ABUNDANCE OF WOODY DEBRIS IN URBAN AND RURAL STREAMS IN THE PIEDMONT AND BLUE RIDGE PROVINCES OF SOUTH CAROLINA

Woody debris from riparian vegetation provides habitat for aquatic organisms, retains sediment and organic matter, and can enhance biogeochemical function in streams. However, few studies have examined the influence of urban land cover on the abundance of woody debris in small streams. This study quantified the abundance of woody debris in urban and rural streams in northwestern South Carolina. First to third order streams were selected from watersheds with one of four predominant land covers: commercial urban, residential urban, rural (mixed forest and pasture), and mostly (>90%) forested. Five streams were sampled in each watershed category. Both large woody debris (LWD; diameter >10 cm) and smaller debris (diameter 5-10 cm) were measured along one 100-m reach of each stream. Also, at 10-m intervals, bankfull channel width and percent canopy cover were measured, as was the trunk diameter of the closest canopy tree within a 5-m radius on either bank. At 20-m intervals, bankfull maximum depth, floodprone channel width, and floodprone maximum depth were measured. Both the volume and density of woody debris along each stream reach were standardized by bankfull channel area. Neither woody debris volume nor density differed significantly among the four watershed groups, although volume and density of woody debris tended to be most variable among commercial and rural streams. Across all 20 streams, woody debris volume (both total and LWD) was significantly positively correlated with percent canopy cover but not with total tree basal area or median tree diameter. Densities of total woody debris and of LWD also correlated positively with percent canopy cover. However, neither total woody debris nor LWD density was significantly correlated with either tree basal area or median tree diameter. Rural streams tended to have higher channel entrenchment (ratio of floodprone width to bankfull width) than streams in the other land cover categories. Across all streams, density of total woody debris (but not of LWD) correlated positively with entrenchment. However, volume of woody debris did not correlate significantly with entrenchment. Our results suggest that the presence of riparian trees can allow woody debris to accumulate in urban streams at levels comparable to those in streams draining mostly forested watersheds.