Morphological effects of local scouring in step-pool streams

Step-pool morphology characterizes many high-gradient streams in a variety of natural settings, but formative processes and evolutionary dynamics are still poorly understood. In this paper, natural step-pool geometry is compared 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 due to falling jets forms plunge pools under each structure, analogous to natural steps determining the formation of pools. In order to test the hypothesis that natural pools are analogous to pools formed below grade-control works with respect to their dimensions, shape and formative dynamics, 37 natural pools and 73 artificial pools were surveyed in 10 mountain streams of the eastern Italian Alps. Pools below grade-control works featured a transitional zone between the scour hole and the downstream sloping bed, marked by a depositional berm. When geometric parameters such as maximum pool depth, length and step-berm distances are normalized to the jet virtual energy, no statistically significant differences were detected between natural and artificial systems. These results lend support to an upstream-forced cascade model for step-pool formation, where the energy of failing jets controls the geometry of the pools, and is therefore regarded as the most important scaling-independent variable. Copyright (c) 2005 John Wiley & Sons, Ltd.