Unit 6 Period 5 Writing-【金版教程】2023-2024学年新教材高中英语必修第二册作业与测评word（外研版2019）

英语 必修·第二册 作业与测评 Period 5　Writing 课时作业(六) Ⅰ　阅读 Beavers (海狸) are famous for building impressive dams in streams. They have a rare ability to transform the landscapes they live in. This is primarily because beavers need to use their dams to hide from predators like bears, wolves, and river otters. Beavers live in dome­shaped constructions within the ponds that can only be accessed by underwater entrances. Inside, they are safe from threats. Besides, they can also store food inside and keep warm in winter. Beavers build dams up to 5 meters high, and the largest one, in Alberta, Canada, is 850 meters long. Most dams are built from pieces of wood, with stones at the base and a sealing layer of mud and plants on the upstream side. The dams are thicker at the bottom with a shallow slope on the upstream side, meaning that the weight of the water pushes down on the dam, keeping it in place and allowing it to hold back large volumes of water. Their engineering work is so effective that beaver dams can last for decades. Beaver dams can promote the creation and development of stable wetland ecosystems, one of the most productive ecosystems in the world, which serve as the home of rare and endangered species. In addition, the dams can help water flow to the newly created ponds, thus, preventing flooding in areas downstream of the rivers where these dams are built. They also protect the land from soil erosion and block pollutants in the water flow downstream into major water bodies. However, Ph.D.student Robert Needham, from the International Centre for Ecohydraulics Research at Southampton, expressed a great deal of his concern about the possibility of beaver dams blocking the upstream and downstream migration of Atlantic salmon (鲑鱼) and trout during their reproduction seasons, as well as affecting habitat quality. Thus, people should control the construction of beaver dams, especially in human­inhabited areas, near agricultural fields and pasture lands. 篇章导读：本文是一篇说明文。文章介绍了海狸筑坝