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The Persistence of Oxbow Lakes As Aquatic Habitats: an Assessment of Rates of Change and Patterns of Alluviation

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The Persistence of Oxbow Lakes As Aquatic Habitats: an Assessment of Rates of Change and Patterns of Alluviation The Persistence of Oxbow Lakes as Aquatic Habitats: an Assessment of Rates of Change and Patterns of Alluviation Pauline L. Dieras 2013 Thesis submitted for the degree of Doctorate of Philosophy Declaration This work has not been submitted in substance for any other degree or award at this or any other university or place of learning, nor is being submitted concurrently in candidature for any degree or other award. Signed Pauline Dieras Date: 05/03/2013 This thesis is being submitted in partial fulfilment of the requirements for the degree of PhD Signed Pauline Dieras Date: 05/03/2013 This thesis is the result of my own independent work/investigation, except where otherwise stated. Other sources are acknowledged by explicit references. The views expressed are my own. Signed Pauline Dieras Date: 05/03/2013 I hereby give consent for my thesis, if accepted, to be available for photocopying and for inter-library loan, and for the title and summary to be made available to outside organisations. Signed Pauline Dieras Date: 05/03/2013 i Abstract Oxbow lakes are of high ecological importance due to the number and the diversity of habitats they provide. They are created after the abandonment of meanders and subsequent sediment infilling leads to their progressive terrestrialisation, taking from a few months up to several centuries. Nonetheless, little is known about oxbow lake terrestrialisation processes, sediment composition, or why such a disparity exists in lakes’ longevity. To understand the controls on oxbow lakes alluviation, field observations, remotely sensed data and GIS analyses were combined. Sediment transfers in oxbow lakes were documented by topographic and sampling surveys of sites in France and Wales. Aerial photographs and maps were used to date cutoff events, analyse oxbow lakes geometry, and understand the controls on oxbow lake terrestrialisation for eight rivers of different characteristics. Findings from this study illustrate that the specific mechanism by which an oxbow lake is formed is critical to its persistence as a lake and to the sedimentary processes experienced. Chute cutoff oxbow lakes filled in 10 times faster than neck cutoffs and showed significantly different sediment deposits. Results also highlighted that oxbow lakes are not only fine-grained sediment stores, as often referred to, but can be significant bed material sinks since a site on the Ain River sequestered up to 34% of the bed material supply. However, the volume of sediment mobilised in the main channel during cutoff appeared to be larger than the bed-load stored in the former channel within the first decade after abandonment (40%). Sedimentary evidence showed that the terrestrialisation of oxbow lakes is driven by several processes: a flow separation zone at the entrance of the channel creating a sediment plug, sediment sorting by flow gradients and decantation in ponded areas. These results have important implications for the management of meandering rivers by providing a comprehensive analysis of depositional processes which also helps to predict oxbow lake longevity. ii Acknowledgements First and foremost, I would like to thank my supervisors Dr Jose Constantine and Dr T.C Hales for their guidance, knowledge and patience. Their energy and enthusiasm kept me motivated throughout my PhD. I wish to thank my co-supervisor Dr Hervé Piégay for his support and kind help. The work carried out for Chapter 3 could not have been possible without his help. A warm thank you to my father who has always been supporting and encouraging in good or difficult moments. Thank you Tom for your love, humour, support and calm which made me happy for the last few years. Thank you to all my friends from the Earth and Ocean Sciences Department who kept me entertained. A particular thank you to Paola, Margit and Sindia for their distracting lunch breaks, amusing stories and for nicely listening to my complaints. I wish to also thank Hannah and Rhian for their friendship and for kindly sustaining me with regular accommodation at the end of my PhD. Thanks to Kate as well, she kept me sane and fit, even during memorable dance-fitness-crazyness classes which were highly confusing. Thanks to Leigh for driving us every day during our 2 months of fieldwork on the Towy River and for your calm facing various types of farm animals. I also wish to acknowledge the UK Resource Centre for Women in SET and the British Society for Geomorphology who awarded me grants which respectively supported fieldwork in France and allowed me to present my research at an international conference. iii Contents Declaration ................................................................................................................ i Abstract .................................................................................................................... ii Acknowledgements ................................................................................................. iii Contents .................................................................................................................. iv List of Figures ........................................................................................................ viii List of Tables ............................................................................................................. x Chapter 1 .................................................................................................................. 1 Introduction ................................................................................................................... 1 1.1 Sediment transfers .................................................................................................. 2 1.2 Meander cutoff ........................................................................................................ 3 1.2.1 Why do meandering rivers cut-off? .................................................................. 3 1.2.2 The significance of cutoffs for meandering rivers ............................................ 5 1.2.3 Cutoff mechanisms ........................................................................................... 6 1.2.3.1 Neck cutoff ................................................................................................ 6 1.2.3.2 Chute cutoff ............................................................................................... 8 1.3 Stages of oxbow lake sedimentation .................................................................... 11 1.3.1 From cutoff to hydraulic disconnection ......................................................... 11 1.3.2 From disconnection to terrestrialisation ........................................................ 14 1.4 Controls on terrestrialisation ................................................................................ 17 1.4.1 Floods and Hydraulic Connectivity ................................................................. 17 1.4.2 Diversion angle, geometry and cutoff mechanism ......................................... 18 1.4.3 River sediment supply .................................................................................... 20 1.4.4 Vegetation....................................................................................................... 21 1.5 The Role of oxbow lakes on meandering floodplain ............................................. 22 1.5.1 Floodplain architecture ................................................................................... 22 1.5.2 Storage of contaminants and flood waters .................................................... 23 1.5.3 Maintaining diverse aquatic habitats ............................................................. 24 1.5.4 Socio-economic role ....................................................................................... 27 1.6 Conclusion and thesis highlights ........................................................................... 28 Chapter 2 ................................................................................................................ 30 Controls on the persistence of oxbow lakes as aquatic habitats ................................ 30 iv 2.1 Introduction ........................................................................................................... 31 2.2 Study sites .............................................................................................................. 34 2.3 Methodology ......................................................................................................... 38 2.3.1 Aerial photographs measurements ................................................................ 38 2.3.1.1 Water Surface Area (WSA) measurement ............................................... 38 2.3.1.2 Error related to the aerial photograph quality ....................................... 39 2.3.1.3 Influence of the river discharge .............................................................. 40 2.3.1.4 Dating the cutoff events .......................................................................... 41 2.3.1.5 Rate of water surface area reduction ..................................................... 41 2.3.1.6 Diversion angle ........................................................................................ 42 2.3.1.7 Difference in slope between former and active channel ........................ 42 2.3.1.8 Initial meander length ............................................................................. 42 2.3.1.9 Statistical analysis ...................................................................................
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