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The Why, When, and Where of Anabranching Rivers in the Arid Lake Eyre Basin Sarah Victoria Eccleshall University of Wollongong

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The Why, When, and Where of Anabranching Rivers in the Arid Lake Eyre Basin Sarah Victoria Eccleshall University of Wollongong University of Wollongong Research Online University of Wollongong Thesis Collection 2017+ University of Wollongong Thesis Collections 2019 The why, when, and where of anabranching rivers in the arid Lake Eyre Basin Sarah Victoria Eccleshall University of Wollongong Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong. Recommended Citation Eccleshall, Sarah Victoria, The why, when, and where of anabranching rivers in the arid Lake Eyre Basin, Doctor of Philosophy thesis, School of Earth and Environmental Sciences, University of Wollongong, 2019. https://ro.uow.edu.au/theses1/554 Research Online is the open access institutional repository for the University of Wollongong. For further information contact the UOW Library: [email protected] School of Earth and Environmental Sciences Faculty of Science, Medicine and Health The why, when, and where of anabranching rivers in the arid Lake Eyre Basin Sarah Victoria Eccleshall BSc (Hons.) University of Wales, Aberystwyth; MSc Stockholm University This thesis is presented as a requirement for the conferral of the degree: Doctor of Philosophy of the University of Wollongong June 2019 ii Abstract Multi-channel or anabranching planforms are a common river planform found in arid regions and nowhere is this more prevalent than in the 1.14 M km2 endorheic Lake Eyre Basin (LEB) of arid Australia. Of the19 main rivers in this basin, 14 anabranch for large proportions of their length yet with different multi-channel planform styles occurring in different parts of the basin. This thesis has three primary aims. The first is to constrain the timing for the onset of anabranching in the LEB whilst the second assesses the long-term sediment dynamics in such systems. The final aim investigates the characteristics of sandy ridge anabranching found in the NW of the LEB. The three aims provide an overarching objective, that of understanding the why, when and where of anabranching in the low- gradient arid LEB. The aims are addressed through primary and secondary data analysis including the use of terrestrial cosmogenic nuclide and optically stimulated luminescence dating to produced geochronological data from ~1 Ma years ago to the present. Using a compilation of 94 luminescence ages of fluvial sediment from the Channel Country region of the LEB from MIS 5 to present data was plotted via a number of different methods, and compared to randomly generated data for a signal vs. noise comparison. This approach shows that a transition from laterally active, sandy bedload systems to muddy anabranching systems occurred after MIS 5. From MIS 5 to MIS 3 there was a tendency for meandering as a planform to decline and anabranching to increase. Between MIS 3 and the LGM the transition from a laterally active to anabranching planform progressed to completion. Independent palaeoclimatic data over the same period, but mostly collected from elsewhere in the basin, suggests that this planform metamorphosis from meandering to anabranching was climatically induced with an overall decline in river energy and often (but not everywhere) associated with a shift from more sandy to more mud dominated systems. However climate is not the sole control on channel planform as the lower reaches of Cooper Creek and the Diamantina River do not anabranch but remain largely single thread, even though they are under the same climatic regime. Here channel slope has iii progressively increased due to the uplifting Innamincka and Gason domes and the lowering of base-level due to the decline in the water volume of Kati Thanda - Lake Eyre that was associated with increasing aridity. Understanding long-term sediment dynamics is investigated here using cosmogenic radionuclides on bulk detrital sediment from the modern channel and from Quaternary outcrops. Concentrations of 10Be in Cooper Creek vary little throughout the system implying a uniform exposure history. When used to calculate denudation they give rates of ~1.2 – 5.1 mm ka-1, some of the lowest globally. For much of Cooper Creek the ratios of 10Be/26Al deviate from the steady state value of 6.7 implying burial is occurring throughout the system. The apparent burial duration of in-channel sediment in the Cooper Creek system varies from ~ 0.05 to ~1.2 Ma and of dune sediment from ~1.06 to ~1.29 Ma. Duration does not appear to have a direct correlation with distance downstream or with proportion of sediment cover in the upstream area but appears instead to be associated with accommodation space. Those locations with greatest potential accommodation, such as the subsiding Cooper-Wilson syncline, store