Geomorphic controls, landforms, and processes of discontinuous dryland rivers

<p dir="ltr">Alluvial rivers are in semi-arid and arid inland regions (i.e., drylands) are diverse and provide a plethora of ecosystem functions and services in otherwise dry landscapes to local human and wildlife populations. An important but understudied feature of dryland rivers are discontinuous channels – unchannelised reaches with floodouts, or terminal zones on the floodplain – usually characterised by intermittent or ephemeral flow regimes and sporadic sediment erosion, transport, and deposition processes. These rivers play a crucial role in providing water, supporting biodiversity, and maintaining ecosystems, underscoring their indispensable significance across environmental, socio-economic, and cultural contexts. As the direct pressures of anthropogenic activity and indirect threats of climate change continue in water-stressed drylands, our understanding and management of these systems is increasingly significant. This thesis addresses the dearth of knowledge regarding the dominant controls, landforms, and processes of channel breakdown and floodout formation in dryland rivers of eastern Australia and southwestern USA.</p><p dir="ltr">Catchment morphometric analysis, longitudinal profile assessments and quantification of recent channel adjustments were used to investigate the external controls and the internal processes on channel discontinuity for different dryland river types (through-going, intermediate, and terminal) in eastern Australia. Overall, larger catchments with longer trunk streams, wider valleys, and higher stream powers had more active alluvial areas to enable floodout formation. Valley width, long profile concavity, and average stream power contribute to channel breakdown, but do not explain the different types of discontinuous channels and their floodouts in eastern Australia. Longitudinal profile assessments revealed general downstream declines in channel capacity due to reductions in discharge and stream power which coincided with areas where valley confinement eased. There were significant differences between river types in their downstream trends of floodplain, channel width, and channel slope, along with estimated flow velocity, bankfull discharge, and unit stream power. These downstream trends and underlying controls on floodout formation, lead to the development of a conceptual model of channel discontinuity related to long profile maturity in dry landscapes.</p><p dir="ltr">Repeat cross-section surveys along Little Faulkenhagen Creek, western New South Wales, revealed significant channel contraction and upstream migration of an intermediate floodout from the 1990s to present, due to a combination of sediment infilling and diversion of water onto the floodplain and into narrow, head-cutting channels that are reforming on the sides of the floodout. These incisional channels have the capacity to bypass the floodout should they link to the main channel upstream, leading to avulsion/s, and indicating a cycle of channel (dis)continuity in this system that is likely representative of other rivers in the region. Similarly, catchment-scale metrics, downstream geomorphic and hydrological trends, and recent historical channel changes in Polacca and Oriabi Wash, Arizona, reveal a dominance of downstream declining discharge and channel capacity that underpin channel breakdown and floodout formation. Processes of channel breakdown (dominated by sedimentation) and channel reformation (dominated by channel incision and arroyo development) are on par with those documented for dryland rivers in Australia.</p><p dir="ltr">Altogether, this thesis provides new insights into dryland river character and behaviour, including key catchment-scale controls and geomorphic processes that contribute to channel discontinuity and floodout formation. Understanding the external controls and internal processes that drive channel breakdown, and integrating this insight into management practices in arid landscapes, will enhance our capacity to predict fluvial changes in river systems more broadly, within the current geological age and environment is human activity (namely the Anthropocene).</p>