eroded soils and washed sediment from the upper parts of subcatchments into the river and its tributaries. Lake Burley Griffin closed more frequently because of bacteria. Algae in many streams.

Pest invasion

The increase in pest plant (weeds) and changes in pest animal populations in the ACT continues to represent a major problem due to their impact on biodiversity, rural productivity and plantation forestry.

There appear to be more wild dogs and dingoes after the fires in the Cotter, Gudgenby, Paddys and Murrumbidgee River catchments, although that may be because the loss of vegetation makes it easier to identify their presence. On the other hand, during 2000–03 fox and rabbit populations appear to have been at the lower end of their historical range, possibly because of stresses from the drought and the 2003 bushfires.

Data on pest animals are neither reliable enough nor continuous.

There are data to show an increase in the extent of some weeds such as Pattersons Curse, Capeweed and Nodding Thistle because of the absence of competition from other vegetation in the aftermath of the drought and bushfires. Again, good quality spatial data were not available for these or other weeds.

In urban areas, expansion of populations of the fish Oriental Weatherloach Misgurnus anguillicaudatus , the bird Common Myna Acridotheres tristis , and the European wasp Vespula germanica has been recorded.

Are we monitoring adequately?

An ongoing concern is the lack of data for land quality and groundwater in ACT catchments.

While some progress has been made towards compiling accurate soil erosion data, there is still no formal monitoring regime in place. Data collected through one-off surveys or even community activities such as Landcare are not always accessible. Holistic long-term monitoring and coordinated data curation are essential. Dryland salinity and soil acidity are also well-recognised problems in the ACT region, but there is still a lack of good survey and monitoring data within the Territory’s border despite the presence of plants that tolerate conditions of salinity or acidity.

Seven of the 20 weeds of national significance occur within the ACT, but their distribution is unknown due to a lack of data on infested areas.

Groundwater is a part of the ACT’s overall water resource. Is the ACT community using too much groundwater? Are watertables rising in association with dryland salinity? How has the groundwater system been affected by entire catchments being burned? None of these questions can be answered accurately even though previous State of the Environment reports have recommended the ACT Government review and implement a cost-effective monitoring strategy.

While the step to license private and commercial bores is commendable, controls on extraction rates are still considered inadequate given the data gaps and increase in groundwater use during the 2002–03 drought in some parts of Canberra.

No, we are not monitoring adequately!

What does this mean for the future?

Post-fire assessments have highlighted the need to implement a truly holistic, integrated approach to management at the catchment level, initially in the water supply catchments.

Also clear is the failure of existing policy frameworks, apart from the Water Resources Strategy, to account for large-scale extreme events. It will be imperative to refine current policy and management plans to address whole-of-catchment management under both average and extreme events.

Consideration of urban buffer zones is being integrated into planning to reduce bushfire risk along urban fringes. These buffer zones may also play important roles in relation to infrastructure, water quality, river health and pest management.

Water quality issues will remain high on the agenda as sediments currently stored within the Cotter catchment are mobilised during storms. Water treatment infrastructure is currently being constructed at Stromlo, but fluctuations in water quality and supply can be expected until the catchment stabilises.

Meanwhile the main water supply is derived from the Googong Dam, which drains a rural catchment. Water quality will remain an issue with Googong due to high nutrient and sediment loads from intensive rural and rural residential areas. This is exacerbated by the heavy demand on supply under drought conditions.

It is likely that the quality of urban catchments will continue to decline unless sediment and pollution controls improve.

Commissioner's recommendations

In consultation with the Commissioner for the Environment, the Government should:

  • immediately develop and resource long-term research and monitoring programs of at least 30 years duration into post-fire recovery of terrestrial and aquatic components of natural and modified ecosystems as part of a joint program other research providers in States affected by fire
  • seek funding for the long-term research in the above recommendation from the Commonwealth Government on the basis of the National Research Priority 'an environmentally sustainable Australia', announced in December 2002
  • implement an appropriate post-fire works and management program to protect water supply catchment, aquatic habitat and nature conservation values of the upper Cotter River, Murrumbidgee and Molonglo River valleys, and their tributaries
  • assess the contributions of existing ACT Government pest plant control programs to achieving pest plant control, biodiversity consevation, and catchment management objectives and, if appropriate, trial alternative programs
  • undertake a catchment-by-catchment hydrological study of groundwater systems to assess water quality and quantity and its connectivity, spatial distribution and temporal variability
  • extend existing policy and management plans to include extreme event scenarios.

Some recommendations from this issue are found in other issues because of the interactions between various components of the environment.

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