Document Type
Thesis
Abstract
Knickpoints are recognized throughout the James River basin. This study focuses on the rock floor streams of the Tye, Piney and Rockfish basins in the Blue Ridge. The development and movement of knickpoints is controlled by three major factors: lithology, geologic structures, and propagating knickpoint forms responding to base level change. Streams incise in response to base level change. The stream power law for incision rates can be constructed using basin area as a proxy for discharge where area and slope are raised respectively to the m and n power. Area-slope plots of area and slope ratios of streams at junctions were constructed for the Tye River, Piney River, and Rockfish River to determine the ratio m/n. Similar m/n values of 0.20 and 0.23 were calculated respectively for the Tye and Piney basins. The Rockfish basin m/n value was higher at 0.4. Previous studies of m/n suggest that this may show that erosional mechanisms other than uniform downcutting are operating in the Tye and Piney basins. Longitudinal profiles were constructed for the James River and major tributaries, the Tye River and selected tributaries, the Piney River and selected tributaries, and the Rockfish River and selected tributaries. Knickpoints along the James River and major tributaries span different physiographic and geologic regions. A knickpoint similarly along Ruckers Run and the Tye River was determined to be propagating and not a response to structural or lithological variations. The knickpoint was correlated with a nearby CRN-dated fluvial terrace of 5.7 million years at 550 ft elevation to determine a propagation rate of 4600 meters per million years. The propagation rate is two orders of magnitude larger than the downcutting rate of 15 meters per million years calculated for the fluvial terrace. The high propagation rate suggests that the propagating knickpoint is a significant erosional mechanism in adjusting from an old graded profile to a new graded profile. The prevalence of knickpoints in the James River basin suggests that knickpoints may be a significant erosional mechanism in adjusting to a new graded profile. However, the different m/n values between the Tye basin and Rockfish basin and the scarcity of knickpoints in the Rockfish basin indicate that knickpoints may be a significant erosional mechanism in some basins and not as significant presently in others.
Date Awarded
2001
Department
Geology
Recommended Citation
Dubinski, Ian M., "Knickpoints in the James River" (2001). Geology Senior Theses. William & Mary. Paper 76.
https://scholarworks.wm.edu/geologyseniors/76
