Dams and reservoirs are among the most important water resources structures, as they store floodwaters, ensure the supply of water for drinking and irrigation, and facilitate hydroelectric power generation. However, these structures face a silent challenge known as sedimentation, which causes reservoirs to lose part of their storage capacity year after year. Global studies indicate that the annual loss in storage capacity ranges approximately between 0.5% and 1% on average, making sedimentation one of the major factors affecting the operational lifespan of dams. The process begins in the watershed, where rainfall and surface runoff erode soil particles and transport them through rivers in the form of suspended load and bed load. When this water enters the reservoir, its velocity decreases significantly, causing it to lose its ability to carry sediments. Coarse particles are deposited first near the reservoir entrance, forming a delta deposit, while finer particles travel farther into the reservoir. In some cases, these fine particles may move in the form of turbidity currents, which flow along the reservoir bed toward the dam body. Several factors control the rate of sedimentation, including rainfall intensity, soil characteristics and vegetation cover within the watershed, channel slope, and the reservoir volume relative to the inflow discharge. The concept of trap efficiency is used to describe the percentage of incoming sediment that is retained by the reservoir. It is commonly estimated using empirical relationships such as the Brune Curve. The impacts of sedimentation are not limited to the reduction of storage capacity. It can also lead to the blockage of water intake structures, erosion of power-plant turbines by solid particles, and increased pressure on the dam body. Furthermore, trapping sediments behind the dam deprives the downstream river channel of its natural sediment load. As a result, the relatively sediment-free water released downstream can cause downstream erosion of the riverbed and banks and alter the morphology of the river channel. To address this problem, integrated solutions are adopted, beginning at the watershed level. These include afforestation, soil conservation, and the construction of check dams to reduce soil erosion. In addition, operational measures can be implemented within the reservoir, such as hydraulic flushing by opening bottom outlets to remove accumulated sediments, passing turbidity currents through specially designed outlets through venting, mechanical dredging, and the use of sediment bypass channels. Bathymetric surveys using sonar and numerical sediment-transport modeling also contribute to estimating sedimentation rates, assessing storage-capacity losses, and determining the appropriate time for intervention. In conclusion, incorporating sedimentation assessment into the design stage of dams, together with regular monitoring and watershed management, represents a fundamental pillar for preserving reservoir storage capacity and ensuring the sustainability of water resources. This is particularly important in countries that depend on reservoirs to cope with fluctuations in water inflows and periods of drought. Presented by: Assistant Lecturer Zahraa Hussein Mohammed M.Sc. in Civil Engineering Source: Morris, G. L. & Fan, J., Reservoir Sedimentation Handbook, McGraw-Hill, 1998.