Watershed Degradation and Sedimentation in Eastern Nile Rivers: Impacts and Sustainable Mitigation Measures

摘要

The Eastern Nile Rivers (ENR), within the countries of Ethiopia, Sudan and Egypt, comprises the basins of the three major tributaries of the Blue Nile (Abbay), the Atbara River (Tekezze), and the Sobat (Baro - Akobo), all originating from the Ethiopian highlands in the East. All three rivers meet with the White Nile in Sudan to form the main Nile which ends at the Mediterranean Sea in Egypt. Degradation and sedimentation (aggradation) processes or phenomena are the major environmental concerns of the watersheds or the basins of these tributaries as well as their impact on irrigation and other related hydraulic activities. This paper is intended to give highlights on the magnitude of the watershed degradation and sedimentation in the eastern Nile rivers problem, factors contributing to those phenomena, investigates watershed degradation and sedimentation management experiences in Eastern Nile Basin and recommends the best technique to be used to alleviate their adverse impact. The paper indicates that factors contributing to the problem include: wind and water erosion, and human activities. Wind and water erosion are the serious form of land degradation that leads to desertification and sedimentation of infrastructures. Watershed degradation and sedimentation is also caused by human activities particularly construction of reservoirs, pumps installation along river banks for water withdrawals, etc. Watershed degradation in the investigated area is mainly resulted from the clearance of vast areas in the upper basins of forested lands for cultivation, fuel wood, brick making, and over grazing. Bank erosion along the rivers has contributed to increased sedimentation elsewhere. This is because degradation is always associated with aggradation, which is sediment deposition. During flood period the Blue Nile and Atbara river transport respectively as high as 2. 6% and 3.5% by weight. In one hand, for the Blue Nile it is found that sediment concentration increased from about 700 ppm in early 1940s to more than 6, 000 ppm in 2003. The consequences of that is aggravation of sediment deposition in Roseires and Sennar reservoirs. The annual silt deposits behind the Roseires Dam (Sudan - Ethiopia border) have increased since 1967 indicating close correlation between watershed degradation and high sediment rate. The annual sediment rate at Aswan High Dam increased to more than 140 million tons. Girba The investigation shows that Roseires, Sennar and Girba reservoirs have lost 30%, 40% and 60% of their original capacities, respectively. On the other hand the paper shows that irrigation canals networks have witnessed high sediment disposition. For example, about 60% of the annual sediment entering Gezira scheme - Sudan (880,000 ha) deposited in distributaries and fields. Another impact is that sediment deposition encourages the growth of weeds in channels with the consequences of decreasing water ways capacities and loss of water through the process of Evapotranspiration. The paper identifies that there are various mitigation measures used in the region to control the degradation and sedimentation processes. The most effective mitigation measures adopted to reduce sedimentation in reservoirs is by operational management and maintaining lower levels during flood season and by dredging of the inlets and irrigation canals. The paper concludes with recommendations to help improve managing, monitoring and suggest sustainable mitigation structural and non - structural measures.