*2.2.5.1 Aquatic habitats*

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 deposition after the dam is removed.

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 the United States were dismantled [7].

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