the rivers are not only a vital source of drinking water, 河流不仅是唯一能解渴的重要水源

105. Each year nearly two million animals migrate across the Serengeti plains
     每年都会有将近200万只角马 横越塞伦盖蒂平原
106. in search of fresh green pastures.
     去寻找新鲜草场
107. For these thirsty herds
     对于这群饥渴难耐的角马来说
108. the rivers are not only a vital source of drinking water,
     河流不仅是唯一能解渴的重要水源
109. but also dangerous obstacles.
     也是充满危险的关隘
110. This is one of the largest concentrations of Nile crocodiles in Africa,
     这里是非洲一处最大的尼罗鳄聚集地
111. giants that grow over five metres long.
     这些大怪物能长到5米多长

来自《行星地球 第一季 第三集 Planet Earth S01E03 淡水》第 108 行