Which issue is most directly caused by decomposing organic matter in hydropower reservoirs and releases methane?

• A. Sediment buildup behind dam
• B. Habitat alteration
• C. Social displacement
• D. Methane emissions from reservoirs

Answer: (D)

When organic matter is submerged in a hydropower reservoir, it decomposes in oxygen-poor sediments. Microbes that thrive without oxygen produce methane as a byproduct, so methane can be released from the reservoir into the atmosphere. This direct release of methane is the main environmental issue associated with decomposing flooded material in reservoirs. Other effects, like sediment buildup behind the dam, arise from how the reservoir traps sediments; habitat alteration comes from the broad ecological changes caused by damming, and social displacement is a human impact tied to dam projects rather than the methane production process.