The lower reaches protected by dikes.
Dikes had been built to control the river's flood runoff
for millennia. The extend of the flooded region can be
seen in the top picture. [The dikes are indicated by two red lines on the two sides of the river (blue line) flowing in the northeasterly direction.] Around 200 A.D. an effort
was made to link up all the dike sections into a complete system.
It was indeed a marvel in civil engineering, comparable
in grandeur to the building of the Great Wall.
The dikes successfully protected the river's lower reaches
for eight centuries, with only relatively infrequent dike
breaches. However, it is not a simple matter to interrupt
nature's plain-building process.
The heavy sediment load brought down with the flood water
started to accumulate in the floodplain between the dikes,
raising the riverbed to such an elevation that it would be
many meters above that of the surrounding land,
turning it into a "suspended river." [A planned water-diversion project with a water conduit
traversing across the river must tunnel under the present river bed.]
During the last millennium over a thousand
floods ravaged the region.
The extend of the 1933 flood is shown in picture below. The bottom of the picture shows the river bed which is higher than the surrounding land and is drained of flood water, inundating the land in the top part of the picture.