*2.2.5 Habitat impact*

*Natural Hazards - Risk, Exposure, Response, and Resilience*

and shipping [3].

*2.2.3 Impact of topography and landform*

deposition amount, and dam removal mode [4].

the river and the erosion of the floodplain surface.

*2.2.3.1 Erosion in the reservoir*

*2.2.3.2 Downstream adjustment*

*2.2.4 Impact on water quality*

the downstream river.

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

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,

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

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

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

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

In the early stage after dam removal, the downstream channel adjustment results

sediment remains in place, unwashed downstream by the current.

sediment release can be approximately simulated by sand wave model.

**128**
