The dominant dimension of the step-pool bed form is its spacing, which is determined by the locations at which large, step-forming grains are deposited. We conducted laboratory experiments involving active transport of widely sorted sediment, including large grains whose deposition readily formed steps. Direct observation of step development indicates that deposition of step-forming grains was predominantly associated with bed roughness impeding transport or with earlier flow and transport conditions not contemporaneous with step development. Neither mechanism is likely to occur at regular locations on the bed. Measured step spacing was found to follow a Poisson distribution, supporting the conclusion that steps do not develop in preferred or regular locations. This contrasts with previous arguments that regular step spacing develops in response to deposition forced at regular locations beneath water surface waves. We observe neither the regular distribution nor the associated depositional mechanism, although the observed step distribution is well defined with a strong mode that could be interpreted as indicating regular spacing.