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Reply to comment by Luis A. Giménez‐Curto and Miguel A. Corniero Lera on “Characteristic dimensions of the step‐pool bed configuration: An experimental study”

[1] We thank Giménez-Curto and Corniero Lera [2006] for their interesting application of jet flow modeling to the problem of step-pool spacing. We find that their extension of results from sand bed streams to gravel-bedded step-pool streams provides a useful physical explanation for the minimum spacing between steps, but argue that conclusions regarding the mean spacing are not consistent with our observations. [2] Curran and Wilcock [2005] defined the exclusion zone as the region immediately downstream of a step within which step-forming clasts are unable to deposit and develop a new step. The exclusion zone effectively establishes a minimum step spacing. While it is clear from our observations that the pool immediately downstream of the step is part of the exclusion zone, the presence of the pool does not explain the total length of the zone. Combining an equation for maximum bed form steepness with flow resistance in jet regime flow, Giménez-Curto and Corniero Lera develop a relation (their equation (5)) in which minimum step spacing is a function of step height, flow velocity, and bed slope. We used this expression (their equation (5)) to calculate the minimum step spacing for all of steps observed in our runs, and there is good agreement with our observed values. With only a small number of exceptions, the calculated minimum step spacing values fall between 20 and 30 cm (average 23 cm, range of 18–58 cm). Giménez-Curto and Corniero Lera have provided a useful physical explanation for the existence of the exclusion zone and a means for predicting the minimum step spacing that will result for a specified step height, bed slope, and discharge. [3] To develop a relationship describing mean step spacing, Giménez-Curto and Corniero Lera combine expressions for minimum spacing and maximum bed form steepness. Giménez-Curto and Corniero [2003] introduces an expression for the maximum possible steepness of a bed form in a steep channel where flow separation occurs and the jet flow regime dominates. Although developed for sand bed forms, Giménez-Curto and Corniero Lera extend the relation to steps in gravel bed channels. A bed form has a limiting maximum steepness, and hence step height, determined by the angle of repose of the bed sediment. We find that the angle of repose is not likely to be a dominant parameter in step-pool systems where the step height is controlled primarily by the size of the step-forming clast. The step heights reported by Curran and Wilcock [2005] scale directly with the size of the step-forming grain. This finding is in agreement with numerous other studies of steps pool systems from both the laboratory and the field [Abrahams et al., 1995; Billi et al., 1998; Chin, 1999; Grant, 1994; Grant et al., 1990; Judd, 1963; Judd and Peterson, 1969; Lenzi, 2001]. To support their hypothesis of a limiting bed form steepness, Giménez-Curto and Corniero Lera plot the ratio of L/Lmin against H/L, and derive a formal relationship (their equation (7)). Figure 1 reproduces their Figure 2 but includes data from all the step formations observed during our runs (described by Curran and Wilcock [2005]). Giménez-Curto and Corniero Lera