Recently in many rivers, vegetation covers wide parts of floodplain of fluvial-fan segment of particularly with a dam on the upstream. Rich vegetation is disadvantageous from the viewpoint of safety against flood but also it is not necessary good for ecosystem of fluvial-fan rivers. When the changes of river landscape of the Tedori river (with dam) and the Abe river (without dam) are compared with each other by aerial-photo analysis, the growth of vegetation after dam construction is remarkable in the Tedori river. When the records of floods of the two rivers are compared with each other, it is clarified how the major floods were controlled by a dam in the Tedori river. The previous studies by us show that repetition of medium-size floods and low-stage waters promotes vegetated area to expand. On the other hand, major floods sometimes destroy the vegetation. Usually the growth and destruction are in balance. Thus the obvious growth of vegetation in the Tedori river probably implies the decrease of chances for vegetation to be destroyed by major floods. Then, models to simulate destruction of vegetation by flood and growth of individual trees have been proposed to explain the difference of landscape changes in the Tedori and the Abe river.

No abstract