Inland waters: Assessments of river health
Independent report to the Australian Government Minister for the Environment and Heritage
Beeton RJS (Bob), Buckley Kristal I, Jones Gary J, Morgan Denise, Reichelt Russell E, Trewin Dennis
(2006 Australian State of the Environment Committee), 2006
During the nineteenth and twentieth centuries, major changes to Australia’s land and water resource use led to altered flow regimes, increases in sediment and nutrient loads, habitat destruction, the introduction of exotic species and the consequent loss of aquatic biodiversity. Whether river health has been getting better or worse or has been stable at a national scale over the last five years is difficult to assess because of a lack of data.
There is some evidence that the management interventions over the past decade may have stabilised the decline in river health in some regions. In 2005 the Victorian Government provided the first scientifically robust assessment of trends in river health across an entire state. It concluded that there had been no broad change in Victoria’s rivers during 1999–2004, and that the previous deterioration in stream condition had been arrested. The study did report that local changes were likely, with improvements in river health in some places and deterioration in a few others (DSE 2005). While far from scientifically or nationally conclusive, these results do provide some cause for optimism. However, the observations in Victoria make it clear that if there are to be improvements in river health, they will be hard-earned and a long time coming. Along the way, some rivers will continue to decline in health while others improve.
Aquatic biodiversity is particularly sensitive to changes in river health, and it has declined in many river systems since European settlement. For example, one-third of river length in Australia has lost 20–100 per cent of the various kinds of aquatic invertebrates that should live there. Conversely, many rivers or river reaches, perhaps the majority, may not be significantly degraded due to a lack of development (Table 16). Many of them are not suitable for dam construction, or they are located in less populated mountain areas or in the northern tropical regions of Australia. Indeed, there is growing concern that many of these rivers, especially the larger pristine rivers in tropical Australia, will come under increasing pressure as sources of water to support irrigation development are exhausted in southern Australia.
|Period of assessment||Number of sites at each level of diversity compared with reference sites|
|More diverse||Similar to reference condition||Significantly impaired||Substantially impaired||Severely impaired||Total number of test sites|
|1990–2004||195||2 465||1 556||433||56||4 705|
|1994–99 (in SoE2001)||154||1 702||963||254||39||3 112|
Source: Australian River Assessment System (AusRivAS)
Frogs are very sensitive indicators of aquatic ecosystem health and extent. Their populations have decreased markedly in the last decade or so, with 27 species of frog listed as endangered or vulnerable (Tyler 2006).
Riparian vegetation , a key part of riverine ecology, has also declined. Extensive revegetation is required to improve river health, and many areas remain under significant pressure due to the combined impacts of human activity and the drought. For example, in 2003, 80 per cent of remaining River Red Gums on the Murray River floodplain in South Australia were stressed to some degree in 2003, and 20–30 per cent of them were severely stressed; this severe decline had occurred in the previous 12 months. In lowland rivers, ‘de-snagging’ and loss of water plants has also modified much instream habitat. While important locally, the scale and scope of recovery programmes are small compared to the scale of the problem. For example, in a sample of river basins, there was only a small increase of some 3700 kilometres in the extent of riparian vegetation between 1991 and 2004 (Figure 30). There have also been some limited projects aimed at bringing snags , which are essential habitat, back into some rivers.
Note: ILZ – intensive land use zone
Source: ERIN (2005c)
The physical river environment has also been extensively modified as dams and weirs have been built. Not only do these structures physically restrict the movement of native animals, but they also regulate river flow, so there are fewer floods and fewer dry periods, and they divert water out of the river. In New South Wales alone, there are over 4 300 weirs and dams. Only 28 have fishways constructed that are effective for native fish.
The impact on wetlands has been dramatic. As many as 231 nationally important wetlands are under pressure across Australia. Of the 64 Ramsar wetlands , latest assessments indicate that 22 have changed in ecological character or have the potential to change (DEH 2002). In the Macquarie Marshes, for example, there has been a significant long-term decline in river flows as a result of river regulation and subsequent diversions upstream. There are now fewer waterbirds , and fewer species of waterbird, than ever before (Figure 31). In the Lowbidgee floodplain, at least 76 per cent of the area has been destroyed by dams, diversions and floodplain development, reducing the amount of water reaching the floodplain by at least 60 per cent. Waterbird numbers in that area have decreased by 90 per cent. Breeding success has been improved in local experiments in the Macquarie Marshes, by allocating water for bird breeding through environmental flows (Kingsford and Porter 2005).
Source: Kingsford and Porter (2005)
While much attention is being paid to restoring degraded rivers, there is also a growing movement to develop a system of state and national river ‘reserves’ to protect rivers that remain in a healthy, near-pristine condition.
