利根川中・下流域平野の地形発達と洪水

抄録

The Tone River is running through the Kanto Plain which is the largest plain in Japan. Now, the river pours into the Pacific Ocean directly, but in former days the river poured into the Tokyo Bay. According to the construction from 1594 to 1809, the Tone River was jointed to the lower reaches of the Kinu River and became to pour into the Pacific Ocean.
We can divide the river basin in the plain into the middle reaches and the lower reaches by the artficial joint place.
There are distinct regional differences on the geomorphology and flooding between the middle reaches and the lower reaches.
(1) Both the middle and lower reaches were covered with sea water in the transgression which was occured in the middle part of the Alluvial Epoch and the marine layer deposited thick. After that the sea-level was stable for a time, and the peat bog layer deposited. After that the middle reaches was deposited by sand, silt and clay by the Tone River and the lower reaches by the Kokai and Kinu Rivers. But the beginning time of the deposition of the Tone River was faster than that of the Kokai and Kinu. And the marshy land has remained till the comparatively recent time in the lower reaches.
(2) The deposition of sand, silt and clay in the middle reaches was big partly because the ground subsidence has been continued partly because the sand, silt and clay which were transported by the Tone River were much in quantity. And the depth of the layer 11 m thick in the Otone Village.
On the other hand, the deposition of sand, silt and clay in the lower reaches was few partly because the ground subsidence was smaller, partly because the quantity of sand, silt and clay which was transported by the Kokai and Kinu Rivers was little, partly because the age which the Tone River began to pour into the area was recent. And the depth of the layer is only 1.3 m at Ryugasaki and 0.8 m at Odome.
(3) Due to the big deposition of sand, big natural levees had been developed along the former main stream of the Tone and ist distributaries in the middle reaches. So, the nearer to the river the land is, the higher it becomes. When we have floods, the flood water overflows from the river course to the adjacent area.
On the other hand, due to the small deposition of sand, there are few natural levees along the river course in the lower reaches. The ground level is horizontal. When we have floods, the flood water flows down slowly on the plain along the river.