In the stretch of the Green River from Flaming Gorge Dam to the Gates of Lodore, Red Creek and Vermillion Creek, though small, stand out as the two largest tributaries. Draining similar areas with similar annual flow (Grams and Schmidt 2005), one might expect the two creeks to have similar, if not identical, impacts on the Green River. According to the Network Dynamics Hypothesis (Benda et al. 2004), confluence effects can be predicted based on the size of the tributary basin versus that of the mainstem; the larger the tributary drainage relative to that of the mainstem, the larger the frequency and magnitude of the resulting geomorphic impacts. This is expected since larger tributary basins should carry more sediment, and thus create larger fans, while larger rivers should be less affected by the fans and wash them away more easily. Both Red Creek and Vermillion Creek lie in the portion of the Green River in which flow is primarily controlled by Flaming Gorge Dam discharges, and so their drainage relative to the mainstem Green River should be very similar as well.
So how, then, do the geomorphic impacts of Red and Vermillion Creeks compare? At Red Creek (Figures 1 and 2), significant debris fan deposits have accumulated along the channel at the mouth of the canyon and a distance downstream; it does not appear as though post-dam floods are sufficient to significantly rework or remove these deposits. Consequently, the channel has become constricted at the confluence, and for some distance downstream. Vermillion Creek (Figure 3), however, does not appear to have caused any geomorphic impacts of consequence. Some finer sediment has accumulated at the confluence mouth, but no fan has formed, as the mainstem Green appears to have easily shorn off any protruding fine-grained sediment that may have accumulated.
The difference, of course, is that Red Creek enters the Green from Red Canyon, a debris- fan dominated canyon, whose steep canyon walls provide plenty of large sediment along with the high gradient necessary to transport it. Vermillion Creek enters from Brown’s Park, which is dominated by finer alluvial sediment on low gradient slopes. So, obviously, gradient and geologic setting are important factors to consider when predicting the magnitude and frequency of confluence effects.
Figure 1. Red Creek, with very little flow in mid-June, trickling over the debris fan surface. (Photo by Rene Henery).
Figure 2. Green River at top of the Red Creek debris flow constriction. Boulders and cobbles have obstructed the channel, creating a constriction that extends some distance downstream (Photo by Rene Henery).
Figure 3. Vermillion Creek, with little to no flow occurring in mid-June. Some sediment deposition has occurred, but not enough to significantly affect the main channel. Post-dam flows appear to be sufficient to remove finer grained sediment carried through the alluvial valley of Brown’s Park (Photo by Robert Thompson).
References:
Benda, L., Poff, N. L., Miller, D., Dunne, T., Reeves, G., Pess, G. and Pollock, M. 2004a. The network dynamics hypothesis: How channel networks structure riverine habitats. Bioscience 54(5): 413-427.
Grams, P. E. and Schmidt, J. C. 2005. Equilibrium or indeterminate? Where sediment budgets fail: Sediment mass balance and adjustment of channel form, Green River downstream from Flaming Gorge Dam, Utah and Colorado. Geomorphology 71(1-2): 156-181.