GEOMORPHIC CONTROL OF LARGE WOODY DEBRIS ALONG SOUTH RIVER, SHENANDOAH VALLEY, VIRGINIA

Numerous studies on large woody debris (LWD) have focused on forested mountain streams located in the Pacific Northwest. Wood in these streams typically forms, log jams that have a profound impact on stream morphology, promoting channel stability and forcing pools. However, studies are sparse on larger rivers where LWD occur in individual pieces. Located in the Shenandoah Valley of Virginia, the South River is a fourth order, gravel-bed river with a drainage area of over 600 km² at its confluence with the South Fork Shenandoah River. The river can be divided into two reaches, upstream and downstream. The upstream reach is sinuous with slopes between 0.0013 and 0.0017 and the downstream has abundant islands and steeper slopes between 0.0019 and 0.0022. Agricultural practices dominate in the riparian zone of the river and this difference in land use creates a distribution of LWD unlike that of forested streams. Four species of trees are dominant at South River: sycamore (Platanus occidentalis), silver maple (Acer saccharinum), boxelder (Acer negundo) and eastern cottonwood (Populus deltoids). Average diameters of LWD are 32 cm with lengths averaging 6-7 m. At the South River, LWD has no influence on channel morphology, but wood traps fine-grained sediment and associated contaminants making it the most significant mechanism for storing clay and silt within the channel perimeter. LWD accumulations are greater in the upstream reach, with a frequency averaging 67 pieces/km. The downstream wood frequency averages 36 pieces/km. In both reaches, 60-75% of LWD occur in pools, with 10-20% in riffle settings. Most of the wood falls from adjacent banks by both natural processes and agricultural practices. These data indicate reach-scale morphology, in the form of pools and riffles, is the primary controlling factor of the distribution of LWD in this agriculture-dominated setting.