The quantification of pebble shape has been of interest to geomorphologists for decades. Several authors developed parameters to describe pebble shapes from their images. The extraction of this information from images involves two steps: the segmentation of pebble contours and the application of a computational geometry algorithm to estimate shape parameters. When images are taken in the field, unavoidable shadows might hinder the possibility of using automatic segmentation methods. This paper introduces a new method for automatic segmentation of pebbles that improves segmentation accuracy in the presence of shadows. The method is based on the Canny edge detection algorithm which uses a double thresholding process to provide a classification of the strength of the detected edges. The proposed method applies this algorithm with an ensemble of thresholding values, estimating, for each pixel, the probability of being an edge. The resulting pebble contours were analysed using two computational geometry algorithms to obtain shape parameters. The algorithm was calibrated on a sample of five pebbles and then validated on a sample of 1696 pebbles. Its accuracy has been estimated by comparing the resulting shape parameters with those obtained using reference software, which was used as ground truth (GT). The proposed segmentation method was capable of accurately segmenting around 91% of the sample with a relative error for roundness of −1.7% and −0.4%; for elongation of −0.2% and −0.3% and for circularity of 0.2% and 0.1%, when shape parameters were computed using the algorithms of Zheng or Roussillon, respectively. The method could therefore be used to segment images of pebbles collected in the field with low contrast and shadowing, providing comparable accuracy with ‘manual’ segmentation, while removing operator bias.

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Sediment transport in rivers is a complex process whose understanding is still partial. Each sediment particle can be characterized by its size, shape or distance it travelled from its entrance into the river network to its actual position on the river bed. A number of field techniques have been developed to estimate sediment source locations. This research investigates the possibility of using sediment morphometry as a proxy for travel distance. Recent studies on sediment attrition claim the existence of a “universal” relation between particles’ mass loss and specific shape indices. In order to test this hypothesis, we identified a small basin with localized sources of arenites and metabasalts, which we treated as tracers. We identified lithology as a major control on the absolute attrition speed, while downstream trends in shape changes are controlled by sediment production at basin scale.@en

The Lower Mara River and Wetland, Tanzania, is an important ecosystem and unique water resource for a vast semi-arid area. The river, an affluent of Lake Victoria, and the wetland are experiencing morphological and vegetation changes resulting in channel avulsions and wetland expansion. This study analyses the changes over the last 100 years and investigates natural and anthropogenic behaviors to explain the increase of the Mara Wetland area. We collated historical topographic maps and satellite images. We conducted two field surveys in low and high flow condition with an unmanned aerial vehicle, a sonar and an ADCP. We mapped selected areas as well as the bed topography in some stretches of the river, measured discharges, and collected river bed and suspended sediment samples. The analysis of the sediments shows that the wetland system, dominated by papyrus sp., is very efficient in trapping sediment, releasing clear water to the Lake Victoria. The historical reconstruction using topographic maps, satellite images and a multivariable analysis including hydrology and land cover, shows that 4 major avulsions occurred in the last 70 years due to a combination of natural behaviors, hydrological fluctuations and anthropogenic factors such as basin deforestation, farming and grazing along the river banks and in the wetland. Each avulsion led to substantial expansion of the wetland. Combined, they increased the wetland area by a factor of 3.6. Describing the Lower Mara River dynamic behavior, this work provides relevant information for sustainable future water and sediment management in order to preserve wetland habitats and natural resources.

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The transboundary Mara River, Kenya and Tanzania, is the only perennial source of inflowing surface water of a vast area including the Mara-Serengeti region. The river sustains millions of wild animals as well as a human population of nearly one million (McClain et al., 2014; Gereta et al., 2009). The basin receives two rainy seasons: a lighter in October- December and a heavier in March-May. The river is rich in suspended sediments, which has further increased by recent deforestation, change of land uses and mining (Defersha & Melesse, 2012). Before flowing into the Lake Victoria in Tanzania, the river forms a wide wetland that acts as a natural filter, sinking the large amount of suspended load and releasing clean water to the lake (Fig. 1). The wetland is fed by sediments and nutrients transported by the river and represents an essential but, at the same time, fragile ecosystem. The vegetation, here dominated by papyrus, plays an important role in the stability of the wetland system. However forest fire and farming are intense along the wetland and have deeply modified the vegetation spatial distribution.@en

The Mara River is the only perennial river of a vast semi-arid area, including the Mara Serengeti ecoregion in Kenya and Tanzania. The river sustains more than one million inhabitants and millions of wild animals. In its lower reaches, the Mara River forms a wide wetland before flowing into Lake Victoria. The wetland represents a rich ecosystem providing essential services, but it is being threaten by increasing human activities. Farming, grazing, fishing and deforestation to produce charcoal and open new crops have deeply modified the riparian vegetation spatial distribution and the habitat morphology. Additionally, the construction of a new dam is planned immediately upstream of the wetland for irrigation purposes and hydropower.
This work is undertaken to set up a hydro-morphodynamic model to predict the short- and long-term effects of human activities on the Mara Wetland habitat. The model will be a tool to evaluate strategies to mitigate the negative effects of the activities.

The Lower Mara River is poorly gauged and only a few scattered data and observations are available. Therefore, in October - November 2017 (dry period) and May 2018 (wet period) multidisciplinary field work was conducted along a 130 km stretch of the river. An unmanned aerial vehicle (UAV) was used to produce high resolution orthophoto mosaics and digital elevation models of selected areas. The UAV gave topography and ground observations on vegetation type, size and distribution, and other features of unattainable areas. A sonar was used to map the bathymetry of some stretches of river and wetland. River discharge was measured on 4 locations. Bed sediments and water samples were collected from 8 spots to analyse sediment granulometry and suspended sediment concentration. Results suggest that, at wetland inlet, the river is particularly rich in suspended sediment, with measured averaged concentrations of the order of 500 mg L-1 and peaks of 2700 mg L-1. The wetland, thanks to its extent and dense vegetation cover, traps the 90% of the suspended sediments and releases clear waters to the Lake Victoria. The future placement of the dam may have a strong influence: without an adequate management, the dam solid and liquid discharge regulation may further trigger morphological changes and jeopardize the wetland ecosystem.@en

Sediment accumulation hampers optimal water resources management of reservoirs. In the Roseires Reservoir across the Blue Nile River, in Sudan, about 30% of the storage capacity has been lost by sedimentation before dam heightening (2012), despite regular sediment sluicing and flushing. At the same time, increasing soil erosion in the upper river basin in Ethiopia is significantly reducing land productivity.

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