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Findings

The results of the river assessment show the extent of change in Australia's river basins. Change in condition is most strongly linked to:

• intensity of land use;
• increased nutrient and sediment loads; and
• loss of riparian vegetation.

Hydrological change related to impoundments and water extraction is a significant driver of river condition. A hydrological baseline was, however, available for only 25% of the number of river reaches in the assessment (or 30% of the total river length) making it difficult to measure the extent to which flow regulation and abstraction have affected rivers. It was found that the two most affected aspects of hydrology are the flow duration (40% of assessed river length is modified) and the seasonal amplitude (30% of assessed river length is modified). This means that flows are usually for longer periods than pre-European settlement flows, and the seasonal peaks and lows of intermittent systems are evened out to resemble perennial systems (see box p. 59).

Aquatic biota index (macro-invertebrates)

One-third (21 909 km) of the river length assessed is to some degree impaired (has lost between 20% and 100% of the various kinds of aquatic invertebrates that should live there) (Figure 37, Table 11).

• New South Wales is assessed as having the poorest aquatic biota condition; approximately 50% of the river length assessed had impaired aquatic biota. Some of the most affected areas were the Georges River and Wollongong Coast basins.
• Over 35% of the river length assessed in the Australian Capital Territory and Western Australia had impaired biota.
• Between 12% and 24% of the river length assessed in the remaining States and Territories had impaired biota.

Assessment results are underestimates of change in some parts of the country, including the lowland rivers of the Murray-Darling Basin and Western Australia. In these areas the reference sites used as benchmarks have already been modified to some extent since European settlement.

These findings on the biotic condition of Australia's rivers need to be considered in the context of the commitment to ecologically sustainable development. Extensive change to riverine ecosystems has already occurred, and with the assessment of river condition providing only a measure of condition, not trend, it may be that this change is continuing. It was beyond the scope of this assessment to ascertain whether the extent of change detected was ecologically sustainable. Ecosystems have a natural resilience to disturbance. Beyond a certain level of disturbance, ecosystem structures and processes fail. Less desirable ecosystems (e.g. lowland river ecosystems may change from macrophyte-dominated systems to systems dominated by cyanobacterial blooms) may result.

Table 11: Aquatic biota index (macro-invertebrates) results for each State and Territory.

	Total length of reach (km) in each category and percentage of total in parentheses				Percentage of total length with data
	Reference	Significantly impaired	Severely impaired	Extremely impaired
Queensland	9 334 (80)	1 997 (17)	250 (2)	16 (1)	16
New South Wales	11 366 (50)	7 551 (34)	2 801 (13)	690 (3)	38
Australian Capital Territory	169 (64)	76 (29)	17 (7)	0 (0)	97
Victoria	9 347 (76)	2 447 (20)	344 (3)	49 (1)	77
Tasmania	4 248 (75)	1 097 (20)	142 (3)	100 (2)	100
South Australia	7 866 (83)	1 098 (12)	124 (1)	389 (4)	98
Western Australia	4 401 (64)	1 977 (29)	419 (6)	31 (1)	27
Northern Territory	2 063 (88)	247 (10)	47 (2)	0 (0)	11
Total	48 793 (69)	16 490 (23)	4 144 (6)	1 275 (2)	34

Environment index

The environment index combines the subindices of catchment disturbance, habitat, hydrological disturbance, and nutrient and suspended sediment load. Some components were not sufficiently comprehensive to fully characterise the driver of river condition (e.g. a water quality index would ideally consider important drivers of water quality, such as nutrients, suspended sediments, salt, turbidity, temperature, dissolved oxygen concentrations and toxicants). Comprehensive data was available for nutrient and suspended sediment loads only.

Increases in nutrients and suspended sediment loads, and decreases in the extent of riparian vegetation have resulted in 85% of the river length being assessed as substantially or moderately modified from natural condition (Table 12, Figure 38). In the Northern Territory, two-thirds of the river length assessed is in largely unmodified condition. In all other States and Territories except Tasmania, more than 80% of the river length assessed is substantially or moderately modified. One of the objectives of the assessment of river condition is to provide information on possible causes of degradation and inform management decisions on appropriate courses of action. Examination of the subindex results that make up the overall environment index is useful and can assist management as detailed in following sections.

Table 12: River environment index results for each State and Territory.

