Historical archives of grey-scale river channel imagery are extensive. Here, we present and test a methodology to extract detailed quantitative topographic date from such imagery of sand-bed rivers. Extracting elevation information from rivers is difficult as they are characterized by a low relative relief (less than 4 m); the area of interest may be spatially extensive (e.g. active channel widths > 500 m in large braided rivers); the rate of change of surface elevation is generally low except in the vicinity of individual channel banks where the rate of change is very high: there is the complication that comes from innundation: and there may be an added complication caused by blockage of the field of view by vegetation. Here, we couple archival photogrammetric techniques with image processing methods and test these for quantification of sand-bed braided river dynamics, illustrated for a 500 m wide, 3 km long reach of the Spouth Sasketchewan River, Canada. Digitial photogrammetry was used to quantify dry areas and water edge elevations. A methodology was then used to calibrate the special signature of inundated areas by combining established two media digital photogrammetric methods and image matching. This allowed determination of detailed depth maps for inundated area and, when combined with dry area data, creation of depths detectable from sequential digital elevation models. The result was a series of elevation models that demonstrate the potential for acquiring detailed and precise elevation data from any historical aerial imagery of rivers without needing associated calibration data, provided that imagery is of the necessary scale to capture the features of interest. We use these data to highlight several aspects of channel change on the South Saskatchewan River, including bar movement, bank erosion and channel infilling

Given the persistence of microplastics in the environment and their potential toxicity to ecosystems, understanding of likely microplastic accumulation ‘hotspots’ in rivers is urgently needed. To contribute to this challenge, this paper reports results of a microplastic survey from a heavily urbanised catchment, the River Tame and four of its tributaries, which flows through the city of Birmingham, UK. All sediment sampled was found to contain microplastics with an average abundance of 165 particles kg−1. While urban areas generally have a greater abundance of microplastics as compared with rural, there is no simple relationship between microplastic numbers and population density or proximity to wastewater treatment sites. The greatest change in microplastic abundance was due to the presence of a lake along the course of the River Tame—i.e., flow velocities are reduced on entering the lake, which promotes the deposition of fine sediment and potentially microplastics. This suggests that the greatest concentrations of microplastics will not be found in-channel but rather on the floodplain and other low velocity environments such as meander cutoffs. We also identified a new mechanism of microplastic fixation in freshwater environments through ecological engineers, specifically caddisflies, that incorporated microplastics into their casing. These results highlight the need to explore further hydrodynamic and ecological impacts on microplastics fate and transport in rivers.

To date, published studies of alluvial bar architecture in large rivers have been mostly restricted to individual bar case studies and single locations. Relatively little is known on how the composition of km-scale bars varies within a multi-km reach or over several 100s km downstream. This study presents ground-penetrating radar(GPR) and core data from 11, km-scale bars from the Río Paraná, Argentina. The investigated bars are located between 30 km upstream and 540 km downstream ofthe Paraná-Paraguay confluence, where a significant volume of fine-grained suspended sediment is introduced into the system. Bar-scale cross-stratified sets with lengths and widths up to 600 m and thicknesses up to 12 m, which are diagnostic of large river deposits, are only present in half the surface area of the bars. The majority of these bar-scale sets (~90%) are found on top of laterally-extensive, fine-grained layers of ripple sets that have been deposited in the troughs downstream of bars. Bar-scale sets make up as much as 58% of the volume of the deposits in small, incipient mid-channel bars, but this proportion decreases significantly with increasing age and size of the bars. Contrary to what might be expected, a significant proportion of individual sedimentary structures found in the Río Paraná is similar in scale to the structures found in much smaller rivers. In other words, large river deposits are not always characterised by big structures that allow a simple interpretation of river scale. However, the large scale of the depositional units in big rivers causes small-scale structures such as ripple sets to be grouped in thicker co-sets, which indicate river scale even when no obvious large scale sets are present.The results also show that the composition of bars differs between the studied reaches upstream and downstream from the confluence with the Rio Paraguay. Relative to other controls on downstream fining, the tributary input of fine-grained suspended material from the Paraguay causes a marked change in the composition of the bar deposits. Compared to the upstream reaches, the sedimentary architecture of the downstream reaches in the top ~5 m of mid-channel bars shows (i) an increase in the abundance and thickness (up to m-scale) of laterally extensive (100s of metres) fine-grained layers; (ii) an increase in the percentage of deposits comprised of ripple sets (to >40% in the upper bar deposits); and (iii) an increase in bar-trough deposits and a corresponding decrease in bar-scale cross strata (<10%).In contrast to the bar-top deposits, even just downstream from the Paraguay input, the thalweg of the Río Paraná is covered with m-scale dunes and its deposits are composed of dune sets even in areas where bar-top deposits are dominantly fine-grained. Thus, changes in bar composition due to a tributary point-source of fine grained sediment are expressed primarily in the composition of the bar-top deposit