With so much of Australia’s land affected by dryland salinity and with so much more at risk (see ‘Land’), it is not surprising that a third of catchments assessed by the NLWRA (2001b) showed signs of increased salt loads in rivers and streams . The problem is particularly severe in the southern Murray–Darling Basin, along the south-east coast of Australia, and in catchments in south-west Western Australia. In other areas, such as the Lower Murray River, salt interception schemes are helping to ameliorate instream salinity levels. Whether river salinity has improved is not yet certain because, with only 8 to 10 years of data in most places, there has not been enough time for any significant trends to emerge from the effects of climate variability and variations in flow.
There is little evidence that increased salt concentrations have yet had a broad-scale impact on aquatic plants and animals, but some wetland ecosystems have shown signs of salt degradation. Impacts on sensitive species should be expected if the salt concentration in Australia’s lakes and rivers increases. While adult Australian fish, for example, are generally believed to be quite salt tolerant, recent research shows that the larval and juvenile stages of certain fish, such as Murray Cod (Maccullochella peelii peelii) are particularly sensitive to salt.
Throughout Australia, soil erosion has significantly increased sediment loads to rivers and estuaries (see ‘Land’). River water from forested catchments, or from catchments with large areas of relatively unmodified native vegetation, tends to be clearer than river water from agricultural and urban catchments. Extensively cleared catchments, such as large areas of the wheat and sheep zones of eastern and south-western Australia, are worst affected. In many parts of Australia, because much of the eroded material has still not yet worked its way through the bigger river systems, large amounts of sediment are stored in river channels in low-gradient areas (Wasson et al 1996, Martin and McCulloch 1999, Prosser et al 2001). Sand slugs in the lowland reaches have smothered various instream habitats, which has had significant impacts on aquatic plants and animals. Rehabilitation techniques for sand slugs are being explored, including placing railway sleepers across a riverbed to increase the natural scouring process (Bond et al 2004).
Erosion of sediments is dramatically increased after fire. The large-scale fires that burned across the Victorian and New South Wales high country and the Australian Capital Territory in 2001 and 2003 caused greatly increased erosion in these areas, leading to poor water quality and the deposition of large amounts of soil and other materials in stream beds and lakes downstream of the fires (DSE 2003). In the Australian Capital Territory, a storm shortly after the 2003 fires washed debris and the equivalent of 27 years’ worth of sediments into the water supply dams (Carey et al 2003). Erosion and sedimentation rates were still high in 2005, especially after rainfall, and the Territory has had to upgrade its water treatment facilities to maintain drinking water quality to the city (ACTEW 2005). For all of these areas, the risk of further deterioration will remain until the vegetation recovers.
Excess nutrients were a major water quality issue in about 60 per cent of basins assessed by the NLWRA in 2001. For most rivers, the largest source of increased phosphorus loads is gully and stream bank erosion, rather than farm fertilisers (Martin and McCulloch 1999, Prosser et al 2001). In contrast, increases in nitrogen levels are typically from fertiliser use, animal wastes and sewage discharges (Caraco and Cole 1999). Nearly 19 000 tonnes of total phosphorus and 141 000 tonnes of total nitrogen were estimated to be transported down rivers to the coast each year from areas of intensive agricultural activity.
|Major exceedances||Significant exceedances||Number of basins assessed|
|Nutrient: total nitrogen||19||19||50|
|Nutrient: total phosphorus||40||20||75|
|Salinity: electrical conductivity||24||18||74|
Source: NLWRA (2001b)
Blue-green algae have become more widespread as changes to instream habitat and catchment land use have altered the stream metabolism of inland waters. All the changes seem to favour increased toxic algal blooms, and more frequent periods of oxygen depletion in the bottom waters of pools, weirs and dams. The current cost of algal blooms is in the range of $180 million to $240 million per year (Atech 2000). Between 2001 and 2005, SA Water recorded the occurrence of 19 algal blooms in 12 freshwater bodies in South Australia. Most blooms occurred during the summer–autumn months and some lasted several months (Baker 2006).
Introduced species appear to be favoured by the large-scale landscape and waterscape management practices found across large areas of the continent. Introduced pests—such as carp (Cyprinus carpio), mosquito fish (Gambusia holbrooki) and oriental weatherloach (Misgurnus anguillicaudatus), and weeds —have continued to expand their range. In the Lower Murray–Darling catchment, for example, exotic species make up 56 per cent of the total biomass of fish (Gilligan 2005). There has been a corresponding decline of native species , with little evidence of effective control of invasive species. Species such as carp not only compete with native species but also alter habitats, and their management is not straightforward. Native fish are more successful in breeding in wet years, and the reintroduction of environmental flows and overbank flows is needed for ongoing improvement.
There are some possible solutions being developed, such as a research programme aimed at producing ‘daughterless carp’ to help reduce carp numbers and distribution. Another technique uses the ability of carp (unlike most native fish) to ‘jump’ over instream barriers. This ‘capture’ technique is proving effective in preventing carp from moving into some floodplain lakes and wetlands.