	Total length of reach (km) in each category and percentage of total in parentheses				Percentage of total length with data
	Largely unmodified	Moderately modified	Substantially modified	Extensively modified
Queensland	8 743 (13)	48 214 (71)	10 599 (16)	0 (0)	93
New South Wales	1 619 (3)	39 232 (68)	17 089 (29)	18 (0)	97
Australian Capital Territory	43 (16)	191 (71)	36 (13)	0 (0)	100
Victoria	3 085 (20)	9 042 (60)	3 099 (20)	0 (0)	97
Tasmania	2 028 (37)	3 250 (59)	194 (4)	0 (0)	98
South Australia	299 (4)	4 666 (61)	2 635 (35)	0 (0)	79
Western Australia	1 487 (7)	15 927 (78)	2 929 (14)	12 (1)	80
Northern Territory	9 165 (66)	4 630 (34)	0 (0)	0 (0)	67
Total	26 468 (14)	125 152 (66)	36 581 (19)	31 (1)	90

Catchment disturbance

Approximately 85% of river length assessed has catchments that have been changed from a natural condition (Table 13). Changes to land use are considered to have the most potential to affect rivers.

Both extent of recent land clearing and land use change are considered by this subindex. Changes tend to be widespread and relatively uniform, reflecting the predominance of broad-acre agriculture. Urban development, and more intensive agriculture have impact on rivers in some localised areas, particularly along the east and south-east coasts. Included in the measure of catchment disturbance is the influence of infrastructure (roads, power lines, railways) on rivers. Infrastructure had a less significant effect on river condition than land use, but was very important in densely populated areas.

• A high proportion of the rivers assessed in Tasmania and the Northern Territory have unmodified catchments.
• Other unmodified areas include the Australian Alps and parts of the coast protected in National Parks.
• Areas most affected are those close to urban areas, particularly Sydney and Melbourne, where infrastructure is dense and there are areas of intensive agriculture.

Table 13: River catchment disturbance subindex for each State and Territory.

	Total length of reach (km) in each category and percentage of total in parentheses				Percentage of total length with data
	Largely unmodified	Moderately modified	Substantially modified	Extensively modified
Queensland	5 119 (7)	66 623 (93)	300 (0)	0 (0)	98
New South Wales	5 773 (10)	52 343 (90)	216 (0)	32 (0)	95
Australian Capital Territory	158 (59)	105 (39)	7 (2)	0 (0)	95
Victoria	1 716 (11)	11 479 (74)	2 208 (14)	87 (1)	95
Tasmania	2 455 (44)	2 918 (52)	213 (4)	0 (0)	96
South Australia	463 (5)	8 422 (90)	519 (5)	70 (0)	94
Western Australia	6 038 (24)	19 149 (76)	8 (0)	12 (0)	94
Northern Territory	8 752 (43)	11 739 (57)	0 (0)	0 (0)	97
Total	30 474 (15)	172 778 (83)	3 470 (2)	202 (0)	99

Habitat change

Over half of the rivers in the assessed area are affected by changes to riverine habitat, with the most modified areas occurring in the Murray-Darling Basin, South Australia and parts of the Western Australian wheatbelt (Table 14, Figure 39). The main indicators linked to this degradation are loss of riparian vegetation and increased sediment loads in rivers; a third habitat measure used was change to upstream-downstream and overbank connectivity.

Riparian vegetation plays a number of key roles in river ecosystem processes, particularly in those rivers with extensive floodplains (e.g. the lowland sections of the rivers flowing west from the Great Dividing Range). In these extensive floodplain areas, the loss of riparian vegetation is high.

Increased sediment loads in streams have led to:

• smothering of habitat - a widespread problem that can be expected to worsen as bedload material that has already been eroded and is stored in river channels continues to move through the river network.
• Gully erosion - significant sediment source across south and south-western Australia.
• Bank erosion - significant sediment source in south-eastern Australia.
• Hillslope erosion - predominant sediment source in northern Australia.

The bedload condition in about 66% of the rivers in the assessed area is either only slightly modified or unchanged. These reaches are not considered susceptible to accelerated deposition of sand and gravel because they are in areas of low catchment disturbance, have high sediment transport capacity or are far away from the source of bed material.

Change to upstream-downstream and overbank connectivity is most affected where reservoirs have been built - in the mid-slopes region along the Great Australian Divide and in Tasmania.

Table 14: River habitat subindex for each State and Territory.

