University of Wollongong Research Online University of Wollongong Thesis Collection University of Wollongong Thesis Collections 2003 Fluvial geomorphology of the Nambucca river catchment: late quaternary change, post-settlement channel degradation and proposals for rehabilitation Christopher J. Doyle University of Wollongong Recommended Citation 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 Research Online is the open access institutional repository for the University of Wollongong. For further information contact the UOW Library: [email protected] NOTE This online version of the thesis may have different page formatting and pagination from the paper copy held in the University of Wollongong Library. 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FLUVIAL GEOMORPHOLOGY OF THE NAMBUCCA RIVER CATCHMENT: LATE QUATERNARY CHANGE, POST-SETTLEMENT CHANNEL DEGRADATION AND PROPOSALS FOR REHABILITATION A thesis submitted in fulfilment of the requirements for the award of the degree DOCTOR OF PHILOSOPHY from University of Wollongong by Christopher J. Doyle B.Env.Sc (Hons) Land Resources School of Geosciences 2003 CERTIFICATE OF ORIGINALITY I, Christopher J. Doyle, declare that this thesis, submitted in fulfilment of the requirements for the award of Doctor of Philosophy, in the School of Geosciences, University of Wollongong, is wholly my own work unless otherwise referenced or acknowledged. The document has not been submitted for qualifications at any other academic institution. (Signed)...« Christopher J. Dotfe June 23, 2003 Fluvial Geomorphology of the Nambucca River Catchment ACKNOWLEDGEMENTS Most sincere thanks must go to Professor Gerald Nanson for giving me the opportunity to undertake this study. His friendship, ideas and guidance have been of immense value. My understanding of rivers has much to do with his insight and dedication to the field. I must also extend the most special of thanks to my wife Ulla-Maija. On the day this project commenced I did not know her at all and in the time since she has become my girlfriend, fiancee, and now beloved wife of five years. Her assistance and motivation has been a force behind this completed work. I must also thank the field assistants I have had over the life of the project, especially Richard Walsh, Maria Coleman, Steven Tooth, Geoff Black, Tim Cohen, Ulla-Maija Doyle and Rob Storrs. Between us all we drove over 20,000 km, ventured onto more than 50 different properties, completed 15 km of surveying, sieved 1.5 tonnes of sediment, measured the dimensions of 3600 stones, examined 42 river rehabilitation works and augured or drilled through almost 1 km of sediment! Many thanks also to David Price and Jose Abrantes for their analysis of the TL samples. I would like to extend my gratitude to Steve Perrens and his consulting staff for assistance in collating land use and hydrology data. Thanks also to Andrew Brooks for his assistance with the project and his perceptions and expertise in the area of riparian vegetation. He was the inspiration behind the vegetation chapter of this thesis. I would also like to thank the residents of the Nambucca Shire for opening their gates and their homes and for passing on the knowledge they have gained. My penultimate thank you goes to my family. To mum and dad for their unconditional support, love and prayers, and to my sisters Angela, Gabrielle, Loretta, Louise and Collette and their families for their consideration. Finally, I have been much blessed during the undertaking of this work. Thanks and credit go to Him who has guided me and answered so many prayers. Fluvial Geomorphology of the Nambucca River Catchment ABSTRACT 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. Fluvial Geomorphology of the Nambucca River Catchment 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. Fluvial Geomorphology of the Nambucca River Catchment TABLE OF CONTENTS ACKNOWLEDGEMENTS..................................................................................................................... i ABSTRACT...........................................................................................................................................