Reach-Scale Channel Adjustments to Channel Network Geometry in Mountain Bedrock Streams

Channel network geometry (CNG) is a critical determinant of hydrological response and may significantly affect incision processes within the Appalachian Plateau near Richwood, West Virginia. The Williams, Cherry, and Cranberry Rivers share drainage divides and their lower reaches flow atop resistant, quartz-rich sandstones. The lower two-thirds of the Cranberry and Williams Rivers display linear profiles atop the sandstones; whereas the Cherry is concave upwards atop the sandstones. Because lithologies and geological structures are similar among the watersheds, we tested whether differences in CNGs explain the profile shapes and reach-scale channel properties. Specifically, we quantified CNG by calculating reach- specific area-distance functions using DEMs. The area-distance functions were then converted into synthetic hydrographs to model hydrological responses. The Cherry River exhibits a classic dendritic drainage pattern, producing peaked hydrographs and low interval transit times. The Cranberry River displays a trellis-like drainage pattern, which produces attenuated hydrographs and high interval transit times. The upstream reaches of the Williams River have a dendritic drainage pattern, but the lower two-thirds of the watershed transitions into an elongated basin with trellis-like CNG. Reach gradients are steeper in the lower reaches of the Williams and Cranberry Rivers where hydrographs are attenuated. In contrast, peaked hydrographs within the Cherry River are associated with lower reach gradients despite resistant sandstone channel beds. Trellis-like CNG may restrict the ability of downstream reaches within the Williams and Cranberry Rivers to achieve the critical discharge needed to cause incision during floods (all other things being equal). If so, increased reach gradients may be hydraulic adjustments that compensate for comparatively low discharges. We are now applying the synthetic hydrographs to HEC-RAS flow models generated from field channel surveys in order to analyze whether stream power and shear stress are adjusted to reflect CNG at the reach- scale. These models are compared to those with discharges calculated using drainage area and precipitation totals alone. We conclude that gradients in bedrock mountain streams may reflect basin-scale hydrology (CNG) and not simply local geological or geomorphic factors. This challenges the conclusions of others who ascribe local channel adjustments to: i) lithology and structure alone, or ii) local colluvium grain sizes.