sediment for the longest duration and those with little or no potential for storage, such as the narrow Innamincka Choke, show no conclusive deviation from the steady state erosion ratio. These results differ to other recently conducted research in the tectonically/isostatically different western LEB where duration is closely correlated to the distance downstream and the upstream sediment cover. By forming sandy islands and ridges, the Marshall River on the Northern Plains exhibits a very different style of anabranching to that in the mud-dominated Channel Country. Single grain optically stimulated luminescence (OSL) dating has established that the depositional islands and ridges have formed over ~ 500 years. OSL ages of ~3000 years obtained from beneath the ridges suggests that these represent only the most recent ridges with previous ones having being eroded and replaced on a cyclical basis. It would appear that anabranching in this instance is a means by which to maintain equilibrium for sand transport in straight channels of low gradient where it is not possible to increase slope through straightening or down cutting. This thesis concludes that anabranching in rivers in the LEB varies both temporally and spatially. It has been a means of maintaining equilibrium in response to iv climatically-driven reductions in flow over the last glacial cycle. The mud-dominated systems in the east of the basin appear to have been progressively increasing their anabranching habit throughout much of the late Quaternary. The same may be true of the sandy systems in the north-west of the basin but the cyclical formation and destruction of their more erodible ridges and islands has left no evidence beyond ~500 years in age. The climate-induced metamorphosis from meandering to anabranching in the low-relief LEB appears to occur during glacial-interglacial cycles of great duration similar to global fluctuations in other fluvial systems, such as alternations in incision and deposition seen in high relief rivers. In contrast, cyclic island and ridge form anabranching can only be verified as occurring during the current depositional cycle of ~ 500 years. Flat arid landscapes, such as those investigated in this thesis, store sediment on route to their endorheic depocentres for 106 years and have adopted different anabranching styles to accommodate the late Quaternary history of diminishing flows over low gradients in different topographic and sedimentary settings. v vi Acknowledgements Those to whom I offer thanks are numerous. There are so many people to whom I owe thanks, on many levels. Firstly to my rabble of supervisors: John Jansen, Gerald Nanson, Tibi Codilean, Toshi Fujioka, Bert Roberts and particularly Tim Cohen who stepped is as my primary supervisor and saw this work come to fruition. The University of Wollongong provided me with both tuition fee and living cost scholarships, without which I would never have had the opportunity to undertake this work. Thanks must also go to Brent Peterson and Josef Stocker with limitless assistance in fieldwork and sample collection. To Nick Whitelaw for his assistance in the field, Bert Roberts, Zenobia Jacobs, and Terry Lachlan for their assistance in the UOW luminescence lab, and Steve Kotevski and Charles Mifsud for all the assistance at ANSTO. All the PhD students, past and present, who have made this experience enjoyable should be reminded that the value of comradery cannot be overstated. I shall not list you all for fear of missing someone. Some of you deserve a special mention: Brent, Daniela, Deirdre, Martin, Nathan, Meagan, Maria. I have been lucky enough to have some very special people in my life who have been so supportive of me (some of whom I have previously mentioned), some of you I’ve known a long time and others I have met more recently but you all have a special place in my heart: Rachel, Rachael, Jess, Deirdre, Brent, Nathan, Brendan, Rhi, Rob, Laura, Jessi, Heather - you’ve all been fantastic and I am so appreciative. Guinea Pig Rodeo, you have been my regular comic relief and never let me take anything too seriously – if in doubt it was probably Harold Holt, or Camembert. Sam, you appeared on the scene fairly late in the game, but I’d never have got this monster across that final line without you, thank you! Lastly, and most definitely by no means least, I would never have made it this far without the support of my Mum and Dad, for everything you’ve done for me, I am eternally grateful. vii viii Table of Contents Declaration .................................................................................................................... i Abstract ....................................................................................................................... iii Acknowledgements ...................................................................................................
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