CHANNEL WIDTH VARIATIONS ALONG ROCKBED CHANNELS CROSSING MULTIPLE LITHOLOGIES, UPPER JAMES RIVER BASIN, VIRGINIA

We examine variations in channel width across variable rock resistance in rock-bed channels in the upper James River basin, Virginia. Fluvial bedrock erosion is a primary geomorphic process driving landscape evolution, and a key component of landscape evolution models. However, relatively little empirical data is available on how and why channel geometry, particularly channel width, varies in rockbed channels. If a river is to reach steady state with respect to incision rates, unit stream power must vary in response to spatial variations in rock resistance and/or uplift rates. Variation in unit stream power is accommodated through changes in river discharge, width, and/or slope. In this study, we focus on how width varies in rockbed rivers crossing rocks of variable resistance, and the relative importance of slope and width in controlling unit stream power. At roughly 200 rockbed stream cross-sections in the upper James River basin, we have obtained channel width during the 100 year flood from FEMA Flood Insurance Study maps and channel slope from profiles constructed from USGS 7.5’ quadrangles. Combined with estimates of the 100 year flood discharge made using drainage area above each cross-section, we calculate unit stream power for each cross-section. Measured channel widths vary from 60m to 200m, and bedrock lithologies include sandstone, limestone, dolomite, and shale. Assuming erosion rates are similar in each cross-section, stream power may be used as a proxy for resistance, allowing us to examine the extent to which stream width varies as a function of bed rock resistance. We will also examine the relative importance of width and slope variations in determining unit stream power along stream reaches.