MODERN WATER LEVEL CHANGES OF LAKE ABAYA, SOUTHERN ETHIOPIA

Lake Abaya is a graben fill in the southern section of the Main Ethiopian Rift Valley. To the south a sill covered by swampy forestland of approx. 1 km width separates Lake Abaya from neighbouring Lake Chamo. Altogether, drainage of Lake Abaya – Lake Chamo system covers a watershed of approx. 18,600 km² while lake areas total c. 1,550 km². Lake level of Lake Abaya is measured since 1970. During that period lake level was subject to repeated changes. But at least, water levels of Lake Abaya since the middle of 1980ties continuously increased. Climatic conditions during that time also repeatedly changed, but, resuming results of climate data time series analysis, climatic conditions as the only factor influencing lake level changes have to be excluded - although they are significant. Additionally, analysing Lake Abaya’s lake level changes it has to be considered that latest since the 1970ties dramatic population growth, changes in land-ownership, clearing of forests and bush-land as well as changes in cultivation manners caused dramatically increasing sediment yield of the Lake Abaya tributaries, thus, influencing basin bathymetry and volume. Consequently, lake level changes of Lake Abaya are controlled reciprocally by : climatic oscillations influencing regional water balance and human impact causing increase of soil erosion rates and, coinciding, increase of the tributary's sediment yield and, thus, causing changes in basin volume. Because of its shallow depth (max. depth of 26 m) lake level of Lake Abaya reacts quite sensitive to changes of water and sediment input and, thus, is an ideal subject-matter to analyse complex pattern of climatic and human impacts on lake level changes. Nevertheless, as Lake Abaya is located in the Main Ethiopian Rift Valley also neo-tectonics at the southern sill influencing outflow to Lake Chamo have to be kept in mind. Investigating relief of sub-aqueous areas of accumulation and character of lacustrine and fluvio-lacustrine sediments, the inter-fingering of climatic and man’s impact is pointed out. In the study presented results are shown exemplarily for the northern lake area where sediments originating from the Bilate River have the predominating influence on composition of lacustrine sediments.