Understanding the discharge capacity of channels and changes in hydraulic properties during large storms is essential for prediction of flash floods. However, such information is limited for steep mountain channels because of their complex nature and the lack of measured data. Thus, we obtained detailed water-level and surface-velocity data during large floods of a steep mountain channel, and documented how complex channel morphology affected water flow during large storms. We installed water-level and surface-velocity sensors at a cascade and at a pool that was 10 m downstream at the Aono Research Forest of the Arboricultural Research Institute of the University of Tokyo Forests in Japan. We successfully obtained 1-min interval data for a major storm with total precipitation of 288 mm that fell over 59 h and a maximum rainfall intensity of 25 mm/h. During the storm, height of the water surface from the deepest point of each cross section ranged from 0.35 to 1.57 m and surface velocity ranged from 0.35 to 4.15 m/s. As expected, the changes in flow depth, surface velocity, and velocity profiles were complex and differed even between the cascade and adjacent pool cross sections. Dramatic changes in flow conditions first occurred at the cascade when discharge increased to a certain point, when water suddenly stagnated at the foot of the cascade and submerged flow might have occurred. Thereafter, the water level increased remarkably but surface velocity and the velocity profile stayed almost constant at the cascade cross section. At the downstream pool, where most rocks were submerged at a mean water depth of 0.7 m, surface velocity suddenly increased dramatically and the velocity profile changed as very slow flow developed in the lower portion of the profile, while water levels increased only slightly. When the rainfall diminished, first, the surface velocity markedly declined, then the velocity profile returned to its original state at the pool, and then submerged flow at the bottom of the cascade ceased. These temporally and spatially detailed flow measurements effectively document and characterize flow conditions during a large flood in a steep mountain channel. These field data confirm the long held hypothesis that marked changes in flow conditions occur when steps become submerged. (C) 2016 Elsevier B.V. All rights reserved.