Cosmogenic Erosion Rates and Calibrated Incision Models in the Northern U.S. Cordillera Reveal the Progression and Drivers of Landscape Transience

Abstract:

Landscapes change over time due to variations in factors such as tectonics and climate. By understanding landscape evolution, we can use landscape morphology to extract information regarding geologic history. The ongoing transient incision in the Clearwater and Salmon watersheds in central Idaho (U.S.) offers such an opportunity. The north to south increase in incision over this region may indicate spatially variable rock-uplift rates, which could be caused by interactions between the lithosphere and nearby Yellowstone plume. Here, we present 10Be cosmogenic radionuclide concentrations for 17 new samples of fluvial sediment taken from relict and adjusted streams from across the region. We also combine our new samples with 32 10Be samples from previous studies. These concentrations reveal significantly higher erosion rates within the adjusted landscape than within the relict. For example, adjusted erosion rates along the mainstem Salmon River (from Riggins, ID to North Fork, ID) are about 0.09 mm/yr, while the relict erosion rates in the area are about 0.045 mm/yr. Conversely, adjusted erosion rates along the Upper Salmon (~17 km east of Stanley, ID) are only about 0.05 mm/yr. This variation in erosion rates may reflect drainage capture or spatiotemporal rock-uplift patterns due to plume-lithosphere interactions. Using these relict and adjusted erosion rates, we calibrated stream power incision models to explore spatial variations in the timing of incision. We use a misfit function to assess model accuracy over time, where the estimated onset of incision is provided by the time at which models best reproduce the observed transient profiles. Although many studies assume a linear relationship between fluvial erosion and channel slope, our results indicate a nonlinear relationship in central Idaho rivers, with erosion scaling with channel slope to a power less than one. We show that combining landscape morphology analyses with cosmogenic erosion rates and calibrated incision models can reveal the progression of landscape transience across a large region. Finally, we illustrate that the character of this progression can be used to distinguish between different drivers of transient incision.