The sediment discharge and its grain size distribution were measured by the observation facility which installed at Bozudaira sado dam in Yotagiri River of Tenryu River System. The 34 floods which had over 0.5m depth included maximum 1.76m were measured from 2001 to 2007, and the characteristics of bedload discharge were mainly considered based on these data. Then the relationship between non�|dimensionally tractive force and non�|dimensionally bedload discharge from grain size distribution of river bed near the upstream of the observation point have a similar property to the bedload discharge formula such as Ashida, Takahashi and Mizuyama's formula from a quantitative standpoint. But the grain size distribution of bedload was different by the observation and the calculation by bedload discharge formula. These results show there is a possibility that bedload discharge and its grain size distribution are strongly affected by the conditions of the sediment yield and discharge in upper river basin. Therefore the impact of the change of the sediment yield and discharge on the characteristics of bedload discharge was considered from the temporal variation of bedload discharge on the same tractive force. As the result, it was shown that the sediment discharge was increase two or three order after the occurrence of debris flow even if the scale is small, and the bedload larger then about 2mm of particle size was on a declining trend since the Iijima No.6 Sabo Dam 40m high was completed in upstream of observation point in February 2006.

Key words�Fsediment discharge, field measurement, bedload, grain size distribution, temporal variation