Abstract
Step pool morphology characterizes many high-gradient streams in a variety of natural settings, but its formative processes and evolutionary dynamics are still poorly understood. In this paper step pool geometry is investigated from the perspective of a comparison with steep alluvial channels where grade-control structures such as check-dams and bed sills make the stream profile resemble a natural stepped stream. Along these channels, local scouring phenomena due to falling jets form plunge pools under each structure, as natural steps determine the formation of pools. In order to test the hypothesis that natural pools and pools formed below grade-control works present analogies with respect to their dimensions, shape and formative dynamics, 37 natural pools and 79 artificial pools have been surveyed in 11 mountain streams of the Eastern Italian Alps. Pools below grade-control works featured a transition between the scour hole and the downstream sloping bed marked by depositional berms. When geometric features such as maximum pool depth, length and step/berm distances are normalized to the jet virtual energy, almost no statistically significant differences have been detected between natural and artificial systems. These results lend support to an upstream-forced cascade model for step pool formation, where falling jets energy controls the geometry of the system, hence representing the most important scaling independent variable.