SEDIMENT SIZE DISTRIBUTIONS IN FORESTED HEADWATER STREAMS OF THE NORTH CAROLINA PIEDMONT

Headwater streams constitute more than 70% of total stream channel length in North America, yet geomorphic controls on such streams are still poorly understood in many regions. For example, close coupling between hillslope, debris flow, and fluvial processes in headwater streams may counter general downstream-fining trends. The goal of this study was to perform an intensive analysis of sediment size distributions in moderate-relief headwater streams, something rarely performed at such a small (<5 km2) scale. Twenty-seven Wolman pebble counts were conducted on 13 first to third-order streams within 3 km of each other in Gaston County, North Carolina. The watersheds are forested and represent relatively undisturbed conditions for the Carolina Piedmont. The underlying rock types are felsic metavolcanic rocks and quartz-sericite schist, and each stream drains only one lithology. Ongoing data analysis will relate sediment size distributions to watershed area and channel slope. Median (D50) grain size in all the reaches where Wolman pebble counts were performed ranged from 12 – 46 mm, and averaged 26.9 mm overall. Of the ten streams where multiple counts were conducted, six display a distinct trend toward downstream coarsening. Grain sizes in the lower reaches of two streams may be influenced by a possible backwater effects from the South Fork of the Catawba River. Uniformity coefficients ranged from 1.89 – 9.00, showing a pattern of increasingly well-sorted bed material in the downstream direction in eight of the ten streams sampled multiple times. The data also show that woody debris jams lead to accumulations of poorly sorted sediment with low D50 values relative to the mean D50 value. Between-stream variability approaches the magnitude of longitudinal variability in any single stream. This suggests that extremely local geomorphic history exerts a strong influence on headwater stream sediments.