Abstract
The Mara River Basin in East Africa is a trans-boundary basin of international significance experiencing excessive levels of sediment loads. Sediment levels in this river are extremely high (turbidities as high as 6,000 NTU) and appear to be increasing over time. Large wildlife populations, unregulated livestock grazing, and agricultural land conversion are all potential factors increasing sediment loads in the semi-arid portion of the basin. The basin is well-known for its annual wildebeest (Connochaetes taurinus) migration of approximately 1.3 million individuals, but it also has a growing population of hippopotami (Hippopotamus amphibius), which reside within the river and may contribute to the flux of suspended sediments. We used in situ pressure transducers and turbidity sensors to quantify the sediment flux at two sites for the Mara River and investigate the origin of riverine suspended sediment. We found that the combined Middle Mara—Talek catchment, a relatively flat but semi-arid region with large populations of wildlife and domestic cattle, is responsible for 2/3 of the sediment flux. The sediment yield from the combined Middle Mara–Talek catchment is approximately the same as the headwaters, despite receiving less rainfall. There was high monthly variability in suspended sediment fluxes. Although hippopotamus pools are not a major source of suspended sediments under baseflow, they do contribute to short-term variability in suspended sediments. This research identified sources of suspended sediments in the Mara River and important regions of the catchment to target for conservation, and suggests hippopotami may influence riverine sediment dynamics.
Highlights
Excessive sediment loads in rivers are a concern throughout the world as suspended sediments can be a major transport medium for chemicals, contaminants and nutrients [1,2,3]
Average discharge over this period of measurement was not statistically different between the two sites, variability was much higher at New Mara Bridge site (NMB)
Maximum discharge at NMB regularly exceeded 30 m3 s-1 and the coefficient of variation was highest at NMB in 17 of the 21 months
Summary
Excessive sediment loads in rivers are a concern throughout the world as suspended sediments can be a major transport medium for chemicals, contaminants and nutrients [1,2,3]. Human and aquatic systems are directly at risk, as downstream impacts of suspended sediment transport on water quality and biota can be significant [4,5,6,7]. Suspended sediment dynamics Mara River environment.yale.edu/tri/) and the Lindsay Fellowship for Research in Africa to CLD; and a fellowship from the Robert and Patricia Switzer Foundation (http://www.switzernetwork.org/) to ALS. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript
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