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Secondary Effects of Dam Construction
How Dam Building Affects the Local Environment
The installation of a hydroelectric dam facility can have a variety of influences on the environment of the river and local area.
   
Dams are built on rivers around the world for hydroelectric power generation. The construction of a dam is a huge undertaking that impacts the local population, economy, and environment. Once building is complete, the presence and operation of the dam itself can also result in positive and negative secondary effects. These effects are generally known prior to construction, and their impacts are considered during the design process.

Settlement Relocation
Depending on the location, the installation of a dam may cause existing homes or other structures to be relocated or abandoned. The construction of the Three Gorges Dam in China has resulted in the relocation of about 1.2 million people. In addition, many archeological sites in the Three Gorges area were flooded when the reservoir was filled, preventing any further excavations at those locations.

Flood Control
The presence of a dam on a river that is prone to flooding can help to reduce the impact of flood waters on downstream locations, particularly in locations with a rainy season. The reservoir behind the dam can be lowered to prepare for the increased water flow of the rainy season, and the flow can be metered throughout the year. Metering is also a benefit to downstream water users, who can access increased water flow during dry seasons.

Sedimentation
Rivers naturally move sedimentation downstream towards their outlet. River deltas are built up with sediment, and flooding can deposit sediment on areas near the river, increasing agriculture production. When a dam is built, this natural sedimentation deposit is halted. In addition, sediment can build up behind the dam, reducing the hydraulic head available for power production. Sediment can enter the inlets to the hydroelectric power plant, causing damage to the turbines. On the downstream side of the dam, the reduction of sediment in the water flow can result in erosion as the natural replenishment of sediment is eliminated.

Improved Water Transport
By flooding a water way, water transport can often be improved, particularly if passage was previously difficult in that area. Deeper channels can be navigated by larger ships carrying larger amounts of cargo. However, passage past the dam has to be considered.

Large dams institute lock systems parallel to move ships past the dam. Despite the time required to navigate a lock system, the greatly increased transport potential makes the cost and time worthwhile. In addition, the increase in water transport often results in a reduction in road and rail shipping traffic locally and a reduction in local greenhouse gas emissions.

Dams are used extensively for power generation worldwide. The installation of a dam for hydroelectric power generation can result in a variety of both positive and negative effects on the local area.