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

Bedrock river morphologies reflect both a range of drivers (e.g., climate, tectonics, and their interaction with lithology) and the erosion processes allowed by these drivers (e.g., abrasion and plucking), yet quantitative relationships between morphology and drivers remain elusive. The Salmon River watershed of central Idaho presents an excellent natural experiment through which to quantify the lithologic influence on bedrock river morphology. This suitability is due to: (1) an ongoing transient adjustment of the landscape leading to a morphological contrast between the relict upper reaches and adjusted lower reaches; (2) the wide range of lithologies within the watershed (e.g., basalt, schist, granodiorite, quartzite, and gneiss); (3) the incision of certain tributary streams into only one rock type throughout transient adjustment; and (4) 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. Channel steepness and concavity from both slope-area and the slope-integral methods are considered. Results derived from either of these two methods generally agree, with some exceptions. Channels for a single rock type can exhibit a wide range of concavity values, but channels in certain lithologies favor relatively high or low values (e.g., basalt channels often have high concavities). For a given concavity value, the differences in steepness between relict and adjusted states are relatively similar for all lithologies. Channels underlain by different lithologies can also have higher or lower critical area values, which are defined by the minimum drainage areas for which slope-area data exhibit a linear trend in log-log space. This lithologic control on the critical drainage area suggests a characteristic balance between advective and diffusive geomorphic processes that is dependent on rock-type. We discuss the implications of these results in the context of different erosion processes. Quantifying relationships between the channel morphologies and drivers of bedrock rivers remains an important challenge in the geomorphological community, and the Salmon River watershed highlights the role of lithology in these systems.