BEDROCK RIVER MORPHOLOGIES IN CENTRAL IDAHO DEMONSTRATE RECENT CHANGES IN ROCK UPLIFT

Bedrock rivers are sensitive to tectonic and geodynamic processes that influence rock uplift, and therefore bedrock river morphology can be used to gain insight into such processes. Erosion is also controlled by base level fall that is not directly related to tectonics or geodynamics, however, such as the drainage of large lakes. Central Idaho is a region where these different considerations are well demonstrated, as the drivers of ongoing transient incision are not well understood. Low-relief surfaces throughout the region could represent either a long-lived highland or a paleo-landscape disrupted by recent changes in rock uplift. Here, we test these competing hypotheses by analyzing the morphologies of individual tributaries in the Clearwater and Salmon watersheds to constrain the pattern of regional incision. We select tributaries underlain by single lithologies (granite, basalt, gneiss, siltite, and sandstone) to isolate the influence of rock properties, evaluate knickpoint elevations and transient incision depths along 70 individual tributaries, and support our interpretations of these data with both numerical model simulations and a previously published analytical model. Knickpoint elevations and incision depths increase from north to south across our study area, with ranges of about 900 to 2200 m for knickpoint elevations and about 300 m to 1200 m for incision depths, and the analytical and numerical models we present demonstrate that such a gradient represents spatial variations in rock uplift. These findings suggest that transience is driven by spatially variable increase in rock uplift that have disrupted a low-relief paleo-landscape and not driven entirely by incision along the Snake River. High steepness values along main drainages suggest that high rock-uplift rates are still maintained to the present. Changes in rock uplift may be related to buoyant support from the Yellowstone plume and/or lithospheric foundering facilitated by the Yellowstone plume, although base level fall from the drainage of the Lake Idaho down the Snake River may be superimposed over these patterns in rock uplift. We show that careful, quantitative analyses of river profiles in geologically complex regions can differentiate between the influences of rock uplift and far-field base level changes.