抄録
In this paper the author tried to define the changes of river bed in the Rivers Mogami, Omono and Yoneshiro in Nishi-Ôu District, each of which having several basins and gorges in the watershed. The results of the . survey are summarized as follows:
1) Contrary to the author's expectation, the records show that the mean low water level in basins or plains in their watersheds has been lowerirg since about 1960.
2) As to the lowest height and the mean height of these river beds, the quantity of degradation has become increasingly remarkable since about 1965.
3) As a whole, it is near the undercut slope in meander or near the junction with tributary streams that the changes of river bed are most remarkable.
4) In these rivers the total amount of deposition or scour in river bed nearly corresponds to the total quantity of gravel mining, and, therefore, the recent degradation of river bed in these rivers may be due to gravel mining.
5) If so much gravel had not been mined in these rivers, the river bed in them would have unchanged (Fig. 6).
