Reservoirs are man-made lakes created to store water during times of excess flow and supply it from storage when the demand exceeds the inflow. The equalization provided by the reservoir makes it possible to satisfy a higher continuous water demand than can be provided from the normally fluctuating river flow. In developing countries reservoirs serve mainly irrigation and hydro-power generation. The major consequences of dams, beyond their various functions, are environmental and social. These constitute the major topics of investigations during the design but are beyond the intended scope of the manual, which is restricted to a brief description of the physical processes of reservoir sedimentation. The stream spreads out on entering the reserovir and loses its identity. Both the velocity and sediment carrying capacity decrease rapidly with distance from the entry point. Consequently, some or most of the sediment brought in by the stream is deposited in the reservoir.