Is it possible to have high frequent flow disturbance and high stream health? Results from a field survey in northern Sydney

A study was undertaken to assess the relationship between Effective Imperviousness (EI) and stream health using Hornsby Council's existing extensive monitoring data. The 15 year data set ranges from bushland reference streams with an EI of zero, to heavily urbanised streams with very high EI. The data set also includes both water quality and biological parameters. EI was, in general, a reasonable predictor of the rapid deterioration of stream health as EI increased from 0 to 5%. However it was observed that selective streams in Hornsby Local Government Area (LGA) had a relatively high level of stream resilience which was correlated to the increase in distance from the last significant directly connected stormwater input. It is often suggested that stream health recovers downstream in waterways due to the in-stream processes of 'disconnection' attenuating frequent flows through in-stream infiltration. However, field investigations have shown that the streams investigated were unlikely to be attenuating frequent flows. This study found that some streams which are likely to have high frequent flow disturbance also have high stream health demonstrating a level of stream resilience. The paper postulates as to the reasons for this observation.