PLANET EARTH Freshwater
The Mara river（马拉河）, snaking across the plains（平原） of East Africa. As the land flattens out rivers slow down and lose their destructive power. Now they are carrying heavy loads of sediment（沉积物） that stains their waters brown. Lines of wildebeest（角马） are on their march. Each year nearly two million animals migrate across the Serengeti plains（塞伦盖蒂平原，位于坦桑尼亚） in search of fresh green pastures.
For these thirsty herds the rivers are not only a vital source of drinking water, but also dangerous obstacles. This is one of the largest concentrations of Nile（尼罗河） crocodiles in Africa, giants that grow over five meters long. From memory, the wildebeest are coming and gather in anticipation. The crocodile's jaws snap tight like a steel trap（捕兽铗） - once they have a hold, they never let go.
It took over an hour to drown this full-grown bull. To surprise their prey crocodiles must strike with lightning speed. Here, only the narrowest line separates life from death. Most rivers drain into the sea, but some end their journey in vast lakes. Worldwide lakes hold twenty times more fresh water than all the rivers. The East African Rift Valley （东非大裂谷）holds three of the world's largest: Malawi（马拉维）, Tanganyika（坦噶尼喀）, and Victoria. Lake Malawi, the smallest of the three, is still bigger than Wales.
