Abstract
In general, a decrease in stream flow (water level) in river-falling stage is accompanied by decrease in load concentration. But, looking over the state of the concentration change in detail, water level that abnormally have a great deal of concentration for a short time were observed several times. It has been considering, that this phenomenon is caused by the influence of the confluent tributaries.
Observation on the River Arakawa in Saitama showed, however, that it is caused particularly by avariance of the secondary (helical) flow motion. Also, the water level emerging notably variable of its motion was about 3 meters on the surveyed are a. In such a water level, concentration of suspended load become abnormally high, because of an influence, the writer thinks, on the increase of the drag force due to turbulence of stream line and on the variance of environment affecting the river bed.
