M. Parsons, M. Thomas, R. Norris
Cooperative Research Centre for Freshwater Ecology
Monitoring River Health Initiative Technical Report Number 21
Environment Australia, 2002
ISSN 1447 1280
ISBN 0 642 54887 0
Australian Governments are increasing their focus on rivers via legislative, research and rehabilitation actions (Ladson et al., 1999). Within this environment, the Victorian Index of Stream Condition (ISC) was developed in response to a managerial need to 'use indicators to track aspects of environmental condition and provide managerially or scientifically useful information' (Ladson et al., 1999, p454).
The ISC evolved in four stages. Stage 1 involved the development of the concept and included a review of stream assessment methods, input from stream scientists and managers, and development of an ISC prototype (Ladson and White, 1999). The desired attributes considered in development of the ISC concept were:
Stage 1 is analogous to the aims of the current Physical and Chemical Assessment Module. Stages 2 and 3 of the ISC involved trialing and refining the concept and Stage 4 involved application of the ISC across Victoria (Ladson and White, 1999). Future stages will involve assessment and further refinement of the method (Ladson and White, 1999).
The ISC measures stream condition within reaches that are between 10 and 30km in length (Ladson and White, 1999). Reaches are defined as 'contiguous sections of stream chosen so that they are approximately homogeneous in terms of the five components of stream condition' (Ladson et al., 1999 p456). Reaches are delineated mainly from 1:250 000 topographic maps or aerial photographs. Within each reach, measurement sites are selected on the basis of:
The ISC consists of five sub-indices, which represent key components of stream condition (Table 2.4.1). Each sub-index consists of indicators, which are calculated using data collected in the field or by desk based methods. Each indicator is then assigned a rating score (see Section 2.4.3). Sub-index scores are calculated by summing the component indicator scores, and the overall ISC score is calculated by summing the sub-index scores (Ladson et al., 1999).
| Sub-index | Basis for sub-index value | Indicators within sub-index |
| Hydrology | Comparison of the current flow regime with the flow regime existing under natural conditions | Amended annual proportional flow deviation Daily flow variation due to change of catchment permeability Daily flow variation due to peaking hydroelectricity stations |
| Physical Form | Assessment of channel stability and amount of physical habitat | Bank stability Bed stability Impact of artificial barriers on fish migration Instream physical habitat |
| Streamside Zone | Assessment of quality and quantity of streamside vegetation | Width of streamside zone Longitudinal continuity Structural intactness Cover of exotic vegetation Regeneration of indigenous woody vegetation Billabong condition |
| Water Quality | Assessment of key water quality parameters | Total phosphorus Turbidity Electrical conductivity Alkalinity / acidity |
| Aquatic Life | Presence of macroinvertebrate families | SIGNAL AusRivAS |
The ISC uses a rating system to assess stream condition. The use of a rating system is designed to provide as much resolution as possible, within the constraint that there is 'limited knowledge of the relationship between a change in the indicator and environmental effects' (Ladson and White, 1999, p10). Values for each indicator are assigned a rating on the basis of comparison with a reference state (Figure 2.4.1). These ratings are summed to produce an overall score that reflects a continuum of stream conditions from excellent to very poor (Figure 2.4.1). In calculating the overall ISC scores, the scores for each sub-index and for each indicator can be weighted, depending on the perceived importance of each, or the availability of data (Ladson and White, 1999).
The ISC is based on the premise that the hydrology, physical form, streamside zone, water quality and aquatic life components indicate the processes and functions that act to influence stream condition. For example, the hydrology sub-index reflects deviation of the current flow regime from natural conditions, the physical form sub-index reflects channel morphology and the provision of biotic habitat, and the streamside zone sub-index reflects the importance of riparian zone and floodplain processes (Ladson and White, 1999; Ladson et al., 1999). A holistic assessment of stream condition is achieved by integrating these components into a single ISC score. However, it is recommended that the scores for the component sub-indices are reported alongside the overall ISC score, because the overall score may be composed of sub-indices that vary in condition (Ladson and White, 1999).
Figure 2.4.1 Assessment of stream condition using the Index of Stream Condition. Derived from Ladson and White (1999).
The ISC was designed to provide an assessment of long term changes in the environmental condition of rural stream reaches 10-30km in length, with surveys conducted at five year intervals (Ladson and White, 1999). As such, the 'level of detail is only sufficient to signal potential problems, suggest their cause and highlight aspects that may need specific investigations' (Ladson et al., 1999, p455). However, the ISC is a tool for determining the success of environmental intervention policies (Ladson and White, 2000) and can be used in a management context to:
The ISC is designed to be a broad scale and long term assessment. As such, the ISC consists of five sub-indices that reflect different components of stream condition. The aquatic life sub-index is the component that reflects overall biotic condition within the sampling reach (Ladson and White, 1999). Macroinvertebrate indicators are used in the ISC, because this group of organisms provides a continuous assessment of the environment which they inhabit (Rosenberg and Resh, 1993; Ladson and White, 1999). As such, the aquatic life sub-index is inherently related to the hydrology, physical form, streamside zone and water quality sub-indices (Ladson and White, 1999). Inclusion of the aquatic life sub-index provides a somewhat independent measure of stream condition, and can be particularly useful in situations where the biota are degraded but the physical, chemical and hydrological indices are not (Ladson and White, 1999). While there is empirical evidence that broadly links degradation in physical, chemical and hydrological factors with degradation in macroinvertebrate communities, care must be taken when comparing scores for the aquatic life sub-index with scores for the other sub-indices. This is because a scoring system may not be a sensitive reflection of mechanistic relationships between environmental factors and macroinvertebrate community composition.