River condition in the Murray-Darling Basin 2001

The National Land and Water Resources Audit 2000 Assessment of River Condition provided the baseline data that enabled a focused assessment of the rivers of the Murray-Darling Basin. The current Murray-Darling Basin Sustainable Rivers Audit is due to report in 2007. Data from both projects were with additional information sourced from published and unpublished reports.

Biotic and environmental condition of reaches in the Murray-Darling Basin in 2001

Indices

Percentage of river length in each category

 

Reference (%)

Significantly Impaired (%)

Severely Impaired (%)

Extremely Impaired (%)

Biota condition

62

28

8

2

 

 

 

 

 

 

Largely Unmodified (%)

Moderately Modified (%)

Substantially Modified (%)

Severely Modified (%)

Overall environment condition

3

69

29

0

Environmental components

 

 

 

 

  • Hydrological disturbance index

43

44

11

1

  • Catchment disturbance index

4

93

3

0

  • Habitat index

28

36

34

3

  • Nutrient and suspended load index

4

41

49

6

Total river length in the Murray-Darling Basin is 77,366 km. Note: Figures may not add up exactly because of rounding

Condition of rivers in the entire Murray-Darling Basin

Biota condition

  • Forty per cent of the river length assessed was impaired, having lost a significant number of aquatic macroinvertebrates expected to occur there.
  • Ten per cent of river length was found to be severely impaired, having lost at least 50 per cent of the types of aquatic macroinvertebrates expected to occur there.

Environmental condition

  • More than 95 per cent of the river length assessed in the Murray-Darling Basin had an environmental condition that was degraded, and 30 per cent was substantially modified from the original condition.
  • Disturbance to the catchment and changes to nutrient and suspended sediment loads were considered to be the greatest contributors to this index of degradation.
  • Most reaches in the central and western part of the Murray-Darling Basin were moderately modified.
  • In contrast, there is an arc of reaches down the eastern side of the basin that were assessed as substantially modified. This was concluded to be largely the result of poor habitat, nutrient and suspended load conditions.
  • More than half the reaches assessed had modified hydrology, with greatest changes found immediately downstream of dams and in lowland reaches used for irrigation supply.

Patterns of degradation

  • An analysis that grouped river reaches with similar characteristics found that many reaches were threatened by multiple stresses, principally land use changes, damaged riparian vegetation, poor water quality, increased bedload, and modified hydrology.
  • The reaches most affected by this complexity of issues were mainly found in the midslopes and lower slopes of the Murray-Darling Basin.
  • All groups of reaches in the Murray-Darling Basin had disturbed catchments and modified water quality (Figure 1). These two issues need to be managed across the entire basin in conjunction with local issues. The other characteristics provided the discrimination between the reaches.

Locations of reaches affected by issues of differing severity in the Murray-Darling Basin. Note that almost all reaches are affected by both catchment land use and water quality.

Locations of reaches affected by issues of differing severity in the Murray-Darling Basin. Note that almost all reaches are affected by both catchment land use and water quality.
Data Sources: NLWRA (2001b). Data used are assumed to be correct as received from the data suppliers. © Commonwealth of Australia 2001

Group 1 reaches did not have any major causes of degradation other than the ubiquitous catchment and water quality effects. These reaches were mostly in the upper parts of catchments and the western Queensland part of the Murray-Darling Basin.

Groups 2, 3 and 7 reaches were affected principally by moderately poor riparian vegetation and bedload condition. The reaches tended to be concentrated on midslopes and lower slopes. Riparian vegetation could be addressed immediately in the short term, but bedload conditions in these regions will not be as easily remediated, especially if left to degrade further. Protection measures in these 1400 reaches may have significant long-term benefits.

Groups 4, 5 and 8 reaches were affected by at least one major issue: poor connectivity or riparian vegetation. These reaches also tended to be found on mid slopes and lower slopes in the Murray-Darling Basin, and affected many of the main stems of the major rivers in the Murray-Darling Basin.

