DRAINAGE INTEGRATION AND CANYON INCISION - A HELLS CANYON STORY
Detailed study of tributaries along the Snake River reveal systematic slope break knickpoints. These knickpoints are spatially distributed on both the east and west sides of the canyon, rising in elevation to the south (upriver). We projected the relict reaches of these tributaries to their junction with the Snake river. These profiles indicate that, at most, ~1 km of incision resulted from the integration of Lake Idaho. Moreover, we reevaluated existing erosion rate data for mixing between pre-capture low-erosion rates in high elevation relict landscapes and high erosion rates in low elevation adjusted landscapes. We applied a simple mixing model to tributaries where erosion rates were measured by cosmogenic nuclides. This model yields erosion rates of 0.01 ± 0.02 and 0.28 ± 0.1 mm yr-1 on the relict and adjusted landscapes, respectively. These results are strong evidence of a transient landscape in Hells Canyon, which is responding to the rapid and catastrophic integration of Lake Idaho. Notably, this capture event alone cannot fully explain the relief in the canyon; however, we can now evaluate how the landscape responded to drainage integration, allowing for the rest of the Canyon’s relief to be evaluated with other methods.