What are ecological impacts of dam construction on rivers?

• A. Altered flow regimes, sediment trapping and reduced sediment transport, disrupted migration of aquatic species, changes to floodplain ecosystems, and potential greenhouse gas emissions from reservoirs
• B. Increased biodiversity, improved water quality, enhanced fish migration, reduced greenhouse gases
• C. No effect on sediment transport, no impact on floodplains
• D. Decreased water temperature uniformly along the river

Answer: (B)

Dam construction changes how a river functions by altering flow, sediment transport, and ecosystem connections. Regulated releases smooth out natural flood peaks and shift the timing of high and low flows, which can reduce habitat availability and disrupt the life cycles of many aquatic organisms that depend on seasonal cues. Sediments get trapped in the reservoir, so downstream reaches receive less sediment; this can lead to channel incision, loss of nutrient-rich sediments, and changes to floodplain and delta ecosystems that rely on regular sediment delivery. Barriers block migration for fish and other species, fragmenting populations and altering community structure. Floodplain ecosystems suffer as periodic floods become less frequent or less intense, changing plant communities and nutrient cycling in wetlands and connected habitats. Reservoirs also contribute to greenhouse gas emissions, particularly methane from decomposing submerged organic matter, adding another layer of ecological impact that varies with conditions and management. Taken together, these interconnected effects explain why dam construction is associated with broad ecological disruption rather than uniform improvements in river health.