*2.2.2 Sediment impact*

Sedimentation is one of the main causes of reservoir removal in China. After the dam is removed, the sediment in the reservoir area will move again. Sediment deposition is the result of the decrease of water flow carrying capacity which is controlled by the backwater effect and velocity of reservoir. If the operation time of reservoir dam is short or the impact of dam on sediment transport is small, the impact of removal on sediment transport is relatively small. On the contrary, the law of sediment transport will change greatly after the reservoir is scrapped. When a small radial reservoir is abandoned, the silt deposited may be mostly carried downstream by the current. After the reservoir with large capacity is scrapped, there may still be a large amount of sediment in place.

Sediment in reservoir area is transported downstream with current, which not only increases turbidity of downstream river segment but also usually leads to sediment deposition in downstream river segment and changes topography of downstream river channel. Fine sediment may cover the original habitat, block the gap between the bed matrices, and destroy the spawning habitat of fish, resulting in the death of fish. It may also block downstream waterways and water intakes, which will adversely affect human production and life.

When pollutants are contained in the sediment, the sediment carrying pollutants to the downstream diffusion after dam removal is bound to have a significant impact on the downstream river habitat. The content of fine sediment and the way of land use upstream are the important factors influencing the pollutant load in the reservoir. This is because fine-grained sediment has a large specific surface area and can absorb more pollutants than coarse sediment. In addition, the upper reaches of the reservoir land use mode can directly affect the reservoir sedimentation, sediment gradation, and pollutant content. Studies have shown that in the basins dominated by agricultural production, the riparian soil is eroded, and the nonpoint source pollution of the river is serious, resulting in a large amount of fine sand and rich nutrients in the silt in the reservoir area. For the watershed dominated by forest land, the sedimentation amount of reservoir is usually small, and the nutrient content of sediment is low [1, 2].

Fort Edward Dam, New York, the United States, was dismantled in 1973. No measures were taken to remove sediment from the reservoir before the dam was dismantled. After the dam was dismantled, serious problems occurred in downstream water quality and navigation. Pollutants—polychlorinated biphenyls (PCBs)—spread with sediment transport and had catastrophic effects on downstream river ecosystems, leading directly to New York state's ban on fishing in the Hudson River in 1976 and posing risks to downstream public health. In addition to pollution, most of the Hudson's waterways, docks, and industrial parks are blocked, reducing the river's ability to cross water, increasing the risk of flooding downstream towns, and causing millions of dollars in economic losses to fishing and shipping [3].

#### *2.2.3 Impact of topography and landform*

#### *2.2.3.1 Erosion in the reservoir*

The reconnection of rivers, the restoration of natural state of river flow, the reservoir area, and the sediment deposited upstream by the erosion of water to the downstream lead to erosion in the reservoir. The main factors influencing sediment transport in the reservoir include channel flow, sediment particle size and its type, deposition amount, and dam removal mode [4].

This is a slow process of development, at the site of the dam, to form a clear groove head, constantly expanding upstream. From the longitudinal perspective, the depth of topographic erosion in the reservoir area gradually increases, and the specific drop of the river course is greatly adjusted until it encounters impervious obstacles or the specific drop reaches a stable state, and finally the upper and lower reaches of the dam site reach a new dynamic balance [5]. The new balance is sometimes similar to that before the dam was built, but in most cases, some of the sediment remains in place, unwashed downstream by the current.

#### *2.2.3.2 Downstream adjustment*

The increase of river sediment content and sediment carrying load forces a series of new adjustments in the lower reaches of the river. At present, it is generally believed that sediment release after dam removal will determine the change of riverbed elevation and sediment transport in the lower reaches, and the process of sediment release can be approximately simulated by sand wave model.

In the early stage after dam removal, the downstream channel adjustment results in the change of bed matrix and channel morphology, and the final result is the evolution of river floodplain system. After a long time, the sediment content of the river reverts to the natural level, which may lead to the transverse movement of the river and the erosion of the floodplain surface.

#### *2.2.4 Impact on water quality*

With the increase of water retention time, the reservoir water has adverse effects of low oxygen content, changes in water temperature and pH value, serious eutrophication, and high pollutant concentration. After the dam is dismantled, the continuity of the river is restored, and the adverse effects on the water quality above are alleviated. However, the removement of sediment deposited in the reservoir will lead to the increase of turbidity of the downstream river body, especially when the sediment adsorption has pollutants, which may seriously affect the water quality of the downstream river.

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*Dam Retirement and Decision-Making DOI: http://dx.doi.org/10.5772/intechopen.84392*

deposition after the dam is removed.

the United States were dismantled [7].

cycles can recover quickly in a short time [9].

this impact vary from place to place.

forming a new wetland habitat.

*2.2.6 Social impact*

*2.2.5.2 Wetlands*

As one of the important characteristics of habitat, bed matrix will change with the adjustment of channel morphology and the change of sediment erosion and

The study found that the fine sediment in the reservoir is eroded by the current, exposing the underlying gravel and pebble layers, thus improving the habitat quality of fish and increasing the biodiversity. After the dam is removed, the habitat quality of fish will be improved, the barrier of fish migration will be removed, fish can reach the upstream spawning area, the number of migratory fish often rises, and the number and diversity of aquatic insects and other organisms may increase [6]. The salmon population, which had been sharply reduced, has been recovered to 80% of what it was before the dam was built, after four dams on the Snake River in

There are studies showing that, for downstream regions, fine sediment deposition in the downstream reduces riverbed permeability; affects the spawning and breeding habitats of fish; reduces the survival rate, diversity, and abundance of aquatic organisms; and brings adverse effects on downstream habitats [8]. After the removal of the Colorado Dam in the United States, a large amount of sediment released was deposited in the deep pool of the river within 12 km downstream, blocking the gap between coarse particles of sediment, resulting in the death of thousands of fish and the reduction of population density and composition change of large invertebrates. Some scholars have found that the above adverse effects can be eliminated naturally and the rate and recovery degree are related to the biological characteristics. For example, organisms with long life cycle and fixed growth are deeply disturbed and slow in self-recovery. On the contrary, species with short life

Reservoir removal will change the hydrological state of surface water and groundwater as well as the law of river sediment transport, thus leading to a variety of changes in upstream and downstream riverside wetlands. The type and scale of

The changes of surface and groundwater hydrological state are the main influencing factors of upstream wetlands after dam removal. Some of these influences are seasonal, while others are long-term. For the downstream wetlands, the law of sediment transport and the change of groundwater hydrological state are the main influencing factors. Reservoir removal causes silt deposition in the lower reaches of the river, which may lead to the invasion of wetland plants in the silt area, thus

The loss of reservoir function, and no other projects to make up for it, may cause serious social problems. For example, if water supply or agricultural irrigation is the main reason for the removal of reservoirs and if the water supply and irrigation needs of residents cannot be effectively solved, serious social problems will arise. In addition, the scour of reservoir area silt may cause the similar problem enters downstream river course along current, silt up downstream channel or channel take water entrance, affect safety of local traffic carriage and production and domestic

*2.2.5 Habitat impact*

*2.2.5.1 Aquatic habitats*
