EXPLORING THE LITHOLOGIC INFLUENCE ON BEDROCK RIVER MORPHOLOGY THROUGH LANDSCAPE TRANSIENCE IN THE SALMON RIVER WATERSHED OF CENTRAL IDAHO

Bedrock rivers exert fundamental controls on both mountain ranges and landscape evolution by transmitting changes in baselevel and transporting eroded material. Bedrock river morphologies reflect both a range of drivers (e.g., climate, tectonics, and lithology) and the erosion processes resulting from these drivers (e.g., abrasion and plucking), but quantitative relationships between morphology and drivers remain elusive. The Salmon River watershed of central Idaho presents a natural experiment through which to quantify the lithologic influence on bedrock river morphology. This suitability is due to: (1) the ongoing transient adjustment of the landscape due to a recent (~10 Ma) increase in rock uplift; (2) the morphologic contrast between the relict upper reaches and adjusted lower reaches; (3) the wide range of lithologies within the watershed (e.g., basalt, schist, granodiorite, quartzite, and gneiss); (4) the transient incision of certain tributary streams into only one rock type; and (5) the relatively uniform climate shared by all tributaries due to their proximity. We present preliminary results comparing the morphologies of relict and adjusted streams underlain by different lithologies. Morphological characteristics considered include channel steepness and concavity from both slope-area and the slope-integral methods. For example, relict channels underlain by both gneiss and basalt have much higher concavities than relict channels underlain by granodiorite (~0.8 vs 0.5). Additionally, we explore the distance knickpoints have propagated and the extent of knickzones (i.e., sharp vs. diffuse) within different lithologies. Finally, we discuss why different rock types might lead to different morphologies. For example, variations in the dominant mechanisms of erosion (i.e., abrasion plucking) may lead to different erosional efficiencies and morphologies. Quantifying relationships between the channel morphologies and drivers of bedrock rivers remains an important challenge in the geomorphological community, and the Salmon River watershed of central Idaho provides an opportunity to explore the lithologic component of these factors.