A capital inside a dyke

Start with geography

Rich soil, dangerous water

The Red River carries a heavy load of sediment out of the mountains of southern China and northern Vietnam and drops it across a broad, flat delta. That sediment is why the delta is one of the most intensively farmed and densely populated places in Southeast Asia.

It is also why the river bed rises. A river that deposits sediment on its own bed gets shallower, so it floods more easily, so people build dykes, so the water is confined and deposits more sediment inside the dykes, so the bed rises further. In parts of the delta the river now runs above the surrounding fields, held in by embankments. The system works only as long as it is maintained.

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