Impact assessment of Mtera and Kidatu reservoirs on the annual maximum floods at Stiegler's Gorge of the Rufiji River in Tanzania
International Water Management Institute, Pretoria, South Africa; University of Kwa-Zulu Natal, Pietermaritzburg, South Africa; IUCN-West Africa Regional Office Project, Komadugu Yobe Basin, Nigeria; Ministry of Agriculture and Livestock Development, Keny
The impoundment of any river causes changes in the downstream flow regime. The effect of a dam on flow regime depends on both the storage capacity of the reservoir relative to the volume of river flow and the way the dam is operated. The most common attribute of flow regulation is a decrease in the magnitude of the flood peaks and an increase in low flows. This paper reports the findings of a study to assess the cumulative impact of two dams on the Great Ruaha River in Tanzania (the Mtera and Kidatu Reservoir System) on flood flows. The method used was to compare the observed annual maximum flows at downstream locations after the construction of the dams with mathematically modeled estimates of the annual maximum flows at the same locations, assuming that the dams were not built (i.e. generating naturalized flows). Contrary to expectations, the study found that annual maximum peaks were supposed to be less than those actually observed. For instance, in 1989, annual maximum peak flow of 1,400 m 3/s was recorded at Kidatu, but the estimated uncontrolled peak flow should have been 800 m 3/s, signifying an artificially-induced flood wave. Although, there was a significant impact on the peak flows at Kidatu owing to the dams, the impact was found to be minimal on the flow peaks at Steigler's Gorge. © 2006 International Water Resources Association.