What impact do dams have on river ecosystems?

Dams can significantly impact river ecosystems because they alter the natural flow of water and its associated processes. They block fish migration, which can prevent species from accessing spawning grounds upstream, disrupting their life cycles and reducing fish populations. Additionally, the construction of a dam changes sediment transport dynamics; sediment that would normally flow downstream gets trapped behind the dam, leading to erosion in other areas downstream and affecting habitats.

Moreover, dams typically create a reservoir, which can change the temperature and chemical characteristics of the water. Water within the reservoir can become warmer and lose oxygen, affecting aquatic life. Such changes can lead to a decline in biodiversity and shifts in species composition, as organisms that are adapted to free-flowing river conditions may not thrive in the altered environment created by the dam.

The other options presented do not encompass the comprehensive range of effects that dams have on river ecosystems. While some dams may create recreational areas, this benefit does not address the fundamental ecological impacts they impose.