What major challenge do environmentalists often cite regarding dam operations?

Environmentalists frequently highlight the alteration of natural water flows as a significant challenge associated with dam operations. Dams can drastically change the natural hydrology of rivers and streams, leading to a wide array of ecological consequences. This alteration affects not just the water itself but also the surrounding ecosystems, including fish habitats, sediment transport, and the overall health of aquatic life.

When water flows are altered, it can disrupt the migratory patterns of fish and other wildlife, hinder the natural reproduction cycles of species, and lead to changes in water temperature and chemistry that might not be conducive to native aquatic organisms. Furthermore, the changes in flow patterns can impact surrounding vegetation and land use, resulting in broader environmental and socio-economic effects.

In contrast, while increased irrigation capacity, hydroelectric efficiency, and economic benefits to local communities are often discussed in the context of dam operations, they are more aligned with the advantages and motivations behind building and maintaining dams rather than the environmental challenges that arise from them.