The author treated first the changes of river bed in the lower stream (0_??_32km.) of the Ôi River in Shizuoka Pref. by considering a) fluctuations in the mean heights of river bed and b) quantities of deposition or scouring in river bed. Then he dealt with the relation between the changes of river bed and gravel mining in the same stretch.
The mean heights of river bed and cross sectional areas of 116 gauge stations were measured and recorded by the Chûbu Branch Office of the Ministry of Construction. Seven measurements were carried out during past 9 years (1955_??_1963).
Conclusions are summarized as follows:
(1) When the mean heights of river bed (divided into 23 sections at intervals of 1 km.) in 1963 are compared with those in 1955, it can be recognized that the former has been considerably lowered in contrast with the latter overall sections. The average degradation for all sections is 33.6 cm., although values of over 70cm. have occurred locally.
Fig 2 shows the fluctuations in the mean heights of river bed during the period 1955_??_1963 in 11 sections and Fig, 3 shows the fluctuations in the longitudinal profile regarding the mean heights of river bed in 1955 as a standard.
(2) The yearly records of the amount of deposition and scouring in river bed show a huge amount of scouring for every year since 1958 excepting 1958 and 1961, the total amount of deposition and scouring recent 6 years (1958_??_1963) reaches about 3.7×10⁶m³ of scouring. Fig. 4 shows yearly fluctuations of the amount of deposition or scouring in river bed during these 6 years.
(3) The gravel mining in the lower stream of the Ôi River suddenly increased in the recent period. The total quantity of gravel mining during the 6 years (1958_??_1963) was estimated to be about 6.4×10⁶m³, which was twice the quantity allowed by prefectural government.
(4) Regarding the causes of the degradation of river bed, it can be considered that the effect of grvel mining in river bed is far direct and decisive as compared with other causes. Based on the analysis of data from 1955 to 1963, the author pointed out that there was a considerable correlation between quantities of scouring in river bed and quantities of mined gravels in the reaches of 0_??_18km., excepting a few sections (Fig. 5A). Moreover, there is also a considerable corrlation in the same reaches between the lowering of river bed calculated from the quantity of mined gravels and the degradation based on the actual measurement (Fig. 5B).
(5) It is to be seen, therefore, that gravel mining has been excessively undertaken in recent years, and yet, the scale of mining is still expanding. It this trend in mining is not reversed in the immediate future, the gravel mining in the lower stream of the Ôi River will be completely forbidden within ten years, for the possible amount of gravel reserves at the end of the 1963 fiscal year is estimated to be about 8.5×10⁶m³.