• The Evolution of Corryong/Nariel Creek since European Settlement: Implications for On-going Management Prioritisation

      Teo, Elisha A.; Marren, Philip M.; University of Melbourne (7th Australian Stream Management Conference / asn events, 2014)
      Geomorphological stability is a useful starting point to inform river management priorities, as it is critical to other river health parameters such as ecology and water quality. A key debate in channel stability is the relationship between climate and human activity. Corryong Creek is an ideal setting to study the interaction between climate and anthropogenic changes on channel evolution as it has experienced significant levels of both. Catastrophic floods have been induced by high rainfall, the floodplain has been completely cleared, the riparian zone is almost entirely invaded by willows, and every reach of the channel has experienced some form of channel modification. The impacts of both climatic and anthropogenic factors are visible in our channel change data, although at different spatiotemporal scales. Higher flows during La Niña resulted in channel widening while lower flows during El Niño resulted in channel narrowing. In addition, land clearing had caused the river to evolve into a higher-energy, straighter channel, while spatially variable and temporally irregular factors such as river engineering, willow density and stock trampling tended to intensify erosion on a reach scale. As our analysis shows that periodic increases in erosion during La Niña are expected, the local community needs to first accept and adapt to some level of channel erosion in order to avoid catastrophic damage during floods. As the second priority, since the reversibility of these factors are limited, erosion risk can be mitigated through strengthening willow management, limiting river engineering, practicing bushfire management, and fencing the riparian zone.
    • The impact of dams on floodplain geomorphology: are there any, should we care, and what should we do about it?

      Marren, Philip M.; Grove, James R.; Webb, J. Angus; Stewardson, Michael J.; University of Melbourne (7th Australian Stream Management Conference / asn events, 2014)
      We undertook a review of the potential for dams to impact floodplain geomorphology, using both a conventional literature review and a systematic review using ‘causal criteria’ analysis. The literature review identified potential impacts on overbank flooding, scour and sedimentation, within-channel bank erosion, meander migration and cutoff frequency, and avulsion characteristics and frequency. The temporal scale of impacts ranged from years and decades, through to millennia. The causal criteria analysis indicated that with the exception of reduced meander migration rates, most impacts had been too poorly documented to be confident of their impact at present. We identify a distinction between ‘passive impacts’ (floodplain disconnection) and ‘active impacts’ (changes in geomorphological processes and functioning). Dams do impact floodplain geomorphology: many of the impacts will be subtle, and over very long timescales (1000s of years), but altered overbank sediment loads have the potential to change patterns of scour and deposition across floodplains. Further research is needed that specifically seeks to identify the impacts of dams on floodplain geomorphology, hydrology-geomorphology-vegetation interactions, and floodplain ecological response. Given the practical constraints on overbank environmental flow releases, there is relatively little that can be done to mitigate dam impacts on floodplain geomorphology. The main options include using within-channel flows to maintain meander migration and partial floodplain connectivity. We suggest that the major action should be that once dams come online, efforts should be made to prevent channel enlargement through scour, channel widening and wood removal, so that geomorphological processes can fully reestablish immediately once the dam ceases to operate.