Characterising the late Quaternary facies stratigraphy of floodplains in South Africa

South African river floodplains and their alluvial deposits reflect a diversity of geological and geographical drivers. We use a genetic geomorphic classification system originally developed for dryland wetlands to characterise geomorphic processes and potential successions of sedimentary fill for South African floodplains. Using case studies from the literature, we consider differences between alluvial rivers and mixed bedrock-alluvial rivers in the context of macro-scale geomorphic setting, and evaluate the impact of the setting on floodplain persistence and potential as a palaeo-environmental archive. Sedimentary facies associations represented in South African floodplains, including lateral and oblique accretion, channel, channel infill, levee vertical accretion, floodplain vertical accretion and debris flow deposits, are also evaluated. Floodplains of South Africa’s interior are typically mixed bedrock-alluvial as channel beds are set upon or close to bedrock and sediment thickness is limited. By contrast some floodplains in tectonic basin settings have sediment deposits exceeding 30 m in thickness. The resulting rivers are alluvial, and thus able to adjust their width, depth and slope to accommodate changes in discharge and sediment supply. Similarly, coastal floodplain rivers are alluvial due to downcutting during the last glacial maximum and subsequent sedimentary infilling as sea levels rose. When considering the potential of floodplains as palaeoarchives of environmental change, two considerations emerge. First, floodplain stratigraphy is not a response to a single variable due to complex process-form feedbacks. Rather, floodplain stratigraphy is an outcome of both autogenic and allogenic processes. Second, most South African floodplains are zones of sediment recycling, and as such, preservation potential is typically low. Thus, although floodplain settings of the interior may be a few million years old, the sediment within them may be only thousands to tens of thousands of years old. Our review indicates that research has historically focused on meandering river and mixed bedrock-alluvial anabranching river floodplains, while understanding of other floodplain sub-types remains limited.

Linking rivers to the rock record: Channel patterns and paleocurrent circular variance

Alluvial rivers are the most important agents of sediment transport in continental basins, whose fluvial deposits enclose information related to the time when rivers were active. In order to extract the most information from fluvial deposits in the sedimentary record, it is imperative to quantify the natural variability of channel patterns at the global scale, explore what controls may influence their development, and investigate whether channel pattern information is preserved in the alluvial plains in order to develop tools for recognizing them in the sedimentary record. By surveying 361 reaches of modern alluvial rivers with available water discharge data at a global scale, we present a quantitative channel pattern classification based on sinuosity and channel count index applicable to the recognition in the rock record. A continuum of channel patterns ranging from high-sinuosity single channel to lowsinuosity multichannels is documented, along with the proportion of depositional elements in their alluvial plains and their conditions of occurrence. Preserved barforms in the alluvial plains of these rivers are used to infer and quantify paleoflow directions at the channel-belt scale and result in ranges of paleocurrent circular variance that may lead to channel pattern identification in the rock record. Data from this work indicate that the recognition of channel patterns may be used to predict paleogeographic features such as the scale of drainage basin area and discharge, slope, and annual discharge regimes.

Critical Review and Improvement of Bank Protection Methods in Nepalese Rivers

This paper reviews performance of bank protection works constructed over the last decades along the major rivers in Nepal. The study reveals main reasons of early spurs failure in five major rivers - Koshi, Narayani, West Rapti, Karnali and Mahakali. The overview finds design deficiency as the main reason of failure of bank protection works. The methodology incorporates a combination of field visits and a review of design reports, master plans. The study finds bank protection works along the major rivers in Nepal consist almost exclusively of spurs often combined with revetments in between, made of gabions filled with boulders. Also, the launching aprons are constructed of gabions. The lifetime of these bank protection works is less than 10 years and often much shorter. Designs are based on standard designs from Indian standards, developed for normal alluvial rivers. The study recommends initiating a bank protection pilot project to identify the cause of failure of the bank protection works and to test remedial measures for the sustainability enhancement of river protection works in future.

Laboratory observations on meltwater meandering rivulets on ice

We present a set of observations on meltwater meandering rivulets on ice and compare them (qualitatively and quantitatively) to morphologies commonly found in meandering channels in different media. The observations include data from planned centimeter-scale experiments and from incidental self-formed millimeter-scale rivulets. Our data show pulsed lateral migration features, undercut banks and overhangs, meander bend skewness, and meander bend cutoffs. The data also compare well with planform characteristics of alluvial meandering rivers (sinuosity, wavelength-to-width ratios, and meander bend fatness and skewness). We discuss the (ir)relevance of scale in our experiments, which, in spite of being in the laminar flow regime and likely affected by surface tension effects, are capable of shedding light into the processes driving formation and evolution of supraglacial meltwater meandering channels. Our observations suggest that sinuosity growth in meltwater meandering channels on ice is a function of flow velocity and the interplay between vertical and lateral incision driven by temperature differences between flow and ice. In the absence of recrystallization (depositional analog to alluvial rivers), bends are more likely to be downstream-skewed and channels show lower sinuosities.

