TOEFL統合ライティング

📖 読解文章

The construction of large-scale dams has long been hailed as a solution to energy and water needs, but recent studies reveal significant drawbacks that argue against their continued proliferation. First, dams disrupt natural river ecosystems, blocking fish migration routes and altering sediment flow. For example, salmon populations in the Pacific Northwest have declined sharply due to dams, as they cannot reach upstream spawning grounds. Second, reservoirs behind dams are major sources of greenhouse gas emissions. Decomposing vegetation in flooded areas releases methane and carbon dioxide, and research shows that in tropical regions, dam reservoirs can emit more greenhouse gases per unit of energy than fossil fuel plants. Third, dams are not a sustainable solution for water management because they exacerbate water scarcity downstream by trapping sediment and reducing river flow, which diminishes water quality and availability for agriculture and communities. Additionally, evaporation from large reservoirs causes significant water loss, especially in arid climates. These ecological and climatic impacts suggest that relying on dams is a shortsighted policy.

🎧 リスニング講義

The reading argues that dams are problematic, but these claims are overstated and ignore modern engineering solutions. First, while fish migration is a concern, fish ladders and fish lifts have been effectively implemented in many dams, allowing salmon to reach spawning grounds. For instance, in the Columbia River system, fish passage technologies have boosted salmon runs to near-historical levels. Second, the greenhouse gas emissions from reservoirs are not inherent to all dams. In temperate regions, emissions are minimal, and when dams are built on already-existing lakes or with proper land clearing before flooding, methane production is dramatically reduced. Moreover, hydroelectric power replaces fossil fuel combustion, resulting in net emission savings over the dam's lifetime. Third, water scarcity is actually mitigated by dams, not exacerbated. Dams store water during wet seasons and release it during dry periods, ensuring a reliable supply for irrigation and drinking water. Modern dam operations also manage sediment release through sluicing, preventing downstream flow reduction. Evaporation losses can be minimized by designing deeper reservoirs with smaller surface areas. Therefore, with careful planning, dams remain a viable and sustainable solution.