THE EFFECT OF STRUCTURE AND LITHOLOGY ON THE ASPECT RATIO OF FLUVIAL CHANNELS: A FIELD-BASED QUANTITATIVE STUDY OF THE NEW RIVER IN THREE GEOLOGIC PROVINCES

Fluvial channel geometry is controlled by the interaction of a number of geologic and hydraulic variables. The width of mixed alluvial-bedrock channels generally is a function of discharge, with variations due to local conditions. The aspect ratio (width/depth) of channels is heavily influenced by substrate size and erodibility. How channel width and aspect ratio vary as a function of other variables, such as structure, lithology, slope, large scale valley topography, and rock uplift, has not been fully quantified. The New River is ideal for examining these relationships because it shows considerable variability in width and aspect ratio and flows through three structurally and lithologically distinct geologic provinces. The New River flows northwards through the Appalachian Mountains and crosses the Blue Ridge, Valley and Ridge, and Appalachian Plateau geologic provinces. The Blue Ridge has a NE-SW structural grain with moderate variability in bedrock resistance. The Valley and Ridge also has a NE-SW structural fabric, but exhibits extreme variation in the erodibility of its tightly folded layers of the Valley and Ridge. The Appalachian Plateau consists of undeformed, gently dipping rocks which have been deeply incised due to caps of resistant stratigraphy. Through these provinces, the New River does not follow the expected trends of channel widening with increasing drainage area. Topographic maps show that the channel width of the New River has a significant variation that far outscores an overall widening downstream. Aspect ratios for the New River are also large, approaching 200. We are collecting a field data set of the river's channel geometry, to compare width, aspect ratio, and the other variables at greater detail than allowed by topographic maps. At numerous places in each province we are collecting cross sections to constrain the variations in depth and bankfull height and to characterize channel substrate, bedrock and bank stability. The data set will allow empirical analysis of how width and aspect ratio of the New River are related to different variables, including slope, discharge, flow velocity, curvature, trend, bedrock type, and structure.