GEOMORPHOLOGY OF THE TROPICAL VALLEY BOTTOM LANDFORM SYSTEM IN THE WEST AFRICAN SAVANNA LANDSCAPE

Abstract

Tropical headwater valley bottoms are shallow, flat-bottomed, valleys at the uppermost part of the watershed in which no stream channel is present. Predominant in Africa's low-relief landscapes and other tropical regions, they are important settings for agriculture, grazing, and, less frequently, small-scale mineral deposits. However, debates over their origins and nomenclature hinder a comprehensive understanding of the role these valley bottoms play in landform evolution of the African surface, mineral deposit formation, and a range of ecological functions. Two primary genetic origin theories are identified: the in-situ weathering/deflation hypothesis and the colluvial/fluvial hypothesis. This dissertation investigates the two genetic origin theories of these features at both the continental and watershed scales integrating geomorphological, digital terrain modeling, and sedimentological methods.To address semantic and ontological classification disparities, a continental-scale analysis of watershed basins containing variously named headwater valley bottom landforms is conducted. The study develops and analyzes a database of 22 type localities and explores geology, climate, paleoclimate, and regolith controls to clarify definitions and classifications. Results indicate distinct differences in watershed basins categorized by climate, precipitation, morphology, bedrock age and lithology, and pedological inheritance but not necessarily correlating to regionally different nomenclature. The research further suggests that major paleoclimate transitions influenced the long-term evolution of these landforms across Africa. Transitioning from the continental-scale, the research advocates for watershed-scale studies using quantitative criteria and high spatial resolution geospatial data. Targeted on valley bottoms in the Kohoué River watershed in northwest Côte d'Ivoire, the study addresses the lack of suitable digital elevation model (DEM) data by developing a high-resolution, vertically accurate DEM through photogrammetric workflows and DEM filtering techniques. Next, this high-resolution DEM is used in combination with field mapping and sedimentological analysis to produce a series of geomorphic cross sections, which support the hypothesis that valley bottom forms transition from unchanneled collapse/deflation features in headwater settings into valley flats with discontinuous channels and become channeled colluvial/fluvial floodplain types downstream. Rather than opposition between the two primary origin theories, this work demonstrates the coexistence of both processes at work in a watershed, varying longitudinally, based on quantifiable geomorphic controls.