Final answer:
Dams can have negative impacts on fish populations, such as obstructing migration and causing injury or death to fish. However, the removal of dams can lead to increased fish diversity and improved water quality.
Step-by-step explanation:
Dams can have a negative impact on fish populations. When dams are built, they can obstruct the migration of fish to their upstream spawning areas. This can be partially mitigated by using fish ladders. However, fish traveling downstream can be killed or injured by turbines in the dam. Additionally, the reservoirs and operation of dams can lead to changes in water temperatures, chemistry, flow characteristics, and sediment loads, which can affect the ecology and physical characteristics of the river both upstream and downstream.
On the other hand, the removal of dams can have positive effects on fish populations. Dam removal projects can lead to increased fish diversity and improved water quality. For example, in the Pacific Northwest of the United States, dam removal projects are expected to increase populations of salmon, a keystone species that transports nutrients to inland ecosystems during its annual spawning migrations. Dam removal has also allowed the return of other spawning anadromous fish species in other regions.
Keywords: dam, fish populations, migration, fish ladders, turbines, reservoirs, water temperatures, water chemistry, water flow, sediment loads, dam removal, fish diversity, water quality, salmon, keystone species, anadromous fish species