Influence of step composition on step geometry and flow resistance in step‐pool streams of the Washington Cascades

Abstract

Step‐pool streams dissipate flow energy primarily through spill resistance. We compared the geometry, step characteristics, and flow hydraulics of 20 step‐pool reaches without large woody debris (LWD) to 20 step‐pool reaches with LWD. Non‐LWD streams exhibited significantly shallower flows, lower steps, shorter step spacings, greater percentages of water‐surface drop created by steps, larger grain sizes, and smaller Darcy‐Weisbach friction factors. Grain resistance was negligible in both stream types. Form resistance created by irregularities in the channel shape associated with steps contributed more to the total flow resistance in LWD reaches. Although both stream types showed poor correlation between step height and flow resistance, the significant positive correlation between flow resistance and step height/length ratio in the non‐LWD reaches demonstrates the increasing effect of spill resistance with increasing step height. The lack of such a trend in the LWD‐loaded reaches suggests that spill resistance was highly influenced by a few large log steps in these reaches. LWD creates deep pools and increases flow resistance along step‐pool streams. It thus stabilizes channels and stores sediment in steep headwater streams recently scoured by debris flows.