This programme focuses on freshwater habitats. Only 3% of the world's water is fresh, and Attenborough describes the course the Amazon, starting high up in the Andes of Peru, whose streams flow into the great river. Young rivers are by nature vigorous and dangerous: they flow fast and form rapids, thick with mud and sediment. They accumulate sand and gravel en route, and this erodes all but the hardest surrounding rocks. The Yellow River of China carries the most sediment of any river. By the time it has settled down and fallen over its last cascade, the water becomes tranquil and rich with nutrients from its banks. It begins to form lakes, and where the water flows into basins created by geological faults, they can be immense. When water reaches such areas, it loses its impetus and drops its sediment, potentially making it very fertile. Lake Baikal in Russia is the deepest: 1,500 metres. In addition, 80% of its inhabitants are unique, including the Baikal Seal. There are many examples of creatures that thrive in such an environment. Predators lie in wait above the surface (kingfishers), below it (turtles), on it (water boatmen), and at its edge (fishing spiders). In its final stages, a river's tributaries are liable to burst their banks and flood. However, some have made a virtue of this: the Marsh Arabs of Iraq construct their buildings on rafts of reeds. This allows fish, pelicans and humans to flourish in a single community.