CHARACTERIZING CHANNEL HEAD RESPONSE TO ANTHROPOGENIC LANDSCAPE MODIFICATION

Anthropogenic modification of hillslopes disturbs the landscape by removing vegetation that formerly intercepted rainfall. The removal of vegetation subjects exposed soil to increased direct precipitation, which in areas with clay-rich soils, may lead to rapid hillslope gullying and sediment aggradation upon the adjacent valley floor. Across the Piedmont of eastern North America, the arrival of Euro-American settlers resulted in significant upland soil loss and the aggradation of meter-thick deposits of “legacy” sediment upon valley bottoms as basin sediment yields increased up to 25 times background following colonization and forest clearing, primarily for agricultural purposes. We hypothesize that such landscape alterations impacted headwater streams by displacing channel initiation points down valley.

We investigate the drainage network in William B. Umstead State Park near Raleigh, NC to determine the influence of this additional legacy sediment on channel head migration. Using statistical relationships between upslope contributing area and local valley slope, we determine the spatial positions of predicted channel head locations across the landscape and compare them to their observed position. Coupling these analyses with field observations we find that channels are initiating downslope of their statistically predicted locations. This indicates that in an effort to restore equilibrium after the influx of legacy sediment, down-valley displaced channel heads are now migrating back up into unchannelized valley reaches. The current up-valley migration will not only lead to localized landscape dissection, but will also result in a large flux of sediment and possibly nutrients that may impose significant impacts on hydrologic, biologic, aquatic, riparian, and chemical functions in downstream environments. We present a quantification of the physical and chemical consequences of channel head migration in such environments aimed at improving our understanding of sediment and nutrient additions to Piedmont streams and coastal watersheds, many of which experience high suspended sediment loading and sporadic to chronic eutrophication.