The interplay between extrinsic and intrinsic controls in determining floodplain wetland characteristics in the South African drylands

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.

Identifying threshold responses of Australian dryland rivers to future hydroclimatic change

“…As such, rivers in these regions may also respond to these hydroclimatic changes by transforming from through-going to more discontinuous river types. In southern Africa, for instance, river morphology varies across strong hydroclimatic gradients, with through-going, maintaining rivers more common in subhumid catchments, and through-going, declining and discontinuous, declining or terminating rivers more common in semiarid and arid catchments 21,22 . Nevertheless, along many of these rivers, varying degrees of bedrock (lithological, structural) control influence the river's 'degrees of freedom'; for example, resistant rocks such as dolerite or quartzite commonly crop out in river bed and banks and influence the depth of incision or extent of lateral migration that can occur in response to hydroclimatic drivers [e.g.…”

“…Alternatively, indirect approaches to discharge estimation in dryland rivers 52 could be used to generate supporting hydrological or hydraulic information to help establish modern climate-hydrology-geomorphology relations across hydroclimatic gradients, as has been done successfully for different dryland rivers worldwide (e.g. 10,22,53 ). By applying ergodic reasoning, these relations could then be used as the basis for predicting river response under altered hydroclimates.…”

“…In this study, these river types have not been considered, as Australian dryland rivers are not braided and are dominantly alluvial in their middle and lower reaches. Previous research has suggested that regional continua of dryland river types may be related to hydroclimatic gradients, with greater aridity leading to a greater propensity for channel breakdown 21,22 . To refine these concepts and identify thresholds of change relevant for Australian dryland rivers, we characterise the hydrology and geomorphology of 29 rivers draining >1,800,000 km 2 of continental eastern and central Australia, and establish the hydroclimatic conditions under which different river types persist.…”

Cascades of sub‐decadal, channel‐floodplain changes in low‐gradient, non‐vegetated reaches near a dryland river terminus: Salar de Uyuni, Bolivia

“…Measurements of these parameters were made for each kilometre reach starting from the Río Colorado bridge (Figure (C), see red dot in lower right). In this ungauged river, the width measurements provided a robust basis for calculating bankfull discharge using Bjerklie's () model, as has been done successfully for lengthy reaches of other dryland rivers (Larkin et al, ).…”

Product vs. process? The role of geomorphology in wetland characterization

“…A C C E P T E D M A N U S C R I P T 4 2015a). Some research has highlighted the role of process geomorphology in understanding wetland distribution and change; for instance, in mangrove swamps (Balke and Friess, 2016), temperate upland swamps (Cowley et al, 2016), ciénegas (Heffernan, 2008), and lowland floodplain wetlands in Australia (Ralph et al, 2011) and southern Africa Larkin et al, 2017a). Moreover, geomorphological insights have contributed to floodplain and wetland management (McCarthy et al, 2010;Ralph et al, 2016).…”