Ground penetrating radar (GPR) surveys of unit and compound braid bars in the sandy South Saskatchewan River, Canada, are used to test the influential facies model for sandy braided alluvium presented by Cant & Walker (1978). Four main radar facies are identified: (1) high-angle (up to angle-of-repose) inclined reflections, interpreted as having formed at the margins of migrating bars; (2) discontinuous undular and/or trough-shaped reflections, interpreted as cross-strata associated with the migration of sinuous-crested dunes; (3) low-angle (< 6°) reflections, interpreted as formed by low-amplitude dunes or unit bars as they migrate onto bar surfaces; and (4) reflections of variable dip bounded by a concave reflection, interpreted as being formed by the filling of channel scours, cross-bar channels or depressions on the bar surface. The predominant vertical arrangement of facies is discontinuous trough-shaped reflections at the channel base overlain by discontinuous undular reflections, overlain by low-angle reflections that dominate the deposits near the bar surface. High-angle inclined reflections are only found near the surface of unit bars, and are of relatively small-scale (< 0·5 m), but can be found at a greater range of depths within compound bars. The GPR data show that a high spatial variability exists in the distribution of facies between different compound bars, with facies variability within a single bar being as pronounced as that between bars. Compound bars evolve as an amalgamation of unit bars and other compound bars, and comprise a facies distribution that is representative of the main bar types in the South Saskatchewan River. The GPR data are compared with the original model of Cant and Walker (1978) and reveal a much greater variability in the scale, proportion and distribution of facies than that presented by Cant and Walker (1978). Most notably, high-angle inclined strata are over-represented in the model of Cant and Walker, with many bars being dominated by the deposits of low- and high-amplitude dunes. It is suggested that further GPR studies from a range of braided river types are required to properly quantify the full range of deposits. Only by moving away from traditional, highly generalized facies models can a greater understanding of braided river deposits and their controls be established.\u

The depositional stratigraphy of within-channel deposits in sandy braided rivers is dominated by a variety of barforms (both singular ‘unit’ bars and complex ‘compound’ bars), as well as the infill of individual channels (herein termed ‘channel fills’). The deposits of bars and channel fills define the key components of facies models for braided rivers and their within-channel heterogeneity, knowledge of which is important for reservoir characterisation. However, few studies have sought to address the question of whether the deposits of bars and channel fills can be readily differentiated from each other. This paper presents the first quantitative study to achieve this aim, using aerial images of an evolving modern sandy braided river, and geophysical imaging of its subsurface deposits. Aerial photographs taken between 2000 and 2004 document the abandonment and fill of a 1.3 km-long, 80 m-wide anabranch channel in the sandy braided South Saskatchewan River, Canada. Upstream river regulation traps the majority of very fine sediment and there is little clay (less than 1%) in the bed sediments. Channel abandonment was initiated by a series of unit bars that stalled and progressively blocked the anabranch entrance, together with dune deposition and stacking at the anabranch entrance and exit. Complete channel abandonment and subsequent fill of up to 3 m of sediment took approximately two years. Thirteen kilometres of Ground Penetrating Radar (GPR) surveys, coupled with 18 cores, were obtained over the channel fill and an adjacent 750 m-long, 400 m-wide, compound bar, enabling a quantitative analysis of the channel and bar deposits. Results show that in terms of grain size trends, facies proportions and scale of deposits, there are only subtle differences between the channel fill and bar deposits, which therefore renders them indistinguishable. Thus, it may be inappropriate to assign different geometric and sedimentological attributes to channel fill and bar facies in object-based models of sandy braided river alluvial architecture