Dams are important structures which are built for flood control, irrigation, municipal and industrial water supply, hydropower production, sediment control, environmental flow regulation, recreation, and navigation. One of the most important physical characteristics of dams and their reservoirs are area-capacity curves. These curves are used for reservoir flood routing, reservoir operation, determination of water surface area and capacity corresponding to each elevation, reservoir USA and 12 dams in Iran is used. This study showed that the obtained equation for reservoir's depth – capacity is more precise in comparison with the obtained equation for reservoir's area-depth. For the studied reservoirs, the mean square error between the normalized depth – capacity equation and normalized depth - capacity data is 0.0001 to 0.0044. While, the mean square error between the normalized area – depth equation and normalized area - depth data is 0.001 to 0.0121. For derivation of appropriate model for prediction of reservoir sediment distribution in Iranian dams, the sedimentation survey data of 11 reservoirs in different parts of Iran is used. Four phases of sedimentation are recognized from analysis of reservoir coefficient variation with sedimentation for these reservoirs. These phases are named as: initial, transition, equilibrium and discharge. Initial phase begins from reservoir operation and continues until arrival of sediment front to the dam site. In this phase, the reservoir coefficient decreases with sediment accumulation. The transition phase begins after arrival of sediment front to the dam site. In this phase, the reservoir coefficient increases with sediment accumulation until reaching a the constant value. Equilibrium phase begins after transition phase. In this phase, the reservoir coefficient is constant with sediment accumulation. The discharge phase begins after rising of sediment elevation to the first outlet gate. In this phase the outlet gate acts as a control point and prevents increasing of sediment elevation at the dam site. In this phase, the reservoir coefficient decreases with sediment accumulation. The result of study showed that in equilibrium phase, the reservoir coefficient is constant about 0.57-0.61. In equilibrium phase the reservoir coefficient is constant, while in discharge phase the reservoir depth is constant. This characteristic is combined with the obtained equations and a new method is developed for predicting sediment distribution in equilibrium and discharge phases of Iranian dams. Key Words: Reservoir coefficient, Sedimentation, Dam, Area - Capacity curve
