Dryland rivers and hydroclimatic change: past, present and future

Rivers and wetlands in drylands are vital components of the landscape that provide countless beneficial services to humans and ecosystems. The hydrological and geomorphological diversity of rivers across time and space makes comprehensive understanding of fluvial systems and prediction of future change very difficult. A long-term perspective, however, can improve our ability to assess the sensitivity of dryland rivers to extrinsic forcing, such as climate-driven hydrological (hydroclimatic) changes, and contextualises the natural range of variability defining modern rivers. This thesis aims to understand: 1)how dryland rivers have responded to past hydroclimatic changes, 2) the processes that define the n atural range of variability of modern rivers, and 3) how dryland rivers will respond to future hydroclimatic change. Hydroclimatic change has affected dryland rivers differently around the world,but evidence from eastern Australia and southern Africa suggests a relatively synchronous period of enhanced fluvial activity during the early to mid-Holocene (~8–4 ka) followed by relative quiescence of fluvial activity in the late Holocene. An inter-continental approach allows an improved understanding of how hydroclimatic changes manifest in dryland rivers in very different settings. The fan-shaped alluvial plain of the lower Warrego River in eastern Australia has preserved evidence for large (~160 m), sinuous, laterally migrating palaeochannels that were transporting ~20 times the discharge of the modern river and were active until ~5 ka. The modern Warrego River has a markedly different plan form,with narrower, straighter anabranching channels in the equivalent position on the plain. Similarly, in the Panhandle region of the Okavango Delta, northern Botswana, large (~120 m), sinuous, laterally migrating palaeochannels conveyed up to ~9 times the discharge of the modern system during a periodof enhanced rainfall over the headwaters that lasted until ~3.5 ka. The response of the Okavango Riverto subsequent declining discharge has been to contract considerably and, in the Panhandle, processes of channel adjustment by avulsion now dominate over lateral migration. A range of hydroclimatic drivers,related to orbital variations and climatic teleconnections, have likely been responsible for declining fluvial activity and dramatic transformations of river style in the late Holocene. The geomorphological sensitivity of the Warrego and Okavango rivers to relatively modest Holocene hydroclimatic change highlights that improving understanding of how rivers will respond to future climate change is of critical importance. Prediction of future fluvial change in dryland settings requires consideration of the intrinsic processes of river adjustment and the influence of extrinsic, hydroclimatic forcing on these processes. Catchment aridity strongly influences the geomorphology of Australian dryland rivers. By defining thresholds of aridity and modelling future climatic conditions, this research indicates that many Australian dryland rivers will likely undergo profound changes to their physical character and behaviour over coming decades/centuries, most likely in the form of increased likelihood of flow disconnection and channel breakdown.