	Total length of reach (km) in each category and percentage of total in parentheses				Percentage of total length with data
	Largely unmodified	Moderately modified	Substantially modified	Extensively modified
Queensland	45 389 (66)	18 184 (27)	4 263 (6)	130 (1)	94
New South Wales	15 724 (27)	19 695 (33)	21 100 (37)	1 845 (3)	98
Australian Capital Territory	148 (55)	68 (25)	54 (20)	0 (0)	100
Victoria	8 301 (53)	3 488 (23)	3 489 (23)	211 (1)	98
Tasmania	3 296 (59)	998 (18)	1 177 (21)	114 (2)	100
South Australia	1 384 (17)	2 764 (35)	3 809 (47)	30 (1)	83
Western Australia	7 522 (34)	9 887 (46)	4 190 (19)	286 (1)	86
Northern Territory	9 134 (62)	2 589 (18)	2 790 (19)	15 (1)	71
Total	90 899 (47)	57 673 (30)	40 873 (1)	2 631 (1)	92

Hydrological disturbance

• Regulated reaches are those with a dam or weir upstream; an estimate of natural flow is necessary to calculate changes to flows.
• Unregulated reaches have no major regulating structures upstream, but flow may be affected by abstraction for use; an estimate of the volumes extracted on a seasonal basis is required to calculate changes.

The natural flow baseline was available for only 25% of reaches in the assessment (or 30% of the total river length) making it difficult to measure the extent to which flow regulation and abstraction have affected rivers. Of the regulated and unregulated rivers that could be assessed, over 80% are modified to some extent and nearly 30% are substantially modified (Figure 40). This may be an overestimate of actual changes to flow in the assessment area because the availability of hydrology data is often associated with the level of resource use.

Unregulated rivers

Almost 90% of the river length in the assessed area is unregulated (diversions occur, but flows are not regulated and existing total annual extractions are less than 0.5% of the mean annual flow). Twenty-six percent of this unregulated river length was assessed using data from the Australian Water Resources Assessment 2000 (NLWRA 2001f) (Table 15).

There were no hydrological data to determine the subindices of the hydrological disturbance index for many unregulated river basins. Reaches in these basins were classified as 'largely unmodified', based on an assessment of mean annual flow and where water extraction was less than 0.5% of mean annual flow. This approach allowed over 20% of unregulated river reaches to be assessed. The Stressed Rivers Assessment Program has developed a hydrological stress indicator to assess unregulated basins in New South Wales. These results differ to the Audit findings for the Richmond, Hunter (regulated in the Audit assessment), Paroo, Snowy, Genoa and Lake George river basins. The discrepancies reflect differences in the data available to the assessments and warrant further investigation.

Regulated rivers

Eleven percent of the river length in the assessment area is regulated and hydrological disturbance could be assessed for approximately half of this length. In regulated rivers that could be assessed, the hydrology of approximately 30% of the river length has been modified. The two components of the hydrology most affected are flow duration - one measure of flood frequency (40% of river length is modified) - and seasonal amplitude (30% of river length is modified). This means that the rivers usually flow for longer periods than natural, and seasonal peaks and lows of intermittent systems are evened out to resemble perennial systems (see box p. 59).

Some regulated reaches have well-vegetated catchments, good riparian vegetation and good water quality. These include rivers such as the upper Snowy River and other rivers in alpine regions and central Tasmania.

Table 15: River hydrological disturbance subindex for each State and Territory.

	River length km	Regulated (unregulated) %	Assessed regulated (unregulated) %	Largely unmodified %	Moderately modified %	Substantially modified %	Extensively modified %
Queensland	74 475	8 (92)	48 (22)	86	10	3	1
New South Wales	62 208	18 (82)	65 (7)	13	66	20	1
Australian Capital Territory	283	51 (49)	61 (0)	0	100	0	0
Victoria	16 556	22 (78)	70 (7)	18	49	30	2
Tasmania	5 843	29 (71)	21 (22)	6	51	43	0
South Australia	10 205	10 (90)	48 (31)	30	49	16	5
Western Australia	26 900	4 (94)	0 (29)	-	-	-	-
Northern Territory	21 140	1 (99)	0 (90)	-	-	-	-
Total	217 610	11 (89)	54 (26)	19	52	28	1

Nutrient and suspended sediment loads

Nutrient and suspended sediment loads are greater than natural levels for over 90% of the river length in the area assessed, and are severely modified in almost 10% the total river length (Tasmania and the Australia Capital Territory have no reaches in severely modified condition) (Table 16, Figure 41). Increases in total phosphorus and suspended sediment loads are strongly linked to degradation of water quality.

Total phosphorus loads in the rivers assessed have increased on average 2.8 times above natural levels. The average annual export of total phosphorus to the Australian coast from the assessed rivers is estimated as nearly 19 000 tonnes. Over 80% of the river length has suspended sediment loads that are 10-200 times natural loads. Several thousand-fold increases have been estimated in areas where gully and streambank erosion generate high sediment loads.

