Quantifying Transient Incision to Determine the Timing and Style of Baselevel Change in Central Idaho

Abstract:

Throughout the tributaries of Salmon River watershed, there is an observable break in slope that separates the low relief (<400m deep valleys) headwaters from the high relief (1200- 1600m deep valleys) main stem of the Salmon River gorge. This transience was triggered by baselevel lowering along the Salmon River sometime during the late Cenozoic. However, the timing of the incision is unknown as is the mechanism that drove it. To provide insight into the drivers of landscape evolution in this region, we integrate field data with numerical modeling to constrain the timing of incision and gorge formation. We present ¹⁰Be cosmogenic radionuclide concentrations from bedload sediment in ten different tributaries of the mainstem Salmon located between the South Fork and the Little Salmon. The samples record basin wide erosion rates ranging from ~0.05mm/yr in relict topography to ~0.15mm/yr in refreshed topography over the last 10³-10⁴ yrs. These erosion rates combined with topographic metrics using channel slope and drainage area are used to calibrate a river incision model for rivers draining the Idaho Batholith rocks. Using the calibrated model, we quantify the migration of knickpoints from the mouth of the tributary to their present location in order to constrain a date for the regional baselevel change. Preliminary model results suggest an increase in the incision rate of the Salmon River occurred roughly 8 million years ago continues at the same rate today. Finally, we discuss the implications of this date in relation to the cessation of the Columbia River Basalts and the integration of the Snake River basin.