Over the centuries since, water management has been perfected, with the development of dikes, overflows, drainage pools, pumping stations and storm flood barriers. Without the coastal dunes and river dikes, two thirds of the country would be inundated at times of flood. About 25 per cent of the Dutch territory is below sea level with the lowest spot at 6.7 metres beneath sea level. Flood defences are therefore absolutely vital. Human intervention has, however, given an artificial permanence to the situation.
Modern rivers are no longer free to meander, to push back their banks or alter their courses.
They are hemmed in by dikes, groynes and ripraps. Nevertheless, each time the Rhine or the Meuse overflows its banks, one readily appreciates the power of nature. In the landscape one can still see the telltale marks of occasions when the water has broken through. During the seventies, the Rhine and the Meuse were seriously polluted. The pollution has affected the river beds and flood plains where contaminated sediments have been deposited in recent decades.