By Dr Hudson NKOTAGU University of Dar es Salaam Tanzania

1. Sediment Discharge and Its Consequences By Dr Hudson NKOTAGU University of Dar es Salaam Tanzania

2. Justification for study • Increase in deforestation in the catchment of Lake Tanganyika is assumed to have caused increase soil erosion and that the rivers carry this sediment load in increasing quantities downstream to the lake. • The perception has been that the input of higher than usual amounts of suspended matter into the lake is a primary threat to the biodiversity of the lake which evolved there over many millennia of relatively low input of suspended matter.

3. Approach • SedSS set out to try and understand the links between catchment factors which affect erosion (rainfall, vegetation, slope, soil etc.), to understand how and in what quantity these erosional materials are transported to the lake and to attempt to understand their impacts on the lake’s ecosystem.

4. SSS final reports

5. Deforestation (Gatorongoro area, Burundi)

6. An example of a river gauging station established by the project

7. Sediment core being collected using Hendricks-Marrs coring device

8. Sediment deposition rate in the Nyamuseni delta

9. Erosion risk areas of the catchment

10. Findings from river-gauging in Tanzania The impacted watershed has an order of magnitude higher than the pristine environment in the current suspended sediment transport rates.

11. Impacts of sediments on biota

12. Sediment transportation

13. Overall findings • Littoral sites within c. 10 km of the point of discharge of a catchment of the medium size range (50-4000 km²) would appear to be most threatened by any changes in erosion rate within that catchment (most likely caused by deforestation or change in farming practices). • It is likely that considerable changes have already occurred in the lakes biota caused by recent (last 50 years) increases in the inflow of suspended matter to the lake.

14. Management • The general consensus of the SedSS participants is that the problems of excess sedimentation is a major threat to the biodiversity of Lake Tanganyika and further also indicates a problem of soil and nutrient loss in the catchment. • Key causes are deforestation and damaging farming practices. • Development of methodologies to reduce erosion and concurrent detailed monitoring of change clearly is the long term answer. This will both reduce the downstream impact of excessive sedimentation on the lake whilst maintaining the integrity and fertility of the soils in the catchment.

15. Conclusions The key management priority is balancing the needs of the people who live near, and depend on the lake and its catchment, with a reduction in the most damaging practices which may lead to the deleterious effects on the lake.