Group 6 reaches were affected by one of two major issues: poor connectivity or riparian vegetation. This is a small group of 125 reaches, mostly in upland areas where attention to longitudinal connectivity (for example, fish ladders) and riparian planting would be likely to have marked benefits.

Hydrological disturbance

  • Changes to hydrology were extensive throughout the Murray-Darling Basin. Typically, reaches with the most modified hydrologic regimes were downstream reaches of rivers being used for extensive irrigation supply. Examples include, the Murray, Murrumbidgee, Wimmera-Avon, Loddon and Darling Rivers.

Hydrological condition of river reaches in the Murray-Darling Basin

Hydrological condition of river reaches in the Murray-Darling Basin.
Data Sources: NLWRA (2001b). Data used are assumed to be correct as received from the data suppliers. © Commonwealth of Australia 2001

Catchment disturbance

  • Catchments throughout the Murray-Darling Basin have been modified by land use activities that may adversely affect river condition. There was no obvious spatial pattern to this disturbance; much of the Murray-Darling Basin showed the same level of impact.

Habitat condition

  • Habitat condition was found to be degraded in much of the Murray-Darling Basin, with the main causes being loss of riparian vegetation and increased sand and gravel bedload. These two have interacted to produce long-term impacts, with bedload moving predominantly during floods. Even if source erosion were to be stopped today, there are large areas of sand deposition that will continue to progress through river systems. The most marked degradation in the Murray-Darling Basin was in the midslope rivers in an arc from Queensland into Victoria.

Nutrient and suspended sediment loads

  • Increases in nutrient and suspended sediment loads were widespread in the Murray-Darling Basin. Most of the loads have been generated in the upland and midslope areas, but most of the impact was seen in lowland rivers, weir pools and reservoirs where the sediment is stored. In the long term, management needs to focus on reducing sediment supply, but the greatest short-term benefits will come from managing the lowland sediment and nutrient stores.

Condition of the Murray River and lower Darling River

The condition of this part of the Murray-Darling Basin was assessed for each of the seven hydrologic zones defined by the Murray-Darling Basin Commission; from zone 1 in the mountains, to zone 6 at the Murray River mouth, and zone 7 in the lower Darling. Overall biological and environmental condition was assessed as degraded in all seven hydrologic zones, with an increasing trend in degradation towards the river mouth.

Biological assessment

  • Fish populations were assessed as being in very poor to extremely poor condition throughout the Murray River; they were in slightly better condition in the lower Darling River.
  • Macroinvertebrate communities were found to be in generally poor condition, which declined towards the river mouth.
  • Riparian vegetation condition along the entire river was assessed as poor, with grazing and alterations to the flow regime being the major causes.
  • Wetland quality has been significantly reduced, particularly in zones 2–4. Most wetland loss has been attributed to permanent inundation of previously intermittently flooded wetlands.

Environmental assessment

  • Hydrological condition in the river channel was poor for all zones, with the extent, timing and duration of floodplain inundation all significantly impacted.
  • Riverine habitat was found to be poor or very poor through all zones with connectivity, riparian vegetation and bedload all affected by regulation.
  • The condition of floodplain inundation has been assessed as very poor in all zones.
  • Nutrients and suspended sediment loads were excessive and increasing towards the mouth of the river.
  • Salinity concentrations in the lower Murray River were assessed as improving as a result of water diversion for irrigation and salt interception schemes. This trend is predicted to reverse with increasing salinity during the next 50 to 100 years because of land uses upstream and rises in groundwater levels in the lower Murray River.

Multiple impacts

  • The degraded condition of the Murray River and the lower Darling River appears to be the consequence of multiple impacts, with the main impacts related to the operation of dams and weirs throughout the system.
  • Throughout the Murray River and lower Darling River, unseasonal flooding of wetlands, loss of connection with the floodplain, habitat simplification, water quality and bank erosion were all found to be significant issues.

Example of Murray River wetlands between Renmark and Waikerie in South Australia.

Example of Murray River wetlands between Renmark and Waikerie in South Australia
Image by Rob Harman, sourced from SKM

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Last Updated 22/06/2007