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Abstract

Numerous mechanisms have been proposed for the formation of step-pools in steep streams. Which of these mechanisms is responsible for constructing step-pools may depend on flow conditions, with existing theory best developed for near-critical flow conditions. Under subcritical flow conditions, however, channel roughness has been observed to exert control on step formation. To improve representation of step-pool development under sub-critical flow conditions, we develop a quantitative model exploring step-pool construction in response to roughness variation. We incorporate variable roughness into a quasi-2D Saint-Venant Exner model in which steps emerge around roughness patches, consistent with the roughness-based mechanism observed in previous experimental work. We compare model results to a field site that developed step-pools in areas of high roughness in an initially plane-bedded reach. Model results provide a mechanistic link between roughness variation and step spacing. For disturbed rivers similar to our field site, the location of rough patches may help inform expectations for early step-pool evolution following river disturbance.