Characteristics of log and clast bed-steps in step-pool streams of northwestern Montana, USA

The role of log and clast steps was studied in 53 step-pool reaches in 32 streams in northwestern Montana. In each reach, 20 consecutive steps were sampled. Drainage areas ranged from 0.5 to 11.5 km2. Mean step spacing was found to be proportional to channel gradient, width of the bankfull channel, and drainage area; mean step height was proportional to the width of the channel. No significant difference exists between clast and log steps with regard to step spacing and step height. This suggested either that (1) woody debris is mobile in even the smallest channels in the study area, or (2) channel flows arrange clast steps around immobile woody debris. Calculations of theoretical entrainment and transport of sediment suggest that the coarse clasts forming steps are capable of being entrained at bankfull discharge in most channels. The presence of immobile log steps (composed of living roots) in the upstream reaches, an increase in the percentage of log steps with gradient, and a decrease in this percentage with the width of the bankfull channel and drainage area, imply that woody debris is mobile primarily in the downstream reaches. Calculations of flow resistance suggest that most steps serve to maximize flow resistance. These results support the second hypothesis, that the height and spacing of clast steps are adjusted around immobile log steps to maintain consistent step characteristics.