Compiling and Analysing Bedrock River Data Across the USA to Unpick Bedrock River Geomorphology

<p>Active incision of bedrock rivers exerts a vital control on landscape evolution in upland areas. Previous research found that bedrock rivers were typically steeper and sometimes narrower than alluvial rivers. However, most of the literature on partially-exposed bedrock rivers has employed small samples mostly from mountainous regions, so their geomorphological properties remain poorly understood. In contrast with the existing literature, a large-sample analysis of bedrock river channel properties would allow the controls on bedrock river width and slope to be unpicked and reveal whether or not the existing literature is biased towards pristine, mountainous bedrock rivers. Overall, such an analysis could improve the reliability of upland landscape evolution models.</p><p>Here we present an analysis of 1,924 river sites from the EPA National Rivers and Streams Assessment to assess the geomorphological differences between bedrock and alluvial rivers. The influences of lithology and uplift on bedrock channel properties are examined using external datasets. We find bedrock rivers to be significantly steeper and wider than alluvial rivers. Sedimentary bedrock rivers were seen to be significantly wider than igneous/ metamorphic bedrock rivers, consistent with findings from Ferguson et al. (2017). We estimated shear stress and critical shear stress for each river site and assessed correlation with bedrock exposure. We found that exposed bedrock could not always be explained by local sediment transport exceeding local sediment supply, indicating that bedrock exposure may be controlled by other factors in some bedrock rivers. Currently, uplift data are being compiled for further analysis.</p>

Analysis of geometric relationships of bedrock and alluvial channels: a comparison between rivers from the Scottish Highlands and San Gabriel Mountains (USA)

<p>Sediment transport in rivers depends on interactions between sediment supply, topography, and flow characteristics. Erosion in bedrock rivers controls topography and is paramount in landscape evolution models. The riverbed cover indicates sediment transport processes: alluvial cover indicates low transport capacity or high sediment supply, and bedrock cover demonstrates high transport capacity or low sediment supply. This study aims to evaluate controls on the spatial distributions of bedrock and alluvial covers, by analysing scaling geometric relations between bedrock and alluvial channels. A Principal Component Analysis (PCA) was conducted to evaluate correlations between river slope, depth, width, and sediment size. The two principal components were used to implement a clustering analysis in order to identify differences in alluvial and bedrock sections. Spatial distributions of mixed bedrock-alluvial sections were investigated from two datasets - Scottish Highlands (Whitbread 2015) and the San Gabriel Mountains in the USA (Dibiase 2011)-, representing different environmental conditions, such as erosion rates, lithology, tectonics, and climate. The rock strength of both areas is high, and therefore it is excluded as a factor that explains the difference between the areas. The results of the cluster analysis were different in each environment. The main sources of variation among river sections identified by PCA were slope and width for the San Gabriel Mountains, and drainage area and depth for the Scottish Highlands. The rivers in the Scottish Highlands formed clusters that differentiate bedrock and alluvial patches, showing a clear geometric distinction between channels. However, the river analysis from the San Gabriel Mountains showed no clusters. Bedrock rivers are typically described as narrower and steeper than alluvial rivers, as demonstrated by rivers in the Scottish Highlands (e.g. slope was around 0.1 m/m for bedrock sections and 0.01 m/m for alluvial sections). However, this may not be always the case: both bedrock and alluvial sections in San Gabriel Mountains presented similar slope around 0.1 m/m. The inability to demonstrate significant geometry differences in bedrock and alluvial sections in the San Gabriel Mountains may be due to the frequency and magnitude of sediment supply of that region, which are influenced by tectonics and climate. A major difference in the supply of sediment in rivers of the San Gabriel Mountains is the frequent occurrence of debris flow. Non-linear interactions between hydraulic and sediment processes may constantly modify the geometry of bedrock-alluvial channels, increasing the complexity of analysis at larger temporal and spatial scales. This study is part of the i-CONN project, which links connectivity in different scientific disciplines. A sediment connectivity assessment in different environments and scales may be useful to evaluate the controls on the spatial distribution of bedrock and alluvial rivers.</p><p> </p><p>Dibiase, R.A. 2011. Tectonic Geomorphology of the San Gabriel Mountains, CA. PhD Thesis. Arizona State University, Phoenix, 247pp.</p><p>Whitbread, K. 2015. Channel geometry data set for the northwest Scottish Highlands. British Geological Survey Open Report, OR/15/040. 12pp.</p>

Role of Longitudinal Lithological Contrast On the Channel Evolution of Piedmont Rivers

<p>The cut-and-fill terrace sequence, resulted from aggradation and incision cycles of alluvial rivers on the piedmont of active orogens, is a common tectonic geomorphological feature observed across different continents under different climatic regimes. Such aggradation and incision cycles are identified on both the orbital and sub-orbital cycles, which poses a question about their origins. Efforts have been put into disentangling the contributions from tectonics, climate and other autogenic sources. In this study, we investigated it by exploring the cut-and-fill terrace sequences along the Jingou River on the northern piedmont of Chinese Tianshan, where numerous terraces are seen along tens of alluvial rivers flowing through the fold-and-thrust belt. More than ten terrace flights, are observed where Jingou River flows across the active Huoerguosi anticline. We collected sediment samples for OSL dating to decipher the building and abandonment processes of these terraces and made topographic measurements to evaluate the paleo-slope of this section of Jingou River. Detailed field survey of sedimentary structure and luminescence dates unambiguously unveil the aggradation and incision cycles on sub-orbital cycles since the last interglacial. Down-cutting of no less than 80 meters is identified during the last ten thousands of years. We tentatively evaluated the possible roles of regional climatic variation, anticline deformation and the autogenic processes. Of all these factors, we detailedly investigated the role of longitudinal contrast of lithologies along the river due to the deformation of the Huoerguosi anticline, which always promotes the channel incision.</p>