In order to identify the process of formation and the form characteristics of step-pool systems in mountain streams, movable bed experiments were conducted by changing the rate of flow, the channel gradient and the bed materials. The findings are 1) the Talbot type grain size distributions of bed surface materials with an index n=1/2 to 3/4, is essential to the formation and stabilization of step-pool systems; and 2) medium-scale bed undulation, which is similar to alternating bars as often found in alluvial rivers, can develop in mountain streams, and formation of the medium-scale bed undulation subsequently promotes the formation of a step-pool system. In addition, 3) a law of resistance of flow during the formation of step-pool systems was specified, and formulae for estimating a wave length and a wave height of a step-pool system by using the resistance law were proposed.
