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Since European colonisation, many coastal rivers of New South Wales (NSW) have been highly modified by channelisation, flow regulation, sediment mining, intensive grazing and agriculture, deforestation, and riparian vegetation and wood removal. Since the late 1980s, nearly 55% of these rivers have undergone a significant 're‐greening' and geomorphic recovery with changes to instream and riparian roughness. However, little research has been undertaken to quantify if there have been any coeval changes in flow hydrology. Approximately 7000 flow hydrographs with one‐hour time‐steps from 117 gauges on 45 study rivers (17 of 20 coastal catchments of NSW) were used to assess changes to in‐channel and overbank hydrology over decadal timeframes. Hydrograph shape and attenuation characteristics for three morphologically defined flow stages were analysed: in‐channel fresh, high flow and overbank flood. Time‐series analysis was used to quantify changes in flow hydrology from the early‐20th century to present and identify trends occurring between known degradation phases (1910s–1940s and 1940s–1980s) and a recovery phase (1990s to present) for these rivers. Our findings indicate that, on average, and for some key examples, changes in flow hydrology are occurring. Flows are slowing and attenuating as indicated by decreases in flood wave celerity and hydrograph kurtosis, skewness and rate of rise and increases in peak‐to‐peak travel time, flood peak attenuation and flood wave attenuation index. The most significant changes have occurred since the 1980s with the most noticeable effects on the behaviour of high flows (around the bankfull stage). However, there is not a consistent direction change in these indicators in all places. We use these findings to categorise the flow mitigation signal of the study regions and catchments and discuss the implication for river and flood management. [ABSTRACT FROM AUTHOR]

Palaeochannels of lowland rivers provide a means of investigating the sensitivity of river response to climate-driven hydrologic change. About 80 palaeochannels of the lower Macquarie River of southeastern Australia record the evolution of this distributive fluvial system. Six Macquarie palaeochannels were dated by single-grain optically stimulated luminescence. The largest of the palaeochannels (Quombothoo, median age 54 ka) was on average 284 m wide, 12 times wider than the modern river (24 m) and with 21 times greater meander wavelength. Palaeo-discharge then declined, resulting in a younger, narrower, group of palaeochannels, Bibbijibbery (125 m wide, 34 ka), Billybingbone (92 m, 20 ka), Milmiland (112 m, 22 ka), and Mundadoo (86 m, 5.6 ka). Yet these channels were still much larger than the modern river and were continuous downstream to the confluence with the Barwon-Darling River. At 5.5 ka, a further decrease in river discharge led to the formation of the narrow modern river, the ecologically important Macquarie Marshes, and Marra Creek palaeochannel (31 m, 2.1 ka) and diminished sediment delivery to the Barwon-Darling River as palaeo-discharge fell further. The hydrologic changes suggest precipitation was a driving forcing on catchment discharge in addition to a temperature-driven runoff response. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

• Hydrogeomorphic processes drive wetland-catchment connectivity dynamics. • Climatic and anthropogenic features influence landscape connectivity patterns. • The upper catchment region is well-connected and contributes to high runoff and erosion. • Identifying hotspots of catchment connectivity change can inform management. Wetlands in semiarid regions are important systems that provide numerous ecosystem services in otherwise dry environments. Their functioning depends on the connected transfer of materials from source to sink which is determined by the degree of landscape connectivity. A combination of catchment characteristics, climatic variables and human activities are important factors driving landscape connectivity; however, wetland management rarely considers all these factors when conserving these systems. In this paper, we evaluate the landscape connectivity of the Mara River catchment in East Africa to understand the hydrogeomorphic processes driving material transfer to the floodplain wetlands. This semiarid catchment hosts the UNESCO World Heritage Serengeti-Mara ecosystem providing a unique opportunity to assess the influence of protected areas on wetland-catchment connectivity patterns. We use the index of connectivity and Emerging Hot Spot analysis to assess the hillslope-channel connectivity and identify specific subcatchments which have contributed positively to the wetland's connectivity over time. The catchment-scale analysis highlights significant spatiotemporal variability in connectivity patterns and associated sediment dynamics. The results reflect that the Upper catchment is highly connected owing to the steep slopes and high rainfall and is likely responsible for the greatest sediment inputs into the fluvial network. Under greater future climatic and anthropogenic pressures, this region is expected to contribute increased sediment inputs into the river which will likely affect the geomorphic response of the downstream floodplain wetlands and may need to be prioritised for management. The protected areas are generally poorly connected due to intact vegetation and their local sediment contribution to the fluvial network is comparatively lower. Consideration of their placement in the catchment is important for managing these ecologically significant areas. Understanding the patterns and variability of hillslope-channel connectivity and the broader-scale landscape connections can help to identify and prioritise existing and emerging hotspots of change in large, semiarid catchments. [ABSTRACT FROM AUTHOR]

