Functioning of dams and reservoirs / The ecological effects of dams and reservoirs

For much of recorded history, humans have been engaged in a struggle to tame the flows of large rivers in order to provide the desired amount of water in the right place at the right time. The most conspicuous means by which this has been attempted and, at times, achieved, is by the construction of dams across rivers to control flow and store water. Almost all major river systems are affected, to some extent, by dams. The main period for dam building started in the 1950s, and is continuing today as an essential part of economic and social development. Major dams are generally very costly structures involving heavy engineering. For example, the Three Gorges Dam on the Yangtze River in China, currently under construction, is the largest dam in the world. Dams bring many benefits, but their effects on humans (through displacement) and the natural environment can be severe. For this reason, any new proposals for dam building have to be examined carefully to assess their potential impacts on human society and on the biodiversity of the river systems on which they are built. This is especially true where a series of dams – known as a cascade of dams – are built along the course of a river.

Reasons for Dam Construction--
Dams are built for a number of reasons:

• Hydroelectric power generation – the water stored behind the dam is used to drive turbines that are connected to electrical generators.

• Flood control – the dam is used to store the water coming down river during the floods, and to release it over a period of time at a later date, so that human habitation or agricultural lands are not flooded.

• Agriculture – the water stored behind the dam is released to provide water to crops during the dry season or in times of drought and water scarcity.

• Water transfers – the water stored behind a dam is discharged through pipelines or canals to another river system to make up for shortfalls in flow in the receiving river. The receiving river usually also has a stabilizing dam so that the transferred water can be stored until it is required. This water is then used for domestic, agricultural or industrial purposes.

• Domestic water supply – the water stored behind dams is released into domestic supply systems, usually for towns and cities, over a period of time to ensure a constant supply of water for cooking, drinking washing, etc.

• Navigation – dams are used to regulate the fall of the river, especially in areas of high slope so that boats can pass. In these cases the dams are associated with navigation lock systems. Dams are also used to retain water that can be released in the dry season to maintain sufficient depth for navigation.

Dams may be used to fulfill more than one of these functions. However, dams used for power generation are likely to be very different in design from those designed for flood control or navigation. Power generation requires a consistent regular release of water, whereas the demand from irrigation tends to be highly seasonal.

Functioning of Dams and Reservoirs--
Dams consist of two main elements:

The dam wall is usually a concrete, rock or earth filled barrier placed across a river to control the passage of water. To minimize cost and maximize engineering efficiency, dams are usually built at narrow points in the river such as gorges, where they can be firmly anchored to the banks. From the fisheries point of view, dams can be classified as high dams, which cannot be passed by standard fish ladders, and low dams, which can be passed by fish ladders (see Section 12). Lower structures are known as weirs, and these may be only a few meters in height.

The reservoir or impoundment is the water body retained behind the dam. Reservoirs may be extremely large, forming extensive lakes. Run-of-the-river dams, which constrain the flow of the river without retaining a large volume of water behind the dam structure, usually inundate little more than the original river channel. Run-of-the-river dams are generally used for hydroelectric power generation.

The reservoir behind the dam is generally designed to have a high water level and a low water level. The difference between the two levels is called the drawdown zone. The drawdown multiplied by the area of the reservoir is known as the useable volume. The useable volume is the amount of water available for power generation, irrigation or domestic purposes.

At the end of the rainy season, the dam is generally full, and as the dry season progresses the water level in the reservoir drops to its minimum design level. Where the dam is intended for flood protection, the dam is generally emptied before the wet season, and the useable volume represents the capacity of the dam to absorb wet season flow. If more water arrives than the dam can hold, it is released via a spillway into the river channel below the dam.

Nam Ngum Reservoir Lao PDR
Ubol Ratana Reservoir, Thailand
Ubol Ratana tailwater, Thailand

The water used for power generation or flood control is also discharged into the tailwaters downstream of the dam, but water used for water transfers, irrigation and domestic supply and agriculture is drawn from the river, which results in a decline in flow downstream. In extreme cases, this may lead to drying of the channel; in almost all instances, ecological effects result from the change in timing or magnitude of flows caused by dams.

The ecological effects of dams and reservoirs
Overall effects on river ecology
The most obvious effect of dams is the conversion of an environment that is characterised by unidirectional (one-way) flow of water, to one where there is little or no flow. The result is that the plants and animals adapted to natural flowing conditions of a river may be unable to survive; by contrast, those adapted to standing waters may thrive, and it is under such conditions that certain floating plants (such as water hyacinth; see Section 6) can become a nuisance. Other species that may be viewed as pests, such as certain species of snails that act as intermediate hosts for parasites of humans (see Section 8), may proliferate in the standing waters of reservoirs.

Water flowing into a reservoir from headwater streams slows and deposits its suspended load. This load includes inorganic material, which falls to the reservoir bed as silt, but also suspended organic particles – mainly detritus in the form of FPOM (see Section 7). The deposition of silt in the reservoir means that, gradually, the basin will become filled and the useable volume of water will decline. This has the effect of limiting the life span of the reservoir, and is an especially important consideration when dam construction is planned along a river that carries a high load of suspended solids. Reservoirs generally store water for a period of time. In large reservoirs, this may be for several months but in run-of-the-river dams, the water passes through at a similar rate to water in the undammed river.

As water enters a reservoir from the headwater streams, the water slows down and deposits its suspended load. Thus the upper portion of the reservoir becomes progressively marshier. The deposition of silt in the reservoir (or natural lake) means that any lake has a limited life and eventually will become so filled with silt as to be unable to fulfil the function for which it was built. Experience has shown that the rate with which most dams fill with silt exceeds initial estimates and as a result the useful life of the dam is reduced. In some cases, the economic justification for dam construction is not met due to its reduced life span.

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