Reservoirs are important for flood control, production of hydroelectric power, fishing, water harvesting and storage. Climate is changing, population and anthropogenic activities are increasing which increase sedimentation rate to most of reservoirs reducing their capacity. With climate change in mind water are becoming scarce resources which need proper storage when available and suitable flood control in areas with unpredicted rainfall. Furthermore, energy demand is increasing with population growth suggesting a need of sustainable management of dams so as to control flood, produce more hydroelectric power and store water to use in dry season. Sedimentation is one of the factors influencing storage capacity of the reservoir. Understanding sedimentation rate, source of sediment and carryout proper management measures is one among of the important methods to preserve dams. 137Cs and 210Pb are important radionuclides for investigating sedimentation rate. The method has been used for more than two decades in developed countries with less use in developing countries. In this study a sediment core was sampled at the centre of Nyumba ya Mungu reservoir in Tanzania, the samples were then transported to China for processing and analysis. High-purity Germanium detector was used for analysis. The results showed that rate of sedimentation in the reservoir ranged from 0.21 to 0.65 cm/year with more sediment coming from Kikuletwa than Ruvu tributary. The range is comparable to most of reservoir in tropics of developing country. Proper utilization of this method can bring sustainable management of water, save communities from flood disaster and increase production of electricity.