Environmental Impacts of the Proposed Stiegler's Gorge Hydropower Project, Tanzania
Authors: Mwalyosi Raphael B. B. 
Published: August 2009

Establishment of a planned reservoir at Stiegler's Gorge on the Rufiji River would enable a large amount of highlyvalued energy to be produced. This would represent an important national asset. But although such a large net amount of energy would be realized on a short- to mediumterm basis, its production would be negatively influenced by a variety of reservoir operation options that would be related mainly to irrigated agriculture, fisheries, and water quality, which form trade-offs with hydropower generation.

The most significant effect of the dam would be drastic reduction, by controlled discharge, in the frequency of severe floods in the lower Rufiji valley. Floods in excess of 2,500 cubic metres of discharge per second could be reduced in number from some 14 to 3 in 24 years, or from 167 to 13 during 300 years. The most devastating floods would also be reduced—from once in about 8 years to once in maybe 40 years.

The STIGO Project impact area contains a major wildlife resource in terms of size, density, and diversity. Its accessibility to Dar es Salaam gives it a great potential for tourism development. As an access road to the dam-site would be a necessary prerequisite to implementation of the STIGO Project, it would indirectly help to open up the Selous Game Reserve to tourism, which is currently being hampered by poor communications. Also, a substantial amount of the forest resources identified along this road could be exploited.

No complete populations of animal wildlife would be in danger from direct ecological consequences of river impoundment and dam construction. However, significant proportions of the populations in the STIGO Project impact area of three species (Giraffe, Wildebeest, and Zebra) would be potentially at risk, owing to their need for habitats of restricted range. On the other hand some species, including Crocodile and Hippopotamus, would increase in numbers following creation of the reservoir and improvement of their habitat downstream of the dam (due to swamp drainage).

Not withstanding that the ecological impacts of dam construction are relatively minor, the socio-economic impacts on wildlife and conservation values are potentially great—resulting, for instance, from facilitated access to the heart of the Selous Game Reserve and concomitantly increased conflict between wild animals and Man. These circumstances would reduce wilderness values and disturb animal wildlife, so that, especially, commercially valuable species may be expected to decline unless strict regulations are made and enforced.*

River impoundment would have very negative impacts on floodplain fisheries and agriculture, the latter of which would probably be changed to irrigated agriculture with artificial fertilization, while floodplain fisheries would totally collapse. Some mangrove stands in the Delta would probably be displaced by reeds. Delta fisheries would be very negatively affected, because of changes in the water regime as well as in salinity levels.

Water quality in the planned reservoir and in the downstream area would be negatively affected by the project. The water would often be unfit for human and animal consumption and use, as well as unfavourable for fisheries. The project would also have negative effects on the health of the riparian population, owing to increased potentials for disease vectors.

Overall, a high degree of uncertainty is involved in the project. The effect of the primary project (hydropower) would be negative owing to its drastic consequences for the immediate and more distant impact areas. Its image might improve and probably become positive if development in the Basin were carefully controlled and managed. This would require reliable prediction of the impacts and a thorough analysis of the remedial and/or additional measures to arrive at an integrated development strategy for the Basin. Although several measures are planned to eliminate and/or minimize the negative impacts, their implementation may be difficult in view of the bad economic situation of the country.