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Dams and reservoirs often enable combining solutions to flood control, water supply, recreation, and other community needs. such as recreation, hydroelectric power generation and water supply. Dams and reservoirs are most frequently used on small and moderate sized streams. The large. areas of land required to store the flood flows of major rivers are usually no longer available. Because of their cost, large dams and reservoirs are usually feasible only where they protect urban areas or high value agricultural lands. For some heavily developed urban areas, dams and reservoirs may provide the only practical means for significantly reducing flood damages. The protection afforded by a dam is greatest in the area immediately downstream. Protection further down- s tream is reduced by flows from tribu- taries and by runoff from lands next to the stream. Protection can also decrease over time if the reservoir gradually fills with sediment. A major disadvantage of dams and reservoirs is that downstream residents may not recognize that they only control floods up to some particular size for which they were designed. Such a false sense of security encourages further development and encroachment on downstream floodplains. The value of protection provided by a dam and reservoir may be reduced unless land use controls are enforced to prevent such development. Dams and reservoirs can also have large social and environ_ mental effects and require significant amounts of maintenance. A remote possibility also exists that a dam will fail, causing catastrophic damage and loss of life. Dams can also play a role in the management of coastal floodplains. If constructed across the lower end of streams, they can prevent storm surges from travelling up the stream valley and flooding inland areas. 31