The Periodic Nature of Step-pool Mountain Streams

Rhythmic sequences of steps and pools in steep mountain streams have captured the attention of numerous workers, but whether periodicities exist in the occurrence of step-pools has not been investigated in a comprehensive way, and the implications for such periodicities have not been explored. Spectral analysis was applied to bed elevation data from streams in the Santa Monica Mountains of southern California to investigate periodicity in the occurrence of step-pools. Results indicate significant step-pool periodicities, confirmed by independent field measurements, that vary from 0.43 channel widths to 2.40 channel widths. Spectral analysis of channels affected by external influences tends to indicate multiple significant peaks that reflect higher variance in step-pool spacing. Underlying periodicities are nevertheless detected, suggesting that external factors obscure but do not destroy the effects of more general internal mechanisms. Periodicity in the occurrence of step-pools suggests that step-pools are bedforms that represent a fundamental mutual adjustment between flow, channel morphology, and energy expenditure. A rhythmic channel thalweg supports the notion that steppools are analogous to meandering in the vertical dimension, and implies that, because step-pool channels are not able to adjust energy expenditure in the plane dimension, they instead adjust boundary roughness in the vertical dimension. A continuum of rhythmic gravel bedforms that displays the results of such adjustments, and that shows how step-pools merge into pools and riffles in the stream channel system, is discussed and illustrated with empirical data from the Santa Monica Mountains. Although many differences exist between steep mountain streams and broad alluvial rivers, the findings of this study suggest that generalized principles of uniform energy expenditure may also apply to rhythmic step-pool streams. These findings are potentially valuable in the management and design of steep channels.