Determining sources of sediments at Nkula Dam in the Middle Shire River, Malawi, using mineral magnetic approach

Shire River is the largest and longest in Malawi supporting many livelihoods. Degradation in the river's catchment is now a major problem causing excessive siltation in the Nkula Dam which is a terminal sink for sediments eroded and transported by the river and its tributaries. In this study, source of sediments that are deposited into the Nkula Dam were determined by analysing sediment samples from western and eastern tributaries of the Shire River using mineral magnetic approach. Representative samples were collected from tributaries on both sides of the Shire River and Nkula Dam, and subjected to magnetic measurements on bulk samples and sized fractions (<250 μm and >250 μm). Results show significantly higher ferrimagnetic mineral contents and ferrimagnetic to anti-ferromagnetic ratios in the eastern tributaries than the western side of the Shire River. Lithology and weathering conditions are suggested to be the main cause for magnetic contrast between the two sides of the river. It is concluded that most sediments in the Nkula Dam originate from the western side of the Shire River, presumably due to excessive erosion. This study demonstrates that magnetic method is a promising approach in assessing fluvial sediment source.