The processes causing high phosphorus and suspended sediment loads in rivers are linked because, in most regions, much of the phosphorus load is attached to sediment particles. The most likely principal factor generating high phosphorus and sediment loads is loss of vegetation in the catchment or riparian zone, leading to increased hillslope, gully and bank erosion and suspended sediment loads in the river. Main sources of sediment are gully erosion in degraded areas (particularly in south-western Australia) and hillslope erosion where cover is seasonally low through grazing or tillage of cropped lands (mainly in northern Australia) (NLWRA 2001b).

• Areas with the highest increase in phosphorus and suspended sediment loads include the north-east part of the Murray-Darling Basin, and areas subject to intense summer storms where gully erosion occurs on erodible and bare soils.
• Parts of western Victoria have significant gully and riverbank erosion on highly dispersible soils.
• Some Queensland catchments (e.g. the Herbert River Basin) are severely modified in terms of sediment loads and this reflects the intense summer rainfall and seasonally low vegetation cover associated with large areas of cattle grazing.
• Tropical crops on sloping lands have high soil erosion rates but the total amount of sediment generated by this land use is small compared with grazed catchments because grazing, although lower in intensity, covers a much larger land area.
• In Western Australia, the natural sediment yield is very low, so even a relatively small increase in suspended sediment load can be significant.

Forested regions in north-east Victoria, Tasmania, and north Queensland were the only areas assessed where existing suspended sediment loads were similar to natural loads.

Table 16: Nutrient and suspended sediment load subindex for each State and the Australian Capital Territory.

	Total length of reach (km) in each category and percentage of total in parentheses				Percentage of total length with data
	Largely unmodified	Moderately modified	Substantially modified	Extensively modified
Queensland	2 809 (4)	12 660 (20)	40 347 (64)	7 573 (12)	88
New South Wales	1 692 (3)	23 784 (41)	27 630 (48)	4 678 (8)	97
Australian Capital Territory	9 (3)	89 (33)	172 (64)	0 (0)	100
Victoria	4 419 (29)	5 067 (33)	5 287 (35)	410 (3)	96
Tasmania	3 233 (59)	1 811 (33)	429 (8)	0 (0)	98
South Australia	210 (3)	2 860 (39)	4 112 (55)	203 (3)	76
Western Australia	870 (4)	2 988 (15)	15 759 (78)	461 (2)	98
Total	13 242 (8)	49 258 (29)	93 736 (55)	13 324 (8)	81

Comparison of the aquatic biota and the environment indices

Scores for the two main indices (aquatic biota and environment) would ideally be similar for each basin. The biota index does not demonstrate the same degree of degradation as the environment index. Reasons for this include:

• macro-invertebrates may be insensitive to some environmental changes, including large-scale changes (e.g. changes in connectivity and catchment disturbance), and to changes in some riverine habitat components (e.g. changes in salinity); the inclusion of other biota (e.g. streamside and aquatic plants, algae, fish or water birds) would give a more comprehensive assessment of the cumulative effects of environmental change;
• there may be lags between environmental degradation and biotic condition (e.g. nutrient or sediment loads to streams);
• an environmental component that would explain a biotic response was not measured (e.g. a toxicant); and
• modelled inputs to the environment index may not reflect actual site values or land management practices.

Spatial patterns in the environment index

When the subindices that make up the environment index are statistically analysed, there are patterns of reaches with similar characteristics for habitat, catchment disturbance and nutrient and suspended sediment loads.

The river reaches with the most urgent need for strategic management and rehabilitation are those in highly modified catchments that have lost much of their riparian vegetation and have dams and levees that disrupt movement of biota and material in the river (Figure 42). These reaches are located in parts of the Murray-Darling Basin, south-west Western Australia, western Victoria, and the South Australian wheat-growing areas.

River reaches that have largely unmodified habitat in terms of riparian vegetation, but very high nutrient and suspended sediment loads, and high potential erosion from hill slopes and stream banks are in need of rehabilitation by reducing nutrient and suspended sediment loads (Figure 43). These reaches are located in Queensland, northern coastal New South Wales, western Victoria and south-west Western Australia.

River reaches that are largely unmodified in all aspects (habitat, catchment disturbance and nutrient and suspended sediment loads) are scattered across Australia, and especially in far north Queensland, eastern Victoria and Tasmania. Protective management will ensure their condition is maintained (Figure 44).

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