Introduction: Fluvial Processes in Small Southeastern Watersheds

The seven papers in this special issue resulted from two special sessions by the same name, at the Southeastern Division of the Association of American Geographers (SEDAAG) in Knoxville, Tennessee, in November 2009. The sessions were convened to address research involving fluvial processes and form in small headwater streams, and the papers represent a subset of the authors that were invited to present at those sessions. The sessions were well attended and stimulating, and we were delighted at the large number and high quality of papers that resulted. We believe that this reflects the growing importance of small watershed research and the need to apply this type of research to basins in the Southeast.

Small streams drain most of the land surface in the United States and are strongly coupled to adjacent hillslopes and catchment surfaces, so they drive runoff generation, sediment production, and the cycling of organic materials and nutrients. Yet, until recently, most research on fluvial systems has been focused on relatively large rivers. The study of small watersheds is of growing importance for many reasons. Regulations of water quality and aquatic ecosystems arising from legislation such as the Clean Water and Endangered Species Acts have provided incentives for state and local governments to study headwater streams. Improved flood hazard analyses and flood forecasts require physically based rainfall-runoff models that include realistic channel morphologies and network maps. Restoration and rehabilitation of small streams will only be successful over the long term with an accurate scientific understanding of the watershed processes that control the rivers. Better quantification of terrestrial carbon and nutrient cycling, much of which involves small streams, is becoming increasingly important to an improved understanding of global cycling of carbon and nutrients. Finally, the application and testing of conventional fluvial geomorphology theory to small watersheds is at a critical stage. Most fluvial theory has been developed in much larger systems, so many of the assumed processes and relationships depend upon extrapolations up into smaller drainage areas than where most data have been available. In addition, many of these relationships were developed elsewhere in the United States, so testing and refining these relationships in the Southeast is a logical extension of the present state of knowledge. We do not claim that the seven papers in this issue can accomplish these challenges by themselves, but we hope that they inspire further study along these lines of research.

Papers in This Issue

This issue is divided into three sections concerned with fluvial processes and features in small watersheds. Part One contains four papers concerned with the morphology of channels, Part Two consists of two papers on bank erosion and historical sedimentation, and Part Three has one paper on automated mapping of headwater channel networks.

Part One, Channel Morphology

This section begins with a paper by David Leigh on channel morphology and evolution of small streams (<20 km²) in the southern Blue Ridge Mountains in response to human impacts. Using conventional statistical analyses of channel cross-section parameters, gradients, and bed-material textures with drainage area, Leigh shows a difference in morphologies—especially channel top width—between forested and pastured or grassland reaches. A series of hydraulic geometry regression equations relate channel form to drainage area. Processes of floodplain evolution and a centennial history of floodplain formation and readjustment in the watershed are identified through the analysis of ¹³⁷Cs, optically stimulated luminescence (OSL), and ¹⁴C dating of floodplain and terrace alluvium to distinguish between historical and pre-historical sediment.

The paper by Kristen Blanton et al. examines bankfull channel indicators at 45 sites in small Florida streams ranging in drainage area from 0.52 to 805 km². Flow duration curves and flood frequencies are analyzed to determine the frequency of bankfull discharge. They find that the elevation of a break in slope of the banks or of the valley flat are the most reliable indicators of bankfull stage in this region, with the latter performing best for reaches with wetland floodplains. They conclude that bankfull stage has a duration of ∼25 percent of the time and that...