THE FORM AND FUNCTION OF HEADWATER STREAMS BASED ON FIELD AND MODELING INVESTIGATIONS IN THE SOUTHERN APPALACHIAN MOUNTAINS

Non-perennial headwater channels commonly dominate water and sediment transport in mountainous regions, 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. This is particularly true in the southern Appalachian Mountains, where headwater streams drain the majority of the landscape. Therefore, we determined controls on headwater channel form and function in the Valley and Ridge and Blue Ridge provinces of Virginia and North Carolina. We surveyed nine headwater (0.33 – 2 km^2 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 set was analyzed empirically to determine controls on channel form. We also employed hydrologic modeling to assess bankfull recurrence intervals and the ability of channels to erode their beds during regularly-recurring flows.

Field survey results demonstrate that the channels are dominantly alluvial and vary greatly in their longitudinal form and grain size. This variation in channel form is due mainly to short wavelength differences in bedrock resistance, as well as at the formation level. Channel function during storm flows also varies greatly between and within channels, due to their high sensitivity to hillslope influences such as bedrock resistance, boulder influx, and soil profile development. Bankfull channel size and channel width and depth do not increase regularly downstream, unlike larger lowland rivers. Furthermore, modeled 2-year storm flows do not fill the bankfull channel form at very small drainage areas (<0.4 km^2). This may indicate a shift in channel-forming flow recurrence to rarer events in the upper reaches of the channel network. The mobile percentage of the streambed is higher in channels with erodable bedrock and lower in channels with resistant bedrock, as the resistant bedrock forms knickpoints and mantles the channel with boulders, slowing hillslope erosion. Results indicate that variability is a fundamental feature of headwater streams, and that their form and function generally do not follow established drainage area relations for fluvial systems.