*2.2.1 Hydrological influence*

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

**2.1 Impact characteristics**

*2.1.1 Comprehensiveness*

*2.1.2 Contradiction*

*2.1.3 Space-time continuity*

*2.1.4 Uncertainty*

time-space continuity, and uncertainty.

ecological environment after the dam is dismantled.

of migratory fish but also easy to cause species invasion.

the gradual recovery of sediment movement, and so on.

**2.2 Reservoir removal impact classification**

In history, some cases have made delightful improvement; some of the others have led to irreparable impacts. Avoid going astray by reviewing the past, which is

The research on the impact of reservoir removal on ecological environment involves engineering technology, ecological environment, social economy, human history, and other fields. This paper summarizes the research results and practical cases of reservoir removal around the world. The characteristics of reservoir removal impact on ecological environment are comprehensiveness, contradiction,

As with other water resource management behaviors, the impact of dam demolition is comprehensive. This characteristic is reflected in the comprehensive impact of physical, biological, ecological, social, economic, and cultural factors on the

The contradiction reflects the two sides of the impact of dam demolition, that is, while it is beneficial to one element, it is harmful to the other. For example, the removal of dams to restore the connected state of rivers is conducive to the breeding

The impact caused by reservoir removal can be spatially from the upstream to the downstream of the dam site, from the river where the dam site is located to the river, and even from the river basin. The time span can be days, months, or even decades. Short-term effects have been generated in the process of dam removal, such as sediment release from the reservoir area, pollution caused by sediment output, water oversaturation, etc. Long-term effects include natural water recovery, the reservoir area becoming a flowing river again, the change of river temperature,

As a result of subjective and objective reasons, the impact of reservoir removal is uncertain. The subjective reason is that relevant researches are not in-depth and comprehensive enough, and many problems are difficult to be accurately explained from the mechanism. For example, there are still many disputes about the evolution process and mechanism of river channel in the reservoir caused by sediment output, and how to scientifically determine the goal of ecosystem restoration after dam removal is still an academic problem. The objective reason is mainly the impact of global climate change, which is also a difficult problem faced by all water resource management activities.

Reservoir removal may reshape or even destroy rivers and coastal ecosystems, causing a series of new problems. In this section, hydrological sediment,

significant to generalized dam removal eco-environment influence factors.

**126**

The construction of the dam will lead to changes in the flow rate, change frequency, duration, occurrence timing, and change rate of the hydrological situation of the river. After the dam is dismantled, the flow rate and water level of the river will change with the seasons, get rid of human intervention, and return to the natural hydrological situation, which will completely reverse the hydrological situation of the river.

After the dam is removed, the river will be reconnected, and the lower reaches of the river will be continuously restored. The shrinking condition of the lower reaches of the river will be alleviated, and the groundwater supply in the lower reaches of the river will be basically restored to nature. However, if the downstream channel has insufficient sediment transport capacity, sediment deposition, and riverbed elevation, the downstream section flow capacity decreases, the channel specific decline becomes slow, the flood discharge capacity weakens, and the reservoir regulation and storage protection are lost; the downstream river level is raised during the flood period, increasing the flood risk in the downstream region.
