PHYSICAL EVIDENCE OF DEFORESTATION ALONG A MODERN RIFT LAKE: AN ON LAND AND NEARSHORE COMPARATIVE STUDY BETWEEN TWO WATERSHEDS AT LAKE TANGANYIKA, TANZANIA

Tanganyika is a tropical fresh water lake associated with the East African Rift Valley. Active deforestation over the past several hundred years has resulted in considerable damage to both watershed and nearshore ecosystems. Key areas, protected from fires and anthropogenic activity, such as Gombe Stream National Park, act as analogs to non-deforestated ‘natural' depositional environments. This allows for the impact of regional deforestation to be measured through a comparison of geologic observations from the Kasekera stream/delta located in Gombe Stream National Park, and the nearby deforested Mtanga stream/delta watershed. Mtanga stream flows through a populated and heavily used land typical of many of the watersheds surrounding Lake Tanganyika. Using a multi-indicator approach, a comparison was made between the two similar size/slope watersheds, which both enter the lake laterally on the eastern shore. Our high-resolution 3-D bathymetric maps of both offshore deltas show that there is an increase in sediment supply volumes in the deforested region. There is no measurable difference in total organic carbon in these systems; however, there is a distinct increase in total inorganic carbon in the Kasekera delta sediments due to a greater abundance of shelled organisms. A grain size analysis was made in both streams and deltas. While Mtanga stream is dominated by poorly sorted gravels and sands, in Kasekera there is a clear fining downstream. The Mtanga delta also shows a general fining towards deeper waters, which is not as apparent as on the Kasekera delta. This is interpreted as an effect of increased erosion from the Mtanga stream watershed. Our study has shown that there is significantly more erosion occurring in the deforested region. This is affecting the morphology of both the onshore stream and the offshore delta.