The construction of large dams has long been considered a vital strategy for managing water resources and generating hydroelectric power. However, recent studies suggest that the environmental and social costs of dams often outweigh their benefits. First, dams disrupt natural river ecosystems. By blocking the flow of water, dams prevent fish migration, particularly for species like salmon, which need to travel upstream to spawn. This has led to dramatic declines in fish populations and has harmed the livelihoods of communities that depend on fishing. Second, dams trap sediment, which is rich in nutrients. Downstream areas, including floodplains and deltas, are thus deprived of natural fertilization, leading to reduced agricultural productivity and increased erosion. Finally, the reservoirs created by dams can become significant sources of greenhouse gas emissions. When organic matter, such as trees and vegetation, is flooded, it decomposes anaerobically, releasing methane—a potent greenhouse gas. In some cases, emissions from reservoirs rival those from fossil fuel power plants.
🎧 听力讲座
🎯 模拟考试环境：先点「播放讲座」听完内容，再作答。听力原文默认隐藏。
The professor argues that the reading’s negative portrayal of dams is overly simplistic and ignores important mitigation strategies. While it is true that dams block fish migration, modern fish ladders and fish bypass systems have been successfully implemented in many dams, allowing salmon and other species to reach their spawning grounds. For example, the Bonneville Dam on the Columbia River has fish passage facilities that have helped maintain robust salmon runs. Regarding sediment trapping, dams are now often equipped with sediment flushing or sluicing mechanisms that release accumulated sediment downstream during high-flow periods. This mimics natural flood patterns and restores nutrient delivery to floodplains, preserving agricultural productivity. Finally, the claim about greenhouse gas emissions is misleading. Only reservoirs in tropical regions, with high temperatures and abundant organic matter, produce significant methane. In temperate and boreal regions, emissions are minimal, and hydroelectric power remains a much cleaner alternative than burning coal or natural gas. Moreover, careful site selection—clearing vegetation before flooding—can drastically reduce future emissions.
📝 写作提示 Summarize the points made in the lecture, being sure to explain how they cast doubt on the specific points made in the reading passage.
