THRESHOLDS BETWEEN BEDROCK AND ALLUVIAL CHANNEL FORMS ALONG THE ALPINE CANYON OF THE SNAKE RIVER

The transition along rivers from bedrock-eroding upper reaches to primarily sediment transport downstream is marked by the shift from a bedrock to alluvial channel. This transition is frequently marked by mixed bedrock-alluvial channels with multiple switches between forms. The thresholds and adjustments of gradient, width, sediment transport capacity, and the extent of alluvial cover in such transitions are poorly understood. The Alpine Canyon of the Snake River, crossing the flank of high terrain associated with the Yellowstone hotspot, has a braided, alluvial reach set between an upper and lower bedrock channel, providing a natural laboratory to document these changes and help address their controls. The transition between braided-alluvial and bedrock channels in Alpine Canyon was documented by sidescan sonar, aerial imagery, and sediment size distributions. The change between a bed 100% covered to one that is <50% covered occurs within ~500 m and is coincident with the river’s shifts from flowing across-bedrock strike to along-strike. The upper and lower bedrock canyons have distinctly narrow average widths of 87.2 and 76.5 m, compared to 532.3 m for the alluvial reach. Gradient varies less coherently, broadly increasing from 0.0018 to 0.0028 in the lower bedrock canyon. Thus, differences in unit stream power track width, being 137.3 and 273.0 watts/m² in upper and lower bedrock reaches and 37.4 watts/m² in the alluvial reach. The pattern of modest gradient change suggests it may be partly set by sediment load even in bedrock reaches, whereas width has adjusted strongly to bedrock and tectonic drivers. Sediment transport capacity calculations indicate that the maximum threshold discharge is below the ~2-yr RI, suggesting that the bedload is mobilized nearly every year. Because the bedload-sediment-transport capacity exceeds bedload sediment supply in bedrock reaches, transport capacity appears adequate to predict changes between channel forms here.