The morphologic structure of step–pools in mountain streams

Abstract

Step–pool sequences in streams in the Santa Monica Mountains of southern California impart a characteristic morphologic structure that varies with slope. But despite the strong association with slope that suggests a direct process control, analysis of 464 step–pools in 13 study reaches reveals the relationship to reflect correlation, but not necessarily causality. The direct control on step height is particle size, whereas discharge is the probable causal process determining step wavelength. Because both particle size and discharge generally change with channel slope downstream, a slope variation with step wavelength and height is the apparent indirect geometric result. A conceptual model of process-form linkages is proposed based on these results, whereby the step–pool morphology is produced by positive relationships between wavelength and discharge, and between height and particle size. The model offers new insights for explaining the step–pool morphology, and it is potentially useful in channeling future efforts toward the appropriate scales and variables.