Morphological and sedimentological responses of streams to human impact in the southern Blue Ridge Mountains, USA

Abstract

Morphological and sedimentological responses of streams to basin-scale impact have been well documented for intensively agricultural or urban areas. Sensitivity thresholds of streams to modest levels of disturbance, however, are not well understood. This study addresses the influence of forest conversion on streams of the southern Blue Ridge Mountains, a region that has received little attention with respect to human impact on stream channels. Basins were chosen for this study to represent the end members of the range of human impact in the area, with the forest cover of the basin used as a proxy for level of impact (ranging from about 70–100% regionally). Two pairs of lightly impacted (> 90% forest) and moderately impacted (70–80% forest) sub-basins of the upper Little Tennessee River were identified for comparison. Reach characteristics (e.g., slope, drainage area, and riparian cover) were aligned in each pair to isolate contrasting forest cover as the primary driver of any detected differences in morphology and sedimentology. A suite of standard cross-sectional and longitudinal data was collected for each reach for characterization of the sedimentology and morphology of the streams. Difference of means tests were conducted to identify parameters significantly differing between the lightly and moderately impacted streams in both pairs. Consistent and significant differences within both pairs were demonstrated in bankfull width/depth ratios, baseflow wetted width, and particle size on the stream bed both in the thalweg and throughout the channel bed. The moderately impacted streams are narrower than the lightly impacted streams, and the bed texture of the moderately impacted streams is finer than that of the lightly impacted streams. The moderately impacted streams contain a higher percentage of < 2 mm particles in riffles, a metric which has been shown to be highly correlated with biotic integrity in the southern Appalachian Highlands. Although this study has shown that human impact in these basins has resulted in an overall fining of bed texture, few conclusions can be drawn regarding the morphological response of the streams to the levels of impact affecting the upper Little Tennessee River basin. Levels of disturbance in the southern Blue Ridge Mountains may be below the thresholds of morphological sensitivity or have not persisted for sufficient duration for morphological response to be evident. Additionally, morphological adjustment to disturbance may be more effectively addressed system-wide, as opposed to at the reach scale.