Thermochronology, landscape evolution and hydrogeology of the Katonga Valley in south west Uganda

The reversed river systems of south west Uganda attracted geoscientists to study the geomorphology of the region in the mid 20th century. During the succeeding fifty years the population and GDP per capita have both risen between five and six fold with a consequent increase in water demand. This thesis aims to apply modern quantitative techniques and theoretical developments to improve our understanding of the landscape evolution of the Katonga Valley and contribute to the groundwater resource assessment of associated alluvial deposits. Karoo-age glaciogenic strata were discovered filling the western valley. Subsequent apatite fission track analyses reveal that the currently exposed rocks were reheated to a temperature consistent with over 2 km of burial during the Mesozoic. Therefore, it is inferred that the western River Katonga has preferentially eroded a Gondwanan paleovalley exhumed from beneath the former sedimentary cover. Electrical resistivity tomography of the valley fill has identified three broad cycles of erosion and deposition, including: 1) The Gondwanan palaeovalley with indurated glaciogenic strata; 2) The Neogene relict valley with fluviolacustrine sediments; and, 3) The late Quaternary channel and with recent wetland deposits. Downwarping of the Victoria Basin in the east produced the first drainage divide on the originally westward flowing River Katonga during the middle Pleistocene. Downthrow of the George Basin in the west led to rejuvenation of the western landscape prior to the formation of a second drainage divide due to rift flank uplift. Sand and gravel associated with an old denuded landscape survives in terraces above the water table in the central valley. Variable climate and fluctuating lake levels led to the deposition of fluviolacustrine deposits in the eastern valley. Pumping tests in this silty sand indicate that the transmissivity is almost always adequate for village water use and is sometimes commensurate with town water supplies.