Systematic oversteepening in longitudinal profiles of mixed bedrock-alluvial channels at tributary junctions : Appalachians, Virginia

Abstract

Certain mixed bedrock/alluvial channels located in the Valley and Ridge province of the Appalachians in Virginia were identified as having a pattern of systematic oversteepening of channel gradients at tributary junctions. Where drainage area increased, channel slopes were either increasing or remaining constant. Subsequent investigation of 10m resolution digital elevation models found this pattern to be widespread throughout several large nearby drainage basins (A [approximately] 10⁸m²). Several hypotheses for the causes and constraints of the pattern were tested in two ways: (1) digital profiles were compared to pre-existing data sets for grain-size, channel width, lithology, and drainage area; (2) a short field venture was conducted to test the accuracy of the DEMs and to provide additional data sets such as grain-size and channel width against which to compare the digital longitudinal profiles. Results show that there is some correlation between lithology, drainage area, and a pattern of downstream fining. However, the relationships are not strong and begs an analysis of the region at large to explain this channel gradient phenomenon. Periodically high levels of sediment flux moving through the drainage system, eg. debris flows, are a promising mechanism for the initiation of systematic oversteepening in the longitudinal profiles.