A debris flow due to typhoon No. 11 occurred at Higashinokawa river basin of Wakayama Prefecture southern part in 2001. Debris flow was trapped and the expansion of the overflowing debris flow was controlled by the trees in debris flow deposit area. The form of the trapping of debris flow by the tree is divided into two types. One is that boulders are trapped by boulders interlocking each other between the trees (type 1). The other is that boulders are trapped by boulders by log crosspieces due to splintered trees and fallen trees (type 2). To control overflowing of debris flow in this basin, It was found out that 30m of average forest length along stream line was necessary.
