Down to the river

After almost two weeks of walking along the Great Wall, we finally reached the edge of a huge valley, which we descended on a road of swerving switchbacks. About halfway down, we looked across a gully to see another Great Wall watchtower, so we scrambled across to it, and as we arrived, gasping, at its base, there it was below us: Huang He, the mighty Yellow River.

I had seen the Yellow River once before - in the city of Jinan, when I was cycling down the east coast about seven years ago. There, near the river's mouth, the river was a turgid, brown-yellow mass, and in the stifling, humid summer air, it looked almost boiling. Here, in the middle course, the river was a deep grey and brown colour.

We could see a huge dam upstream across the river, which was frozen solid, although below the dam, the river was flowing again. Huge cliffs well over 100 metres high encased the river, and, as we followed it as part of our 5,000 kilometre walking expedition, we saw a series of twisting cliff roads, villages and occasional factories that ran along the top.

The Yellow River is known as the 'mother of China' because through the millennia it has fertilised and irrigated the land, and it is here that China's earliest documented civilisations arose. But it's also known as 'the sorrow of China', because of its propensity for regular and terrible floods, which through the centuries are thought to have killed millions of people.

The reason for the floods is that the river carries huge quantities of sediment - mostly loess, the soft yellow earth of the river's upper and middle reaches. In fact, it carries more than 36kg of sediment per cubic metre of water, compared with the less than 2kg by the Nile. This sediment is then deposited on the riverbed downstream, meaning the bed is always rising, and so huge embankments are needed to control the river.

While the huge number of dams built in the past 50 years have tamed the river somewhat, it still poses many challenges, not least that the silt is always clogging up the dams. What's more, due to pollution, half of its water is now considered biologically dead, and because of excessive use, since the late 1970s the river has often run dry before it even reaches the sea.