DRAINAGE INTEGRATION AND CANYON INCISION - A HELLS CANYON STORY

Hells Canyon is the deepest gorge carved by a river in North America, with 2700 m of relief from valley floor to the peaks of the Seven Devils in Idaho. The canyon must post-date the 16.4-15.9 Ma Grande Ronde flows of the Columbia River Basalts (CRB) through which it is cut. Previous researchers surmised that canyon formed due to the late Miocene to early Pliocene draining of Lake Idaho and subsequent integration of the Snake River into the Columbia River watershed. This nearly 65-year-old hypothesis has not been thoroughly examined with modern geomorphic tools. We seek a landscape signal of this integration that would have increased the drainage area funneled through Hells Canyon by 2 order-of-magnitude.

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.