Existing knowledge about step-pool structures and their application to river engineering in mountain rivers

Here, we review existing knowledge about step-pool structures in mountain rivers with reference to their morphology, process of formation, destruction, hydraulic function, restoration, and ecological function. First, we present the constructive factors of steppool structures and their appearance, focusing on channel bed profile. Second, we summarize the processes driving the formation and destruction of step-pools, concentrating on current and sedimentation. The untidune and keystone theories are discussed as the key concepts of step-pool formation in relation to the installation of artificial step-pool structures. Third, with reference to previous studies, we discuss the relationship between step-pool structures and river course characteristics. Fourth, we describe the law of resistance and velocity in a mountain river. Finally, with reference to their ecological function, we discuss the use of step-pool structures in restoration projects. Based on earlier studies of step-pool structures, we discuss future issues and propose studies to technologize and standardize step-pool structures for river improvement and restoration.