ABSTRACT
Already in Ancient Times, world populations were drawn towards rivers. Rivers were ideal for transport, supported agriculture and provided drinking water. Especially in developed countries, many cities grew due to the increased commercial activities in the Middle Ages. Cities expanded towards more low-lying areas. Dikes were constructed and even rivers were diverted to give the growing cities better flood protection. By the beginning of the Twentieth century most rivers in developed countries have lost their natural appearance. This can be seen for instance in the Rhine river basin in Europe. Many flood plains were transformed into urban districts or trade centres and led to bottlenecks in the river system. Since the beginning of the Twentieth Century, cities throughout out world have grown substantially. For instance, Tokyo has grown from about 3.7 million inhabitants in 1920 to more than 12.5 million people in 2005.
