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Eberhard Karls Universität Tübingen (2018)

Assessment of Landscape Processes, Forms and Features in the Lake Manyara Region, East African Rift Valley

Quénéhervé, Geraldine Suzanne

Titre : Assessment of Landscape Processes, Forms and Features in the Lake Manyara Region, East African Rift Valley

Auteur : Quénéhervé, Geraldine Suzanne

Université de soutenance : Eberhard Karls Universität Tübingen

Grade : Doktorgrade 2018

Résumé
The rift valleys of the East African Rift Systems form two branches (Eastern and Western rift). Within the Eastern rift, there is a southward propagation in the onset of volcanism, and hence rifting that has led to the formation of a narrow graben commonly referred to as the Gregory rift (in Tanzania). The endorheic basin at its southern end where the present saline Lake Manyara was formed. The lakes largest tributary is the Makuyuni River that is providing fresh water from northwest volcanic highlands. Along the Makuyuni River, where first and second tributaries cut deep into the sediments, the landscape is dominated by erosion features. These sediments reveal exposed and outcropping artefacts and fossils, which have been found starting in the 1930s in archaeological and paleontological expeditions. In the close vicinity of Makuyuni town, two hominin bearing sites were discovered in 2008. As few is known about the landscape itself, this study has set its focus on the surrounding landscapes of Makuyuni and its geomorphic implications. Key drivers of landscape development in Northern Tanzania are the tectonics and tectonically induced processes. The thus disrupt drainage networks and analyses thereof can reveal the effects on the morphology of stream longitudinal profiles. These transformations in turn, have an impact on driving river incision, in general on linear erosion phenomena. The triggered rill and gully erosion features will eventually lead to soil loss. Gully erosion is a major threat concerning landscape degradation in semiarid ecosystems, as they remove fertile topsoil and as such prevent agricultural use. The study showed, that most of the gully systems are there for decades and in stable conditions. There are, however, areas of exposed risk for erosion. A detailed examination of soil loss areas, innovative automatic surface runoff detector (SRD) devices have been installed. They measure runoff height and duration. This data contribute to a better understanding of the water cycle as well as to soil functions in semiarid environments. The specific research questions have been considered in five different publications that describe the applied methodologies and results in detail. In order to understand the different geomorphic processes, different methods and scales have been used. Field surveying was an important factor and surface runoff, soil characteristics and erosion phenomena have been sampled and measured. Specifically, this study examined rill and gully erosion features and underwent a detailed mapping around Makuyuni in order to describe and understand the occurring landscape systems. Laboratory procedures involved soil parameter analysing and digital data analysis involved digital elevation model analysis, remote sensing imagery, sophisticated statistical models and in the end a detailed geomorphic map compiling of the greater area. Many studies have focused on partial aspects of geomorphic processes. The frame-work of this doctoral thesis seeks to answer a holistic approach, in order to understand, interpret and discuss related geomorphic processes, their spatial extents and locations. This work contributes to the knowledge of present geomorphic processes and features and the landscape evolution within the Lake Manyara area in Northern Tanzania. The above mentioned methods and analyses may be applicable to similar areas in the African rift.

Mots clés  : Geomorphologie , Ostafrika , Erosion , Bodenerosion , Gully , Boden , Kartierung , Tektonik , Ostafrikanischer Graben , Tansania

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Page publiée le 12 janvier 2019