Determining sources of sediments at Nkula dam in the middle Shire river, Malawi, using mineral magnetic approach

Abstract

Shire River is the largest and longest in Malawi supporting many livelihoods. Degradation in the river'scatchment is now a major problem causing excessive siltation in the Nkula Dam which is a terminal sinkfor sediments eroded and transported by the river and its tributaries. In this study, source of sedimentsthat are deposited into the Nkula Dam were determined by analysing sediment samples from westernand eastern tributaries of the Shire River using mineral magnetic approach. Representative samples werecollected from tributaries on both sides of the Shire River and Nkula Dam, and subjected to magneticmeasurements on bulk samples and sized fractions (<250mm and>250mm). Results show significantlyhigher ferrimagnetic mineral contents and ferrimagnetic to anti-ferromagnetic ratios in the easterntributaries than the western side of the Shire River. Lithology and weathering conditions are suggested tobe the main cause for magnetic contrast between the two sides of the river. It is concluded that mostsediments in the Nkula Dam originate from the western side of the Shire River, presumably due toexcessive erosion. This study demonstrates that magnetic method is a promising approach in assessingfluvial sediment source.