Controls on the Form and Function of Headwater Channels in Humid, Moderate-Relief Drainage Basins

Non-perennial headwater channels commonly dominate water and sediment transport in mountainous regions. However, little is known about the behavior and evolution of headwater channel systems or their coupled interaction with perennial trunk streams. To better understand landscape evolution or topographic responses to tectonic and climatic change, the controls on headwater channel form and function must be more tightly constrained. This is particularly true in the southern Appalachian Mountains, where non-perennial headwater streams drain the majority of the landscape. In the Valley and Ridge, for example, drainage networks form a trellis pattern that consists of perennial trunk streams draining only the narrow valleys, while ephemeral/intermittent tributaries are responsible for draining all of the adjacent hillslopes. Our goal is thus to gain an understanding of the controls on headwater channel geometry (longitudinal and cross sectional) and function (discharge and sediment transport). We have investigated nine channels in a range of lithologic, structural, climatic, and base-level-controlled relief conditions in the Valley and Ridge and Blue Ridge provinces in Virginia and North Carolina. Several of these drainage basins also provide control on the effect of historic logging, which occurred across the entire Appalachian landscape and must be considered as a potential agent of short-term channel change. The channels are first to third order, with drainage areas of 0.33-1.6 square km and relief of 134-559 m. Longitudinal and cross sectional channel form, channel substrate, and in-channel woody debris were surveyed in detail from the trunk streams to the channel head. Results show that systematic variations of channel geometry with drainage area are disrupted by the presence of resistant bedrock and woody debris. Sporadic resistant units create steep, bedrock-floored knickpoints that control the upstream gradient and narrow the channel banks and adjacent valley slopes. Knickpoints also force finer sediment to accumulate upstream while supplying coarse material for downstream reaches. Short wavelength variations in bedrock resistance (grain size, cementation, etc.) thus control channel form and function and ultimately influence landscape denudation. Woody debris exerts the greatest control in low gradient channel reaches, where it decelerates flow and traps fine sediment, thus altering longitudinal form. Sediment storage by woody debris appears insignificant based on regional erosion rates, however, and surprisingly does not appear to correlate with logging history. Hydrologic modeling of runoff and discharge further demonstrates the influence of bedrock, climate, and woody debris on the recurrence interval of effective sediment-transporting flows.