The Effect of Structure and Lithology on Aspect Ratio of Fluvial Channels: A Field-Based Quantitative Study of the New River in Three Geologic Provinces

Abstract

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. Through these provinces, the New River does not follow the expected trends of channel widening with increasing drainage area. Topographic maps show that 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 500. We collected a field data set of 29 sites of the river's channel geometry, along with characteristics of bedrock, sediment, and confinement. Fifteen of the 29 sites are bedrock reaches. The data set allows 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. Sediment characteristics and confinement of the channel do not affect channel morphology. Bedrock is shown to affect channel width directly through the percent of bedrock exposed in the channel and indirectly through the modified rock mass strength, rock hardness, obliquity to regional strike, dip orientation, and degree of joint intersection.