Detection of Decadal Time‐series Changes in Flow Hydrology in Eastern Australia: Considerations for River Recovery and Flood Management

Since European colonisation, many coastal rivers of New South Wales (NSW) have been highly modified by channelisation, flow regulation, sediment mining, intensive grazing and agriculture, deforestation, and riparian vegetation and wood removal. Since the late 1980s, nearly 55% of these rivers have undergone a significant 're-greening' and geomorphic recovery with changes to instream and riparian roughness. However, little research has been undertaken to quantify if there have been any coeval changes in flow hydrology. Approximately 7000 flow hydrographs with one-hour time-steps from 117 gauges on 45 study rivers (17 of 20 coastal catchments of NSW) were used to assess changes to in-channel and overbank hydrology over decadal timeframes. Hydrograph shape and attenuation characteristics for three morphologically defined flow stages were analysed: in-channel fresh, high flow and overbank flood. Time-series analysis was used to quantify changes in flow hydrology from the early-20th century to present and identify trends occurring between known degradation phases (1910s-1940s and 1940s-1980s) and a recovery phase (1990s to present) for these rivers. Our findings indicate that, on average, and for some key examples, changes in flow hydrology are occurring. Flows are slowing and attenuating as indicated by decreases in flood wave celerity and hydrograph kurtosis, skewness and rate of rise and increases in peak-to-peak travel time, flood peak attenuation and flood wave attenuation index. The most significant changes have occurred since the 1980s with the most noticeable effects on the behaviour of high flows (around the bankfull stage). However, there is not a consistent direction change in these indicators in all places. We use these findings to categorise the flow mitigation signal of the study regions and catchments and discuss the implication for river and flood management. In Eastern Australia, flow and flood hydrology has significantly changed since the 1910s. We use over 7000 hydrographs to quantify changes in flow hydrology such as at-a-station kurtosis, skewness and rate of rise and upstream-to-downstream celerity and attenuation. We find a flow mitigation signal emerging in many catchments. These changes in flow and flood hydrology are occurring coevally with vegetative and geomorphic river recovery that has been occurring along these rivers for several decades.image