Geometric scaling of step-pools in mountain streams: Observations and implications

Understanding the geometric structuring of alluvial step-pools is an enduring problem in mountain stream geomorphology. Many previous studies propose that the key control parameter governing the step-pool form is mean bed slope. As explored through several examples, however, we find that slope control formulations are inconsistently demonstrated for broad data sets. On this basis we reexamine the bed form architecture of step-pools with a new descriptive parameter termed the aspect ratio (α): active stream width divided by step drop height. We test α with data collected from three different mountain settings for step-pools observed over mean bed slopes ranging from 2% to 22%. We find that step wavelength and height are well described by α when these dimensions are normalized by the step drop height; furthermore, we observe that α varies inversely with mean bed slope. Context for our findings is provided by recent field and laboratory work which highlights that the cross-stream component to the overall flow structure in step-pools and scour pools is significant. This is important because it expands upon, and relates our work to formative models which are based on jet scour, and the associated three-dimensional flow structure of step-pools. Our work has practical implications for the design of step-pool channels because existing approaches are based in large part on structuring artificial systems according to a narrowly defined geometry. We observe, however, that under certain conditions this approach may yield constructed step-pools that are unstable. As an alternative approach, we propose that the geometric relationships developed here can be applied to identify artificial step-pool geometries that will be more fully reflective of natural analogs, and thus possibly more stable.