Impact of land use on sediment and nutrient yields to Lake Malawi/Nyasa (Africa)

Thirteen rivers flowing into Lake Malawi/Nyasa from the country of Malawi in southern Africa were studied over 1 year to estimate annual yields of sediments and nutrients from the terrestrial catchments to the take. The river basins ranged in size from 113 to 12,110 km(2), but maximum elevations above lake level were similar in all the catchments yielding quite different mean basin slopes. Flow was seasonally variable with low or no flow at the end of the prolonged dry season. Annual runoff varied from 37 to 617 mm with most rivers having below average discharges in 1997. The river basins were all affected to varying degrees by subsistence farming which is the dominant land use in Malawi. Sediment and nutrient (C, N, P, Si) concentrations, loads, and yields were sensitive to the degree of agricultural land use in the basins. Lowest sediment concentrations, loads, and yields were characteristic of the small, steep catchments with a high proportion of intact forest. Highest concentrations, loads, and yields of sediments were from densely settled catchments with extensive agricultural activity. Concentrations, loads, and yields of nutrients tended to follow the particulate fluxes as dissolved components were less responsive to basin disturbance. Increasing populations and associated agricultural development may already have increased nutrient loading to Lake Malawi by 50% as forests have been increasingly cleared for agriculture.