The Form and Function of Headwater Streams Based on Field and Modeling Investigations in the southern Appalachian Mountains
Adams, Rebecca Hope Kavage
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Headwater streams drain the majority of the landscape, yet little is known about their form and function in comparison to lowland rivers. Better understanding of their morphology and sediment transport processes will improve understanding of landscape evolution and promote a more complete view of fluvial systems. Therefore, the goal of my project was to determine controls on headwater channel form and function in the humid, moderate-relief drainage basins of the Valley and Ridge and Blue Ridge provinces in the southern Appalachian Mountains. I surveyed nine headwater (0.33 - 2 km2 drainage area) streams in a variety of bedrock, climate, base level, and land use conditions and produced a high-resolution dataset on their longitudinal and cross sectional form. This data was analyzed empirically to determine controls on channel form, and used in hydrologic modeling to determine the ability of the channels to erode their beds during regularly recurring flows as well as the recurrence interval of bankfull flows. Field survey results demonstrate that the channels are dominantly alluvial and vary greatly between and within channels in their overall longitudinal form, channel slope values, and grain size. These variations are due to differences in bedrock resistance at the formation level as well as at short wavelengths. Bedrock also controls channel form through its influence on local and regional base level, channel initiation processes, and log jam abundance. Hydraulic geometry, steam competence and bankfull flow recurrence also vary greatly between and within channels. This variation is due to the high sensitivity of the streams to hillslope influences such as bedrock resistance, boulder influx, and soil profile development. Increases in bedrock resistance within a channel create knickpoints that lower stream competence and slow hilllslope erosion. Stream competence is generally higher in channels with erodable bedrock and lower in channels with resistant bedrock, but most channels could entrain the majority of the grains on their bed at 2-year stormflows. Bankfull is a larger, less frequent flow than the 2-year storm at very small drainage areas (<0.4 km2), but is approximately a 2-year recurrence flow at larger drainage areas. Bankfull occurs less frequently in North Carolina Blue Ridge streams, due to deep soils that form on metamorphic bedrock under an more intense precipitation regime and have high rainfall storage capacity. Results indicate that variability is a fundamental feature of headwater streams and that they do not follow channel slope, hydraulic geometry, and bankfull relations developed in lowland river systems.
- Masters Theses