Rivers and resilience: a longer term view from the drylands

In fluvial geomorphology and cognate disciplines, river resilience is defined in various ways and has physical, ecological and socioeconomic dimensions but commonly is taken to mean the ability of a system to: A) withstand disturbance; B) recover from disturbance; or C) adapt and evolve in response to disturbance to a more desirable (e.g. stable) configuration. Most previous assessments of dryland river resilience have tended to focus on potential future adjustments to river flow regimes that may result from global climate change and/or human activities, in some cases with consideration of the implications for river ecology or human land uses in these marginal environments. Commonly, these assessments do not involve consideration of the potential future adjustments to dryland river geomorphology (e.g. natural channel-floodplain structures) that may also influence riverine ecology or human land use, thereby limiting options for river management decision making. How can we close this conceptual and practical gap? In this chapter, we focus on Australian and southern African case studies where a combination of aerial image interpretations, field investigations, palaeohydrological calculations and geochronology has enabled detailed reconstructions of dryland river responses to changing Holocene hydroclimates. We then examine how knowledge of these river responses may provide important insights for improved conceptual definition and practical application of resilience thinking in dryland fluvial geomorphology and cognate disciplines. In particular, we: 1) demonstrate the range of past – and possible future – dryland river responses under changing boundary conditions and forcings; 2) highlight the significance of distinguishing between dryland ‘river behaviour’ (i.e. natural variability within a largely unchanging channel-floodplain structure) and dryland ‘river change’ (i.e. more profound transformation of river geomorphology) when considering resilience; 3) stress the importance of identifying proximity to quantified geomorphic thresholds to help distinguish between ‘river behaviour’ and potential ‘river change’; and 4) identify key conceptual and data gaps that need to be addressed to ensure better uptake of these insights in research and management. Addressing these knowledge gaps will be crucial for improving future management of dryland riverscapes and their multifaceted ecosystem services in a rapidly changing 21st century world.