Abstract
　Mechanisms of debris flow movement were analyzed through observations made in the headwater area of the Urakawa River, one of the debris flow-haunted area in Japan. Debris flows were observed by a visual recording system with video camera set up near the flow source, while the muddy flow associated with the debris flows was sampled downstream and its to 1180 g/l, which is nearly ten times higher than that in flows not associated with debris flows. This marked difference was due to the relatively high proportion of suspended particles of diameter 0.25 mm and larger. The analysis of the recorded video tape that in some of the debris flows, several flow heads appeared near their origin within a few minutes. In some cases the debris flow head was preceded by high density muddy flow. It was estimated that the debris flow develops into fuIl strength as its head alternately advances and hauls. This hypothesis was ascertained later by analysis of debris flow traces.

*名古屋大学農学部
Faculty of Agriculture, Nagoya University