Log step and clast interactions in mountain streams in the central Cascade Range of Washington State, USA

Field surveys of log steps in high-gradient streams investigate the interactions between log steps and clasts to assess whether second-growth wood can form log steps capable of diverting flow and acting as significant roughness elements similar to those formed by old-growth wood. We measure the functional log diameter and step height, as well as visually estimate the clast size in the step of 102 log steps in 15 high-gradient streams in the central Cascade Range of Washington, USA. We compare step height data to measurements of maximum bankfull depth to measure the capability of log steps to divert flow relative to their clast size and log diameter. Step height positively correlates with functional log diameter (p < 0.001) and clast size. Additionally, a threshold clast size exists above which functional log diameter more strongly correlates with step height, indicating that clast size can limit step height. We find that steps with cobble or larger (> 64 mm) clasts in the step are 3.7 times more likely to reach or exceed bankfull depth than steps with smaller than cobble clasts in the step. We conclude that it is necessary for steps formed by small logs to have large clasts in the step in order to reach bankfull depth, act as significant roughness elements, and create flow diversions, implying that the potential for second-growth forests to maintain log steps similar to those formed by old-growth logs depends on the size of bed material.