No abstract

Controls on the characteristics of floodplain wetlands in drylands are diverse and may include extrinsic factors such as tectonic activity, lithology and climate, and intrinsic thresholds of channel change. Correct analysis of the interplay between these controls is important for assessing possible channel–floodplain responses to changing environmental conditions. Using analysis of aerial imagery, geological maps and field data, this paper investigates floodplain wetland characteristics in the Tshwane and Pienaars catchments, northern South Africa, and combines the findings with previous research to develop a new conceptual model highlighting the influence of variations in aridity on flow, sediment transport, and channel–floodplain morphology. The Tshwane–Pienaars floodplain wetlands have formed in response to a complex interplay between climatic, lithological, and intrinsic controls. In this semi‐arid setting, net aggradation (alluvium >7 m thick) in the wetlands is promoted by marked downstream declines in discharge and stream power that are related to transmission losses and declining downstream gradients. Consideration of the Tshwane–Pienaars wetlands in their broader catchment and regional context highlights the key influence of climate, and demonstrates how floodplain wetland characteristics vary along a subhumid to semi‐arid climatic gradient. Increasing aridity tends to be associated with a reduction in the ability of rivers to maintain through‐going channels and an increase in the propensity for channel breakdown and floodout formation. Understanding the interplay between climate, hydrology and geomorphology may help to anticipate and manage pathways of floodplain wetland development under future drier, more variable climates, both in South African and other drylands. Copyright © 2016 John Wiley & Sons, Ltd.

Rivers provide crucial ecosystem services in water-stressed drylands. Australian dryland rivers are geomorphologically diverse, ranging from through-going, single channels to discontinuous, multichannelled systems, yet we have limited understanding of their sensitivity to future hydroclimatic changes. Here, we characterise for the first time the geomorphology of 29 dryland rivers with catchments across a humid to arid gradient covering >1,800,000 km 2 of continental eastern and central Australia. Statistical separation of five specific dominantly alluvial river types and quantification of their present-day catchment hydroclimates enables identification of potential thresholds of change. Projected aridity increases across eastern Australia by 2070 (RCP4.5) will result in ~80% of the dryland rivers crossing a threshold from one type to another, manifesting in major geomorphological changes. Dramatic cases will see currently through-going rivers (e.g. Murrumbidgee, Macintyre) experience step changes towards greater discontinuity, characterised by pronounced downstream declines in channel size and local termination. Expanding our approach to include other river styles (e.g. mixed bedrockalluvial) would allow similar analyses of dryland rivers globally where hydroclimate is an important driver of change. Early identification of dryland river responses to future hydroclimatic change has far-reaching implications for the ~2 billion people that live in drylands and rely on riverine ecosystem services. Rivers are lifelines in climatically variable and water-stressed drylands, the dry subhumid through hyperarid environments that cover 40-50% of the Earth's land surface and host ~28% of the world's population 1,2. Dryland rivers are fundamentally important for human populations, providing a plethora of provisioning, regulating, supporting and cultural ecosystem services 1,3. Yet dryland rivers exist in marginal environments and are threatened by declines in water availability due to the impacts of climate change (e.g. decreased rainfall, increased temperature and evapotranspiration, and greater climatic variability) and other human activities (e.g. river regulation, flow diversion and abstraction, and land use change) 4-6. Rivers are not static conduits of water, sediment and nutrients, but adjust dynamically to a suite of internal and external drivers. Among various external drivers (e.g. tectonic activity, sea level fluctuations, climate), research has shown that late Quaternary hydroclimatic changes have driven substantial geomorphological changes to many dryland rivers globally, including during the mid to late Holocene [e.g. 7-10 ]. Indeed, in tectonically stable settings such as continental Australia, and in reaches where rivers are free from significant bedrock influence, hydroclimatic changes are the principal driver of river response and resulting channel-floodplain geomorphology. To date, however, assessment of the potential likelihood and pathways of hydrological and geomorphological changes in dryland rivers due...

Dust plays a globally important role in supplying biologically essential elements to landscapes underlain by nutrient‐poor substrates. Here we show that dust may play a significant role in sustaining productivity in the vast wetlands of the Okavango Delta in southern Africa, one of the world's richest biodiversity hotspots. Dust accumulates preferentially on tree‐covered islands in the seasonal swamps of the Delta, creating pockets of fine‐grained, nutrient‐rich material within the semi‐arid landscape of the Kalahari Desert. Strontium and neodymium isotopes reveal that this dust likely originates predominantly from the Makgadikgadi salt pans, located 300 km away, and contributes 10–80% of the fine‐grained material present in Okavango island soils. Surface material sourced from the Makgadikgadi Pans contains relatively high amounts of bioavailable phosphorus and iron, potentially influencing Okavango Delta biological productivity. We propose that long‐term ecosystem productivity and nutrient availability in the Okavango may be strongly mediated by regional dust inputs. Understanding the influence of dust deposition on nutrient loads and biogeochemical cycling is thus critical for predicting the response of the Okavango Delta to future changes in climate. We suggest that dust inputs may play a significant role in the supply of nutrients to other large, global wetland systems located in dryland environments. © 2020 John Wiley & Sons, Ltd

Avulsion (relocation of a river course to a new position) typically is assumed to occur more frequently in rivers with faster sedimentation rates, yet supporting field data are limited and the influence of sedimentation rate on avulsion style remains unclear. Using analysis of historical aerial photographs, optically stimulated luminescence dating of fluvial sediments, and field observations, we 1 document three avulsions that have occurred in the last 650 years along the lower reaches of the semiarid Tshwane River in northern South Africa. Study of the modern river and abandoned reaches reveals that a downstream decrease in discharge and stream power leads to reduced channel size and declining sediment transport capacity. Bank erosion drives an increase in channel sinuosity, leading to a decline in local channel slope, and to a further decrease in discharge and sediment transport. Local sedimentation rates >10 mm a -1 occur within and adjacent to the channel, so over time levees and an alluvial ridge develop. The resulting increase in cross-floodplain gradient primes a reach for avulsion by promoting erosion of a new channel on the floodplain, which enlarges and extends by knickpoint retreat during periods of overbank flow. Ultimately, the new channel diverts the discharge and bedload sediment from the older, topographically higher channel, which is then abandoned. Our findings support the assumption that avulsion frequency and sedimentation rate are positively correlated, and we demonstrate that incisional avulsions can occur in settings with relatively rapid net vertical aggradation. The late Holocene avulsions on the semiarid Tshwane River have been driven by intrinsic (autogenic) processes during meander belt development, but comparison with the avulsion chronology along a river in subhumid South Africa highlights the need for additional investigations into the influence of hydroclimatic setting on the propensity for avulsion.