The character and behaviour of dryland rivers are driven by complex interactions between extrinsic and intrinsic controls. A long-term perspective is necessary to assess the sensitivity of dryland rivers to extrinsic forcing, such as hydroclimatic changes, and to contextualise the threshold channel responses. The semi-arid Mara River in northern Tanzania is the only perennial river in the UNESCO World Heritage Serengeti-Mara ecosystem and provides water resources for human and wildlife populations. Satellite imagery and discharge modelling were used to assess the valley-scale controls and hydrogeomorphic processes driving the morphological adjustments of the lower Mara River and its floodplain wetland. Both the modern and palaeochannels undergo downstream morphological and hydrological changes as they traverse the broad, alluvial floodplain. In response to marked downstream declines in discharge (from ~300 m3·s−1 to ~45 m3·s−1) and stream power (from ~40 W·m−2 to ~0.5 W·m−2), the modern channel exhibits a nonequilibrium river response resulting in downstream declines in channel size (from ~60 m to ~10 m wide) and eventual channel breakdown in the floodplain wetland. Palaeochannels in the unconfined reaches are typically larger with higher palaeodischarges than the modern channel. While some palaeochannels exhibit similar downstream trends to the modern channel, others may have maintained a continuous channel farther into the wetland. Comparison of the Mara River to other dryland river systems highlights the importance of valley setting and hydroclimate as long-term controls that influence geomorphic river response over time. The lower Mara River represents a nonequilibrium river response to declining discharges related to transmission losses that leads to channel bed aggradation and avulsion, constituting an inherent condition in many dryland alluvial rivers. An improved understanding of the interplay between intrinsic and extrinsic controls is important for the assessment of river character, behaviour, and floodplain wetland development, particularly under future projections of more variable or extreme climate conditions. [Display omitted] • Channel breakdown and floodplain wetland formation in a dryland fluvial system • The modern channel exhibits a nonequilibrium river response to declining discharges • Long-term controls (i.e. hydroclimate) drove the transition from larger palaeochannels to the modern channel • Catchment land-use change has increased the sediment supply to discharge ratio in the lower reaches • Intrinsic threshold responses lead to contemporary channel changes and breakdown in the floodplain wetland [ABSTRACT FROM AUTHOR]

[Display omitted] • Geomorphic river diversity can be explained by a mix of catchment scale controls. • Imposed controls (slope and valley bottom) do not adjust over geomorphic timeframes. • Flux controls (bed material and stream power) are interactions between fluxes. • Assessing controls along long profile can explain the pattern of river diversity. A mix of catchment-scale controls operate to determine geomorphic river diversity in a catchment. These are contextualised here as imposed and flux controls. Imposed controls do not adjust over geomorphic timeframes and produce the environmental setting in which a river functions. Flux controls are dynamic interactions between flow and sediment that heavily influence geomorphic river diversity. We measure and statistically analyse the network scale mix of imposed and flux controls occurring along longitudinal profiles of various shape. We use this to explain the diversity, pattern and sequence of river types at different positions in a catchment. Using the Richmond River catchment, New South Wales, Australia, as our case study we analyse the imposed controls of slope and valley bottom width and the flux controls of bed material size and gross stream power for five river types, ranging from confined continuous rivers in headwaters to laterally unconfined discontinuous rivers in lowland plains. We find that slope and gross stream power are strong, positively correlated controls on all river types, but act most strongly on rivers with continuous channels. In contrast, bed material size is a dominant control on rivers with discontinuous channels. Slope and gross stream power are also critical for determining the downstream pattern of river types along longitudinal profiles. However, the relationship between these controls is complex as not all rivers are influenced by these controls in the same way. Understanding the mix and patterns of controls operating along longitudinal profiles can be used to explain the variability and pattern of river types we see in the landscape. [ABSTRACT FROM AUTHOR]