Impact of Reservoirs on the Flood Pulse--
Dams have a very strong impact on water balance in rivers. Dams are built for several reasons, for example to generate electricity, to control floods, for irrigated agriculture, to maintain flows for navigation and for domestic water supply. Some dams fulfill more than one function. In all cases water is stored behind the dam in a reservoir and released slowly according to the needs of the user of the dam, however, differences in usage dictate the way the dam is managed. For example, dams for flood control need the maximum available volume before the beginning of the rainy season in order to store the excess water arriving during the rains. Dams for irrigation and electricity generation can maintain greater volume at the end of the dry season and release excess water over the spillway as necessary. The result of the operations of the dam on the hydrograph is to suppress the peak floods during the rainy season and to even out the flow through the dry season. This means that lake-like conditions can be created in the river channel for some distance downstream of major dams as well as in the upstream reservoir.

The suppression of the flood means that the floodplain is no longer flooded, or is flooded to a greatly reduced extent. Lack of flooding changes the nature of the vegetation on the floodplain moving away from flood dependent or flood tolerant species to drier forms. It also means that much of the seasonal fauna disappears. In some areas of the world the almost total control of wet season flow has led to the permanent occupation of the floodplain for human habitation. In years of excessive rainfall, however, so much water may be released over spillways as to re-flood the plain, leading to catastrophic loss of life.

The presence of greater amounts of water in the river channel in the dry season may, at first sight, appear beneficial. However most of the living forms in rivers are highly adapted and dependent on the seasonal rise and flow of the waters. The more stable hydrological regimes that occur in rivers downstream of dams results in considerable changes in the species and types of organisms present. There is, for instance, a tendency for introduced species that may be better adapted to stable conditions, to colonize the waters downstream. The full impact of dams is examined in Section 13.

Impact of Water Transfers--
Transfers of water from one river basin to another, to provide drinking water for cities or water for irrigated agriculture, are becoming increasingly common. Transfers can be made by direct removal of water from a river, but more usually involve some form of flow stabilization structure such as a dam or weir. The recipient river usually has some form of storage for the transferred water, such as a reservoir. In addition to the effects of the dams themselves (see section 13), water transfers involve changes to the hydrographs of both donor and recipient rivers. Water transfers effectively rob one river in favor of another, but in so doing they alter the timing and amount of water in both systems. The donor river receives less water and removal at times of low flow can be particularly damaging to living aquatic organisms downstream of the abstraction point. At the same time, living resources in the river that receives the water may have to adapt to amounts of water far in excess of the normal. This can damage sedentary bottom living organisms, for example insects and molluscs.

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