The flume experiments were carried out to examine the relationship between a log jam and an obstruction in a channel. Cylindrical wood pieces and bridges with a single pier were used as a model of floating woody debris and an obstruction, respectively. The results reveal that a log jam at the model bridge is determined by shaded area of an obstruction in a channel. An empirical equation for predicting the volume of a log jam at an obstruction was proposed. A log jam at the model bridge caused backwater rise. Backwater rise depends on the number of wood pieces trapped by a model bridge. Dependence of loss coefficient on trapped wood pieces is clearly shown. Loss coefficient is determined by blockage ratio of a jam at a model bridge.