QUANTIFYING TRANSIENT INCISION IN CENTRAL IDAHO

Throughout the Salmon River watershed, there is an observable break in slope that separates the low relief (<400m) headwaters from the high relief (1200- 1600m) main stem of the Salmon River gorge. The break in slope is thought to mark the headward advance of the transient boundary between relict topography and refreshed topography that was triggered following base level lowering along the Salmon River during the late Cenozoic. We present preliminary cosmogenic, topographic, and numerical modeling analysis focused on quantifying incision rates and time in central Idaho. The ¹⁰Be cosmogenic radio-nuclide concentrations we present are from bedload sediment from ten different streams of the mainstem Salmon located between the South Fork and the Little Salmon. The samples are from both above and below the transient knickpoints and will be used to calibrate a river incision model for rivers draining the Idaho Batholith rocks. Using the calibrated model, we will quantify the migration of knickpoints from the mouth of the tributary to their present location in order to constrain a date for the last regional base level change in central Idaho.