The recent paper by Larsen et al. (2004) provides interesting new data that extend concepts developed for the Grand Canyon's Colorado River to the Green River, Utah. However, previous work is misrepresented and an internal inconsistency causes confusion about the conclusions.
In Larsen et al. (2004, p. 309 and 312) it is stated that: (1) “According to Kieffer (1985), reworking takes place only during very large discharges,” and (2) “Our data [that of Larsen et al.] do not support the conceptual model of Kieffer (1985) who suggested that large floods are required for reworking.” To the contrary, in discussion of their Figure 12 (reproduced here as Fig. 1), Keiffer stated (1985, p. 402 and 403): “Floods of differing sizes and frequency erode the channel to progressively greater widths, as shown in Figures 12C, 12D, and 12E [here 1C, 1D, 1E]. Small floods (Fig. 12C) enlarge the channel somewhat. Moderate floods (Fig. 12D) enlarge the channel further…Rare large floods (Fig. 12E) carry this process further…The shape of the main stream at a debris fan at any instant of geologic time therefore reflects the contouring that occurred at the maximum discharge of the river since the last emplacement episode in the history of that debris fan, unless sedimentation of the finer-grained, more transient material partially mask the larger scale erosion.” It was never stated or implied in Kieffer (1985) that “large floods are required for reworking, or that “reworking takes place only at large discharges” (Larsen et al., 2004).
In defining constriction ratios “as the ratio of the upstream width to the channel width at the constriction (Kieffer, 1985),” Larsen et al. (2004, p. 311) have misquoted Kieffer. The Kieffer definition was in fact the inverse of the Larsen et al. definition. Kieffer (1985, p. 397) states, “The ratio of the width at a cross section taken through the constriction to upstream width in region 0 (w2/w0) will be called the constriction of the river.” Thus, the statement in the abstract of Larsen et al., “Peak discharges…decreased fan constrictions,” is inconsistent with the text (p. 311) where Larsen stated that “there was 10 m of lateral bank retreat that widened the constriction ratio from 0.25 to 0.33.”
In spite of the confusion caused by the wrong definition of a constriction ratio, several statements in Larsen et al. (2004, p. 309, 311, and 312) directly support the Kieffer (1985) concept: (1) “Fans whose surfaces had been aggraded decades earlier and had already been inundated by prior floods were not significantly reworked in 1996,” (2) “No reworking [of the Wild Mountain Debris fan] occurred during subsequent smaller floods,” and (3) “These results indicate that lateral bank erosion is an important process in reworking. Once significant reworking has occurred, subsequent smaller floods accomplish little geomorphic work.”
These statements support the conclusion by Kieffer (1985, p. 385) that “each new level of record high discharges caused the river to erode a channel of sufficient width to reduce flow velocities below a threshold value required for movement of the larger boulders of the debris fan, thus contouring the fan toward a configuration more in equilibrium with the high discharges.” This is precisely the picture given in Figure 1.