Oral Presentation 9th Australian Stream Management Conference 2018

Using concreted stormwater channels for determining ‘worse case scenarios’ when assigning grades of relative condition to urban waterways (#51)

David J Reid , Carl Tippler

Report Cards are increasingly popular tools for communicating waterway condition to diverse audiences, using easily understood grades from A to F. Grading can be applied across a sliding scale, by applying algorithms to the values of a suite of environmental variables measured from each site. The upper and lower limits of the scale are delineated by using data from minimally disturbed and severely degraded ‘worse case scenario’ (WCS) sites, respectively. Deciding upon appropriate types of sites to determine the limits varies between catchments and is somewhat subjective. In urbanised catchments, concrete channels may be potentially useful for determining WCS for streams as they are common and highly degraded. However, concrete channels are not usually used for monitoring programs, including for determining the WCS, as reporting is focused on ‘natural’ waterways. To determine the effect that WCS selection had upon grading, we compared WCS values for environmental variables (i.e. riparian vegetation, water quality and macroinvertebrate communities) determined from surveys of concrete reaches and ‘natural’ reaches in the highly urbanised Georges River catchment of southern Sydney. WCS values for riparian vegetation were usually lower for concrete reaches than for ‘natural’ reaches. However, WCS values for water quality variables were not uniformly worse in concrete reaches; for example, dissolved oxygen was higher in concrete reaches. WCS values for macroinvertebrate metrics were usually lowest in concrete reaches. However, macroinvertebrate communities were degraded at all sites and the difference in metric values between concreted and ‘natural’ reaches was less than expected. Overall, the use of concreted or ‘natural’ reaches as the lower limit or WCS had little effect on waterway grading. However, using concrete reaches as WCS aided in interpretation and highlighting the severe degradation of urban streams, as ‘natural’ streams graded F had a condition ‘as poor as a concrete channel’.

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