Analysis of changes in the step-pool morphology and channel profile of a steep mountain stream following a large flood

The step-pool morphology of steep mountain streams is reorganized during large floods by the removal of steps, scouring of the channel bed and the formation of a new step-pool sequence by the re-deposition of step-forming grains. In this paper we document these changes caused by the largest recorded flood (return period ~ 50 years) in the steep mountain stream Erlenbach in central Switzerland in 2007 by comparing pre-flood and post-flood surveys of a ~ 650 m long study reach. We show that the flood led to statistically significant changes in the distributions of step height and spacing, most notably an increase in the variance of both variables. Organization at the river reach scale is not apparent, and the correlations between step properties both pre- and post-flood are generally weak. The flood led to widespread erosion of the river bed. Locally up to 2–3 m of vertical channel change was observed. At least 60% of the steps moved, even the largest ones; however they remained constrained to the parts of the study reach where landslides on the adjacent hillslopes are most active. Detailed changes in bed structure were analyzed from the pre- and post-flood profiles with a statistical scaling approach which identifies the complexity of the spatial organization of sediment in the longitudinal profile. Multiscaling found in both step and pool sequences reflects the complex spatial structure which is the result of the superposition of steps formed by the history of flood events.