Doctor of Philosophy
School of Geosciences
Doyle, Christopher J., Fluvial geomorphology of the Nambucca river catchment: late quaternary change, post-settlement channel degradation and proposals for rehabilitation, Doctor of Philosophy thesis, School of Geosciences, University of Wollongong, 2003. http://ro.uow.edu.au/theses/1969
The Nambucca River catchment is on the mid-north coast of New South Wales, Australia, and drains 1,407 km2 of land east of the Great Dividing Range. This study examines the pre- and post-settlement record of channel change in the seven tributaries of the Nambucca catchment and suggests a scheme for rehabilitation based on current geomorphic information and the identified record of channel changes.
The Late Pleistocene history, obtained from 19 river terrace thermoluminescence dates, identifies a remnant terrace of 78 ka from the Colleambally Phase during Oxygen Isotope Stage (OIS) 5. Younger terraces correspond to the Kerarbury Phase (55-35 ka) and the Gum Creek Phase (31-25 ka), both in OIS 3. However, the majority of terraces date during the Yanco Phase (20-13 ka) in OIS 2. This record of late Quaternary activity correlates with periods of fluvial activity identified on the much larger Nepean and Murrumbidgee Rivers in southeastern Australia. No sediment dates have been obtained in the Nambucca catchment for the period 12 ka to 3 ka, probably because extensive flushing removed most of that alluvium in what has recently been termed the Nambucca Phase. Radiocarbon dating of the Nambucca floodplains has provided 15 dates, all but one younger than 3000 yrBP. Between 3000 and 2500 yrBP, the streams changed from gravel, braided and somewhat laterally active, to stable systems forming floodplains by vertical accretion and with channels that underwent occasional avulsion. This laterally stable period continued through to European settlement in the middle 1800’s.
Since settlement there have been four periods of change in the catchment that have shaped the formation of the streams in the catchment:
Phase 1 (1830-1870): Settlers selectively logged the forested catchments for red cedar (Toona australis) but during this phase much of the forest on stream banks and floodplains remained intact.
Phase 2 (1870-1896): Extensive land clearance for agriculture occurred during this phase. A cluster of large floods in the 1890’s triggered a series of nickpoints. The initial channel instability problems probably date to this period.
Phase 3 (1897-1947): The period from the late 1890’s to the late 1940’s was relatively dry with very few recorded flood events. However, the earliest available aerial photographs from 1942 indicate channels straightened with meanders having cut-offs in the lower part of the catchment. The catchment appears to have been primed for major change during the flood dominated phase after 1947.
Phase 4 (1948-Present): The change to this phase was associated with a series of large floods in the late 1940’s and early 1950’s. Streams experienced substantial bed lowering, continual overwidening and the exposure of abundant floodplain sediment, releasing massive amounts of gravel that had been stored for 3,000 years. The exposed gravel bars soon became colonised by Casuarina cunninghamiana, which many landholders believed worsened the problems of channel instability in the catchment. In an effort to restore the streams, government authorities from the 1960’s to 1980’s encouraged the extraction of gravel and the removal of woody debris from the streams.
This investigation of the modern Nambucca catchment identifies tributaries floored with fine quartz gravel, migrating nickpoints, large colonies of Casuarina cunninghamiana and, bankfull channel capacities that can now convey 1-in-10-year floods and greater. An assessment of catchment geomorphology, and review of the many river rehabilitation schemes that have been attempted, recommends that controlling bed levels is a high priority. In this catchment substantial government funding is unlikely and the use of ‘soft’ engineering methods are required to provide longer term benefits for river rehabilitation. The construction of rock ramps appears to be the most suitable method for setting bed levels and arresting nickpoint retreat. Other methods such as pin groynes, brush groynes and jacks have all proven successful in straight reaches experiencing overwidening and bank retreat. Importantly, effective management of the riparian zone is required to encourage growth of native vegetation species in the absence of livestock.
This study provides a comprehensive review of changes in catchment behaviour from the late Quaternary to the modern day; it provides detailed information about the geomorphology and sedimentology of the channels, and it completes a detailed assessment of rehabilitation schemes. As such it presents stream managers with a methodology for making scientifically based decisions on river rehabilitation.